ON THE IDENTITY OF HEAT AND LIGHT

BY M. ABRIA


Dean and Professor of Physics of the Faculty of Sciences of Bordeaux Corresponding member of the Philomatic Society of Paris, Member of the Academy of Sciences, Belles Lettres, Arts of Bordeaux , of the Society of Physical and Natural Sciences of the same city.

BORDEAUX
G. GOUNOUILHOU, PRINTER OF THE FACULTIES.
11 rue Guiraude, 11

1866

ON THE IDENTITY OF HEAT AND THERE LIGHT

The various branches of experimental physics have been, for half a century, the object of theoretical research brought about by the progress of science, research which has led to the discovery of relations as curious in themselves as important by their consequences. We have not endeavored only to better understand each class of phenomena, we have endeavored to deepen those which establish the transition from one class to another, which make it possible to pass from movement to heat, from that with electricity or light. It is to the work undertaken in this direction that we owe many of the discoveries with which science has been enriched in recent years.

It seemed to me that it would not be without interest to summarize those of recent researches which lead to well-established and generally adopted consequences on the connection of the various forces to which the molecules of bodies are subjected. The subject would be much too vast if I treated it as a whole; it is better to restrict it, and limit oneself to considering it from a single point of view.

Among the agents to which are due the phenomena that we observe or that we can reproduce at will, there are two whose properties present obvious analogies, noted especially for about thirty years, and from which flow important conclusions on their origin and on the conditions in which they arise. Moreover, these two agents, heat and light, present this remarkable peculiarity that they accompany each other almost constantly, and that if the source from which they emanate gradually increases in intensity, we first see appearing the phenomena of heat to which those of light are soon added. Thus, all the bodies which are the seat of energetic chemical actions, coal, hydrogen, sulphur, iron, when they combine either with oxygen or with chlorine; all those which are traversed by a sufficiently intense electric current, such as metallic wires or two coal cones interposed between the poles of a strong pile; finally all those who, by any cause whatsoever, on which we can even in certain cases emit only conjectures, in that of the sun and the stars, for example, are brought to a similar state; all these bodies, I say, enjoy the property of simultaneously emitting luminous and calorific rays, of being the source of radiations capable of exerting on our organs two very distinct sensations, perceived: one by all points on the surface of the body, the other only by a special organ. The sensation of heat can in fact be felt by most of the nerves, which, speaking of the cerebro-spinal axis, lead to the various regions of the epidermis; only one, the optic nerve, has the property of transmitting the sensation of light.

If light and heat are clearly distinguished from each other by the difference of the impressions which they produce on our senses, the actions which they exercise on the bodies of nature do not allow them to be confused either. Under the influence of heat, most substances experience very sensible variations of volume, sometimes followed by no less remarkable changes of state. Under that of light, chlorine and hydrogen unite to form hydrochloric acid; the salts of silver, and in general the substances employed today in photography, experience modifications which enable them to undergo reactions which they originally refused.

The sources of light being at the same time sources of heat, the two agents accompanying each other almost constantly, enjoying almost identical properties, propagating through media, reflecting on their surface, refracting in their interior following the same laws, giving rise to phenomena so similar that the verification of the properties of rays of heat analogous to those of rays of light is generally reduced to replacing the eye by an apparatus sensitive to thermal effects, one is led to wonder not only if the mode of production of the two classes of phenomena is the same, but also if there is not complete identity between the two agents; if that which produces on the organ of sight the sensation of light, it is not also what causes us on the other parts of the body the sensation of heat; if the agent which determines certain chemical combinations, it is not also the one which dilates the bodies and makes them change state, and if it is possible to realize, in this hypothesis, the analogies and the differences which the properties of bodies subjected to the action either of light or of heat. This question has been studied by several physicists for a number of years: independently of its own interest, it smacks of natural philosophy at several points. I thought the Academy would listen with some interest to a brief account of the work undertaken on this subject and the results achieved.

I
Our knowledge of the mode of production of light is quite advanced: the many phenomena of optics, the work of which they have been the object, the curious consequences to which we have arrived, by allowing ourselves to be guided by theory, make it extremely probable today, not to say certain, that luminous bodies themselves owe this property to an extremely rapid vibratory movement by which their last particles are animated. It is difficult to form an idea of ​​the rapidity of a movement which, in a single second of time, makes the luminous atoms oscillate six hundred thousand trillion times on either side of their position of equilibrium, and which can propagate while retaining the same character through a very large number of substances. The reality of this movement is not doubted today by any physicist. The numerous verifications to which the fundamental principle of wave theory has been subjected, and which have not only concerned the phenomena envisaged in a general way, but above all the numerical consequences that observation could address, form a bundle of proofs which are far from being encountered in other branches of science. It is not useless to give a brief account of it here. form a body of evidence that is far from being encountered in other branches of science. It is not useless to give a brief account of it here. form a body of evidence that is far from being encountered in other branches of science. It is not useless to give a brief account of it here.

The principal phenomenon from the theoretical point of view is undoubtedly that of interference, that is to say of the increase or decrease in intensity which can result from the meeting at the same point of two rays of light; the coloring of thin sections offers numerous examples of this, already known in the time of Newton, and studied by this great physicist and by his successors. But it is above all to Fresnel that we owe this remarkable property of light by means of simple experiments, beyond all objection. The applications made of it to the production of colors in the case of thin films, either by reflection or by refraction, and in that of certain thick films, are simple enough to be perfectly understood almost without calculation.

Another very remarkable and decisive experiment from the point of view of theory, is that whose principle was exposed in 1839 by Arago, but which was not carried out experimentally until a few years later by MM. Fizeau and Foucault. If light is propagated by waves, its speed of propagation must be less in water than in air or in vacuum, and must in general diminish in proportion as the refractive power of the substance which it passes through increases. Conceived to definitively decide the choice to be made between the two theories of corpuscular and wave which still shared the assent of physicists, this experiment led to the implementation of processes which make it possible to measure the speed with which light travels through very small intervals. When we think that this speed is 298,
The most convincing proofs in favor of the vibratory motion of luminous bodies are drawn above all from the exactness with which, in this hypothesis, we account for all the accidents, all the modifications which the rays of light offer us. Several times the theory has revealed properties which experience had not yet revealed and which it has fully confirmed. Thus, in Fresnel's memoir on the diffraction of light, a memoir in which the formulas which make it possible to apply the calculation to this kind of phenomena are recorded, this illustrious physicist had neglected to examine the consequence to which they led in a remarkable case, that of an opaque disc placed in a thin stream of light. Poisson, who was far from being a partisan of the wave doctrine, noticed that, according to the calculations, the center of the shadow should be as bright as if the screen did not exist. The experiment attempted almost immediately by Arago was crowned with complete success.

The propagation of motion in crystallized substances presents in certain circumstances peculiarities which have only been fully known by the thorough study of the equation to which calculation leads, an equation given for the first time by Fresnel, who contented himself with discussing it in a general way, and who died a few years later without knowing the curious consequences that could be deduced from it. The discussion of this equation, carried out a little later by an Irish geometer, Mr. Hamilton, revealed that a ray of light, penetrating from air or vacuum into a crystal under certain incidences, was to expand into an infinity of rays distributed over a conical surface; and that in other directions a single ray issuing from the crystal must, on the contrary, present in vacuum or in air an infinity of rays distributed in the same way over the surface of a cone. The mode of section of the crystal, the incidence of the rays on the surface, the precautions to be taken, form a set of conditions which one would probably not have discovered if the calculation had not served as a guide. The verification of this remarkable property constitutes one of the best proofs of the reality of the wave motion which gives rise to light. the incidence of the rays on the surface, the precautions to be taken, form a set of conditions which one would probably not have discovered if the calculation had not served as a guide. The verification of this remarkable property constitutes one of the best proofs of the reality of the wave motion which gives rise to light. the incidence of the rays on the surface, the precautions to be taken, form a set of conditions which one would probably not have discovered if the calculation had not served as a guide. The verification of this remarkable property constitutes one of the best proofs of the reality of the wave motion which gives rise to light.

This consequence is corroborated by phenomena of the same order, but of very different appearances, discovered by Arago in rock crystal, by Biot and Seebeck in certain liquids of organic origin, such as turpentine and sugar solutions. , phenomena known as circular polarization, from which elliptical polarization arose a little later. The theory of waves, applied here by Fresnel with the same success, gives a perfectly satisfactory explanation of it, and led this illustrious physicist to the discovery of a particular kind of double refraction, of double circular refraction, which arises when the light propagates in certain substances, and in particular along the axis of the rock crystal. A remarkable consequence of the reasoning with the aid of which these curious phenomena are accounted for, a consequence perfectly verified by observation, is that their manifestation can be made evident when there is a small difference between the velocities of two rays which pass through the body simultaneously. This difference is, in fact, only 1/80,000 in quartz, and is sufficient to clearly separate the two primitive rays from each other.

The reasons which physicists give in favor of the existence of the extremely rapid vibratory movement with which the last particles of luminous bodies are endowed, are really powerful enough to draw the assent of any reasonable mind and to make him admit without hesitation in these body a state of which our organs alone would be powerless to ascertain the existence directly. But before examining "whether these consequences can be extended to hot bodies, and whether we must likewise conclude that heat is due to undulations, it is necessary to make known in greater detail what this movement of luminous bodies consists of. .

which coexist in the body without being confused, and whose extreme durations are approximately in the ratio of two to one. This multitude of vibrations which we are obliged to admit in luminous substances, is a fact with which the mind needs to familiarize itself when it wishes to form as clear an idea as possible of the phenomenon. It is due to the great number of vibrating molecules, and also certainly depends on the conditions to which they are subjected, that is to say, on the very constitution of the body. is a fact with which the mind needs to become familiar when it wishes to form as clear an idea as possible of the phenomenon. It is due to the great number of vibrating molecules, and also certainly depends on the conditions to which they are subjected, that is to say, on the very constitution of the body. is a fact with which the mind needs to become familiar when it wishes to form as clear an idea as possible of the phenomenon. It is due to the great number of vibrating molecules, and also certainly depends on the conditions to which they are subjected, that is to say, on the very constitution of the body.

These various movements are not all, in fact, of the same amplitude: the vibrating atoms, whose masses are very probably different from one to the other, deviate more unequally from their positions of equilibrium, and the shock communicated by each of them to the optic nerve, with the aid of interposed media, depends on the mass and the initial velocity. Hence result variations of intensity in the light emitted by each of them, when this light is decomposed by its passage through a prism which separates it into its various elements. For certain substances and under certain conditions, for sodium in particular, the movement is almost unique, isochronous for all points of the body, and gives a light whose ripple lengths, intimately linked with the durations of the vibrations themselves, vary only between very restricted limits. But for most light sources, the durations of the vibratory movements are multiple, their intensities are more unequal, and it is on this property that the method of spectral analysis introduced into science by MM. Kirchoff and Bunsen.

The vibrations which produce light are not generally reflected in the same proportion by the media which they encounter. Colorless substances which do not alter, in the act of reflection, the color of the light beam, return equal fractions of each of the elementary movements which constitute it. The same thing happens for white surfaces; but this equality in reflection ceases for bodies which possess their own color; these reflecting in greater abundance the rays of the same color, allow the others to penetrate more easily into the interior of their mass.

In the case of colorless substances, it has been possible to find, by theoretical considerations, a relationship verified by experience between the quantity of light reflected at a given incidence, and the refractive index of the substance, i.e. the relationship that exists between the speeds of propagation of the luminous movement in the external environment and in that on which the light arrives. This relation, of extreme importance, will be very useful to us, as we shall see further on, when we have to compare heat and light with each other.
Moreover, these same movements are not transmitted entirely through a given environment; some cross it without changing intensity; the others, on the contrary, are weakened in a more or less considerable proportion, sometimes even completely absorbed. Thus, a red glass lets through without great alteration the particular vibrations, whose wavelength is approximately six to seven tenths of a thousandth of a millimeter, and completely extinguishes all the others, which very probably modify the movement of its molecules. In general, a body. transmits without loss the waves which give it its own color when seen by transmission, and arrest the others; blue waves alone can pass through a solution of copper sulphate; the yellow and green waves are extinguished by a solution of potassium permanganate, which does not stop the extreme blue and red rays. Transparency for some rays does not result, as a consequence, transparency for others, and this because of the great diversity of movements that coexist in the light source, and propagate without being confused. Sensation alone is the result of all the impressions exerted on the optic nerve by each of them, white when all coexist and have been transmitted to the organ without alteration, otherwise composed when only a few can reach him. which does not stop the extreme blue and red rays. Transparency for some rays does not result, as a consequence, transparency for others, and this because of the great diversity of movements that coexist in the light source, and propagate without being confused. Sensation alone is the result of all the impressions exerted on the optic nerve by each of them, white when all coexist and have been transmitted to the organ without alteration, otherwise composed when only a few can reach him. which does not stop the extreme blue and red rays. Transparency for some rays does not result, as a consequence, transparency for others, and this because of the great diversity of movements that coexist in the light source, and propagate without being confused. Sensation alone is the result of all the impressions exerted on the optic nerve by each of them, white when all coexist and have been transmitted to the organ without alteration, otherwise composed when only a few can reach him.

We are therefore authorized to conclude, in the present state of science, that the atoms of luminous bodies are animated by extremely rapid vibratory movements, varying from one to another in duration and energy, or, what comes to the same thing, in amplitude, being transmitted from the light source to the other mediums, preserving the same duration, but experiencing, in their intensity, a weakening which depends on the nature of the medium itself and on the duration of the vibratory movement. The communication of these movements to the optic nerve determines the sensation, the nature of which depends on the number and energy of the elementary movements.

II
The heat is also due to a vibratory movement. This assertion rests especially on the numerous analogies which exist between heat and light and on some direct proofs, proofs to which it is however difficult, in the present state of science, to give all the extent and all the rigor that we would be entitled to demand. The sensitivity of the organ of sight, the sharpness with which one can, with the help of instruments of suitable magnification, distinguish the smallest details, have undoubtedly contributed to the progress of optics. We lack an analogous organ for heat; it is impossible for us to perceive the calorific images as we perceive the luminous images, and we are obliged to make up for this imperfection of our senses by the use of thermometric apparatus, the most perfect of which allow us only to analyze phenomena of a certain energy, the attempts hitherto attempted to equip us means capable of making us study calorific rays of a tenuity comparable to that of light rays which have always failed. But if we review the verifications made for a certain number of years of several calorific phenomena analogous to those offered to us by light, we cannot prevent ourselves from concluding that the cause which gives rise to both is analogous. others. the attempts made up to now to provide us with means capable of making us study calorific rays of a tenuity comparable to that of light rays having always failed. But if we review the verifications made for a certain number of years of several calorific phenomena analogous to those offered to us by light, we cannot prevent ourselves from concluding that the cause which gives rise to both is analogous. others. the attempts made up to now to provide us with means capable of making us study calorific rays of a tenuity comparable to that of light rays having always failed.

But if we review the verifications made for a certain number of years of several calorific phenomena analogous to those offered to us by light, we cannot prevent ourselves from concluding that the cause which gives rise to both is analogous. others.

Of the calorific sources on which our experiments relate, some emit both heat and light: such is the sun, a body brought to incandescence, either by rapid combustion or any other chemical phenomenon, or by a Electric power; the others only send around it heat: this is the case of bodies heated below red, whose temperature does not exceed 600°. By studying the former, we have been able to recognize that the calorific rays enjoy properties analogous to those of the concomitant luminous rays. Thus, they can interfere, that is to say give rise, as a result of their mutual encounter, to periodic variations of intensity; they experience double refraction, rectilinear polarization, molecular and magnetic rotational polarization. Of these various phenomena, the first alone can be considered as furnishing a direct proof in favor of the existence of the undulatory motion which constitutes heat; the others prove beyond doubt that the modifications offered by the rays of light are sometimes found, even in their numerical values, in the rays of heat which accompany them.

It is probable that if it were possible to operate on extremely slender heat beams, this verification could be pushed much further. But in the present state of science, it is certainly permissible to conclude that if we do not find complete identity in the series of properties of the two agents, the cause must be attributed to the inadequacy of our methods of experimentation; and although these properties have only been verified for the heat of luminous sources, that of dark sources having been unable until now, because of its low intensity, to allow analogous verifications, we can, basing ourselves mainly on the induction, establish the similarity of the mode of production of the two agents.

Other properties, important like the one we have just recalled from the theoretical point of view, have been verified not only on the heat of luminous sources, but also on that of dark sources. They have been studied especially by Melloni, and relate either to the reflection or to the transmission of calorific rays.

These rays are reflected, in fact, like luminous rays; but moreover one can measure with enough precision the quantity of heat reflected by a substance; now, when we compare the proportions of heat and concomitant light reflected by certain bodies, we find almost identical numbers. Out of 100 rays, for example, taken in the red part of a solar spectrum, we obtain: for the number of those which are reflected by a plate of brass, 72 luminous rays and 75 calorific; for those in the green part, 62 luminous rays and 63 calorific. This equality leads, as we will see later, to important consequences.

We have been able to recognize, moreover, that the rays of heat differ from each other by a quality quite analogous to the color in the rays of light. Only, as we do not possess a special organ fulfilling for heat the function devolved to the eye in the act of vision, and as we cannot, consequently, assure ourselves by our senses of the existence of images of heat variously colored, verification which is so easy for us for luminous images, we have supplemented it by measurements of intensity, leading moreover without uncertainty to the same consequences.
It is necessary, in order to understand them well, to be helped almost constantly by the appearances offered by the rays of light.

Let us therefore conceive a series of variously colored flames and let the light which they emit pass through different substances, cut if they are solid into parallel plates, enclosed if they are liquid or gaseous in troughs or tubes also terminated by similar plates. A colorless plate, of glass, of water or of another nature, will determine in the transmitted light an attenuation which will be the same for all the luminous sources; but the colored substances will allow themselves to be traversed by beams of varying intensities with their own coloring and that of the rays which they transmit. Thus, the red flames will be seen very clearly through a red glass, and will appear black on the contrary through blue glass; the reverse will take place for the blue flames, whose light transmitted through a blue glass will be very intense, and almost nil on the contrary if one uses a red or yellow glass.

Let us now replace our variously colored flames by several sources of heat, obtained for example by raising a metal to increasing temperatures, from 400° or 500° to that of incandescence, and let us analyze them using our thermometric methods. modifications which will be experienced by the fluxes of heat which emanate from it in traversing our various substances. Those which will act on heat, like white glass, water, etc., on light, will be recognized by this character which they will cause the calorific beams to experience, whatever the source which will have given them birth, the same weakening or the same reduction of their original intensity. Among all the bodies tested, only one, rock salt, enjoys this property which makes its use valuable in all research on radiant caloric. The other solid or liquid bodies tested up to now determine an attenuation which varies from one substance to another and which is also not the same for the rays of heat emanating from different sources. Thus, ordinary glass transmits about a sixth of the heat radiated by a copper plate heated to 400° of the centigrade thermometer, nearly a quarter of that sent by incandescent platinum and two-fifths of the heat beam sent by a lamp. Locatelli.

The heat emitted by the light source passes through two plates, one of potash chromate, and the other of tourmaline in the ratio of two to one, and this ratio becomes, on the contrary, five to unity for the heat sent by the obscure source. We can therefore, with the help of intensity measurements, observe that the calorific rays are transmitted in different proportions through the same substance according to the source which emits them, as would happen for luminous rays coming from flames of various colors, and, moreover, that the nature of the substance interposed on the path of the rays exerts a marked influence on the intensity of the beams transmitted, as would also be verified with plates of different colors placed successively on the path of the rays emitted by various light sources.
This property of the rays of heat has received the name of diathermy or calorific coloring, and may be verified by other experiments which it is well to recall.
Let us transmit the rays of heat emanating from a source through a prism of rock salt. If the source emits light, there will be dispersion of the latter, and a light spectrum will be formed.

Heat will also be refracted like it, and there will also be a heat spectrum. We shall therefore have in the same regions of space luminous rays and calorific rays, and we shall be able, letting ourselves be guided by analogy, to distinguish red, green, and blue rays of heat, like the corresponding rays of light. If we interpose various substances on the path of these beams separated from each other by refraction, the calorific tint of each of them will be easily recognized by the way in which it will behave with each of the beams contained in the spectrum,
By combining the results of the various observations made so far, we can say what is the predominant tint for the heat of a certain number of substances. Rock salt is of all the bodies examined the only one which is sensibly colorless to heat, which behaves with the various calorific rays like glass, water, alcohol with luminous rays of various tints. It appears, however, that it absorbs a very small but perceptible proportion of the less refrangible rays.

This tint, by using for heat the expressions used for light, expressions which allow us to form a clearer idea of ​​phenomena, is found in a certain number of substances. Red dominates in sulphur, fluorspar, solutions of neutral chromate and di-chromate of potash, sulphate, very extensive indigo, carbon black and especially carbon bisulphide containing iodine. in dissolution. Among the green and blue liquids for heat are found solutions of sulphate of iron, pure water or water containing sulphate of copper, alum, &c. The numerous researches carried out up to now on the thermochrose of different bodies have only been able to discover a diathermic substance, that is to say, permeable by all the calorific rays without distinction: it is the rock salt which still presents a slight coloration. It is certainly remarkable that there exist so many substances colorless to light, and that only one has been met with presenting analogous properties for heat. This absence of athermochroic substances (an expression proposed by Melloni to designate colorless substances for heat), makes it difficult to study the properties of calorific radiation, crystals of rock salt not being very widespread. and that only one has been found presenting analogous properties for heat. This absence of athermochroic substances (an expression proposed by Melloni to designate colorless substances for heat), makes it difficult to study the properties of calorific radiation, crystals of rock salt not being very widespread. and that only one has been found presenting analogous properties for heat. This absence of athermochroic substances (an expression proposed by Melloni to designate colorless substances for heat), makes it difficult to study the properties of calorific radiation, crystals of rock salt not being very widespread.

Among the substances whose thermochrosis, that is to say the action on the calorific rays of various species, interests us the most, there are some which deserve a particular examination: these are those which constitute the various media of the eye. , transparent cornea, aqueous humor, lens and vitreous humor. It is important to know how they behave on the rays of heat which present themselves to reach the retina, and in what proportion these are absorbed or transmitted.
From the very precise experiments of M. Janssen, it results that out of 100 rays of heat emitted by a moderator lamp, about 4 are reflected by the transparent cornea, 88 are absorbed by it and by the various media of the eye comprised between the cornea and the retina, only 8 reach this one. Their thermochrose is, moreover, the same as that of pure water, which absorbs in great abundance the least refrangible rays, and allows itself to be traversed only by those which accompany light of low refrangibibility, that is to say light. blue; but it is important to note that they arrive on the retina in a quantity that is sensible and measurable by our thermometric instruments. When our eye is placed in front of a luminous heat source,

III
The analogies between the properties of heat and light are sufficiently numerous for us to be able to regard the mode of production of the two agents as identical, and, consequently, to extend to the first the laws of motion of the second, laws demonstrated by phenomena. so clear and so numerous. Since heat is the result of a vibratory movement, a movement transmitted from bodies to our organs by means of interposed media, and which, by acting on them, determines a special sensation, each calorific ray must have a particular wavelength. , or, which amounts to the same thing, each oscillating molecule must carry out, in the unit of time, a determined number of vibrations. This wavelength, this number of vibrations, exercise a most marked influence in the phenomenon of refraction, or rather in that of the dispersion which accompanies it, the deviation being so much the greater as the wavelength is less considerable.

Let us first study what experience teaches us in this respect, without concerning ourselves with the absolute value of the duration of oscillation of calorific molecules, a duration to which we will return later. Let us examine how this wavelength varies and the amplitude of the atomic movement which gives rise to heat. without concerning ourselves with the absolute value of the duration of oscillation of the calorific molecules, a duration to which we shall return later. Let us examine how this wavelength varies and the amplitude of the atomic movement which gives rise to heat. without concerning ourselves with the absolute value of the duration of oscillation of the calorific molecules, a duration to which we shall return later. Let us examine how this wavelength varies and the amplitude of the atomic movement which gives rise to heat.

The conclusions reached by experimental research are the same as for light. As the temperature of a body rises, the various calorific rays which it emits become more and more numerous; the wavelength of those which appear progressively is continually decreasing, whence results that the oscillatory movements by which the molecules of bodies are animated become more and more rapid. On the other hand, as the temperature also increases, the amplitude of the pre-existing vibrations itself increases. A double phenomenon then occurs: increase in energy of the primitive oscillations, addition of new increasingly rapid oscillations. The hot body thus becomes the seat of an infinity of movements,

To fix ideas, let us conceive a platinum wire whose temperature is gradually raised by means of an electric current; suppose that this temperature is insufficient to make the wire visible in the dark; let us pass the heat which emanates from it through a prism of rock salt, and let us measure the intensity of the calorific beam obtained by refraction. The thermometer thus finds itself immersed in a completely dark portion of the spectrum, which exerts no action on the eye, but nevertheless very perceptibly impresses the instrument on which it falls. Then let us gradually raise the temperature of the wire, and make it pass from the completely dark state to the dark red, to the bright red, finally to the dazzling white. Already, when the wire has a fairly high degree of heat, unable, however, to make it visible in the dark, the instrument shows in the refracted beam a rise in temperature which is constantly increasing.

One can deduce from the numbers furnished by the thermometer the measurement of the successive amplitudes of the oscillating molecules, and one recognizes without difficulty that this amplitude varies within limits which depend on the degree of sensitivity of the instruments which one uses, but which are rather wide since We have found in certain cases, for the extreme values, the ratio of at least ten to unity.

On the other hand, as the temperature of the wire rises and the amplitudes of the oscillatory motion of the first calorific waves increase, new rays appear corresponding to shorter wavelengths, the existence of which is recognized by moving the thermoelectric pile through the various regions of the spectrum. These movements may have originated at the same time as the earlier ones; but their intensity was then too weak for them to be perceptible by the thermometer; their amplitude also increases with temperature, and all of these new rays increase the total heat emanating from the source. However, the maximum temperature is always in the dark part of the spectrum, beyond the red rays.

Let us replace the platinum wire by another source of heat, by the flame of pure hydrogen, and arrange, as in the preceding case, the thermometric instrument in the spectrum obtained by the passage of this flame through a prism of salt gem: the colors of the spectrum are then extremely weak, the flame of hydrogen being very pale, and one is obliged, to put the apparatuses in the suitable position, to make use beforehand of a brilliant flame, that of the lighting gas, temporarily substituted for hydrogen. The maximum temperature is still located in the almost invisible spectrum, beyond the red rays. The temperature decreases on either side of this position, and is almost insensible or at least very weak in the region which corresponds to violet. Thus, in the hydrogen flame, the most intense heat waves or of greater amplitude correspond to the dark region located beyond the red. They are accompanied by other less intense waves, some longer, others shorter, corresponding consequently to durations of the molecular or atomic oscillatory movement, more considerable for the first, more rapid on the contrary for the seconds. But the long-period vibrations predominate and constitute the greater part of the heat emitted by this flame.

If now we introduce a solid body, a platinum wire or a fragment of lime, it is brought to incandescence, and the total heat radiated is increased in a considerable proportion. These long-period vibrations therefore determine in the solid body other vibrations of a shorter duration than those which they themselves possess. The heat which would be lost as a result of the conductibility of the gas is found condensed in a way in the solid body whence it escapes by way of radiation; but one still observes in this case, as in the preceding ones, movements of very different durations and amplitudes.

The interpretation of the results of the experiment by the wave doctrine gives us, as we see, valuable notions on the movements of the last particles of bodies. It allows us to have a more exact idea of ​​them, to grasp their true character, to disentangle them from one another despite the complication they present. Whichever of the two phenomena, heat or light, to which these movements give rise, which one considers, one arrives at the same consequences: successive appearance of increasingly rapid movements, increase in amplitude of pre-existing vibrations. We can go further, and determine for each light and calorific source the relative proportion of the movements which generate either light or heat.
Instead of employing refraction to separate from each other the calorific and luminous rays emitted by our artificial sources, we can take advantage of the diathermansies of the various substances or the unequal absorptions which they exert on them. A suitable choice of absorbing media will enable us to isolate either the calorific rays or the luminous rays, and to determine the proportion of each of them in the total radiation. Two above all seem to enable us to obtain this result easily. The first is the dissolution of iodine in carbon disulphide, a solution which, when sufficiently concentrated, stops all the light rays emitted by a source of heat and light, while it allows itself to be traversed by the corresponding dark radiations.

The second is the bisulphide of carbon itself, perfectly transparent, which does not stop the light rays and allows most of the dark calorific rays to pass. If one interposes on the path of a luminous and calorific beam coming either from a more or less heated platinum spiral, or from the flame of oil or gas, or from the coal tips of a lamp electric, first carbon disulphide, then the. same liquid added with iodine, and if the quantity of heat transmitted is measured in each case, it will be possible to deduce from the comparison of the results the quantity of light associated with the heat in each of the sources tested. Gold, one constantly arrives at the conclusion that for all sources of heat and light, the dark calorific rays form almost the whole of the beam. Thus, the iodine solution absorbs nothing or almost nothing of the heat emitted by a body heated to 100°, by a spiral brought to an obscure red, by the flame of hydrogen. It only begins to exert significant absorption when the heat source itself begins to emit light. This absorption is about three to four per cent for the flame of oil or gas, four and a half for a spiral of white-hot platinum, and ten per cent when experimenting with electric light. the iodine solution absorbs nothing or almost nothing of the heat emitted by a body heated to 100°, by a spiral brought to an obscure red, by the flame of hydrogen. It only begins to exert significant absorption when the heat source itself begins to emit light. This absorption is about three to four per cent for the flame of oil or gas, four and a half for a spiral of white-hot platinum, and ten per cent when experimenting with electric light. the iodine solution absorbs nothing or almost nothing of the heat emitted by a body heated to 100°, by a spiral brought to an obscure red, by the flame of hydrogen. It only begins to exert significant absorption when the heat source itself begins to emit light. This absorption is about three to four per cent for the flame of oil or gas, four and a half for a spiral of white-hot platinum, and ten per cent when experimenting with electric light.

The considerable proportion of dark heat contained in luminous and calorific radiations is confirmed by other experiments in which we can obtain, as it were, heat on the one hand, and light on the other. If we receive on a lens of rock salt or on a silvered mirror the rays emitted by two cones of carbon placed at the two poles of a strong battery, we can concentrate all these rays in the same point, the conjugate focus of the source. radiant. Now, if we place in the path of the rays an opaque solution of iodine, the light disappears at the focus, but all the heat remains there, and although invisible, although incapable of acting on the organ of sight, it is none the less powerful enough to exert an energetic action on all substances. Paper, tinder, the wood ignite quickly. Metals, such as zinc, lead, iron come into fusion there. The charcoal and the platinum are brought there to a white-red, as if the heat had experienced, in traversing the solution of iodine, no diminution in its intensity.

If we place, on the contrary, on the route followed by the rays emanating from the electric lamp, a solution of alum, a perfectly clear and transparent solution, but which, while letting light pass through it, is very little permeable to the rays calorific especially to the less refrangible ones, one obtains at the focus a luminous image of a very bright brilliance, but whose calorific effects are extremely weak, and which, while still exerting a sensible action on the thermometric instruments, can neither melt metals , nor ignite organic substances.
It is therefore demonstrated by experience that when a source emits both light and heat, the proportion of the first which accompanies the second is very small relative to the total quantity, representing only the tenth in the most favorable circumstances.

What consequences can we deduce from these facts, already observed by Melloni, recently confirmed and extended by the remarkable experiments of Mr. Tyndall? What do these curious data of observation teach us about the movements of the last particles of bodies and about the action that our organs experience from them?
The attentive study of the results recalled successively in this work, and which one can consider today as well acquired, leads first, according to us, to this conclusion; that heat and light, considered in the bodies from which they emanate, are one and the same thing. Among the almost infinite number of movements with which the atoms of a substance which emits both light and heat are endowed, let us distinguish in thought those which correspond to concomitant luminous and calorific rays, those for example emitted by sodium vapor, and which it is natural to choose because they are very uncomplicated, their durations being comprised between very close limits. Instead of supposing that they are two distinct rays which accompany each other, two independent movements which are superimposed, it is simpler to admit that it is the same movement which produces on the retina the sensation of light, on the other organs that of heat.

Let us try to recognize if this assertion is in conformity with experience, or if it is contradicted by it.
The question of the identity of light and heat can obviously be posed only for the concomitant luminous and calorific rays emitted by a source which is at the same time luminous and calorific; Basically, it is reduced to investigating whether the movement, isolated by thought, of the atoms which engender these rays, has the same duration, whether we consider the remission of light, or whether we have regard to that of light. the heat. This duration of the oscillatory movement is itself intimately linked with the undulation length of the two rays in a given medium, in a vacuum, for example, this length being only the space traveled during a complete vibration of the atom. ; so that we are led to compare the wavelengths of the two luminous and calorific rays considered. If this length is different, the durations of the oscillatory movement of the molecules which give rise to light and heat are not the same, heat and light are distinct from each other. If these lengths are equal, on the contrary, the movements which produce light have the same period as those which generate heat; there is therefore no need to distinguish them from each other: heat and light are identically due to the same motion. the movements which produce light have the same period as those which generate heat; there is therefore no need to distinguish them from each other: heat and light are identically due to the same motion. the movements which produce light have the same period as those which generate heat; there is therefore no need to distinguish them from each other: heat and light are identically due to the same motion.

The difficulty would easily be solved if we could operate on an excessively thin calorific ray, comparable, by its thinness, to the rays of light that can be isolated from each other in a way, so as to measure the wavelength of each two. But we are obliged to experiment on a calorific beam composed of several elementary rays, and we can only measure the average wave length of the whole, especially of the most intense. Despite this embarrassment, due to the imperfection of our thermometric measurement procedures, the results obtained so far leave, we believe, no uncertainty about the definitive conclusions.

The wavelengths of light rays are between two limits: 0mm.000360 and 0mm.000750, which have been determined with precision. The calorific diffraction and interference experiments made by MM. Foucault and Fizeau, place the hot and cold fringes in the same places as the luminous and dark fringes, and necessarily assign to the mean wavelength of the calorific beams which reach us from the sun, a value equal to that of the concomitant luminous rays. We can therefore first conclude that these undulation lengths are of the same order of magnitude for the two kinds of rays.

Let us now note that the wavelength of a ray intervenes in other phenomena, notably in those of reflection and refraction, and influences, in the first case, the quantity of reflected light, in the second, on the deviation that the ray experiences, or, to use the proper term, on the index of refraction of the substance. If therefore we cause two luminous and calorific rays to fall on a film from the same direction, as isolated as possible, the proportions of heat and of reflected light will have to be equal or different, according to the equality or inequality of the wavelengths of the light. these two kinds of rays. However, as I said above, experiment gives almost identical numbers for these proportions, the differences of which are sufficiently explained by the uncertainties of the observations. It therefore leads to the conclusion that the incident movements are, in both cases, of the same duration.

The refractive index of a substance for a given light ray is the ratio of the wave lengths of this ray in vacuum and in the substance. Observation shows that this ratio itself depends on the absolute value of the wave length in vacuum; it decreases when this wavelength increases, and increases in the opposite case, the lesser wavelengths being more shortened proportionally than the others when the movement is propagated from the vacuum into the substance. However, the value of this index influences the amount of light reflected in the normal direction by a transparent plate with parallel faces, an amount which can be calculated using a formula that is unnecessary to recall here. On another side, Melloni's experiments have revealed with very great precision the intensity of the heat reflected by the two faces of a thin layer of rock salt or glass for all kinds of rays and the conclusions of these experiments can legitimately be extended to elementary rays. If we calculate the quantity of heat which must be reflected, taking for the refractive index of glass or rock salt that furnished by optical processes, we find a result which agrees with experience, namely 0.077.

To sum up, if we apply to heat the experimental methods employed in optics and which suppose the wavelengths of the luminous rays to be known, we constantly find that the lengths are the same for the luminous rays and the calorific rays of the same refrangibility.

These considerations seem to us decisive, and necessarily lead, in our opinion, to the conclusion that two concomitant calorific and luminous rays are produced by the same movement. We cannot refuse to admit it as soon as we consider the heat as due to a vibratory movement and that we have regard to the results of the various experiments accompanied by measurements made by the numerous observers who have dealt with the study of this agent.

We admit, it is true, implicitly that the speed of propagation of heat in vacuum is the same as that of light. The wavelength of a luminous or calorific ray is equal, in fact, to the speed of propagation of light or heat in the medium considered, divided by the number of vibrations effected in the unit of time. the atomic or molecular group that gives rise to it. Experience shows us that this wavelength is the same for two rays, one of light, the other of heat, which experience the same deviation when they pass under the same incidence of vacuum in a medium. If we conclude that the atomic motion is the same for both, we necessarily admit that the third term of the ratio, ie the speed of propagation of the movement in the vacuum, is equal on both sides. Now, although the speed of heat has not yet been measured like that of light, either by astronomical phenomena or by experimental physics procedures, all that we know about elasticity authorizes us to conclude that equality of the two speeds of propagation, and one falls, if one refuses to admit it, into improbable hypotheses.

The speed of propagation of movement in an elastic medium depends, in fact, only on the elasticity and the density of this medium, and in no way on the absolute magnitude of the molecular shock which gives rise to it, when it occurs. acts of shocks of the same order, which one cannot refuse to admit for light and heat, which, as we have seen, follow and accompany each other constantly. There are therefore strong theoretical reasons for thinking that the speeds of propagation of the two agents are the same.

Moreover, the wavelength being the same on both sides, it is necessary, if the speeds of propagation are not equal, that the numbers of molecular oscillations vary in the same ratio as these speeds themselves. . If, to fix the ideas, we consider the luminous portion of the solar spectrum, which is calorific at the less refrangible end, very chemical at the opposite end, it would be necessary to admit that these three species of rays which are superimposed are generated: those of light, by movements whose speed of propagation and wavelength are known to us directly; those of heat, by other movements of a longer duration, for example, and which would also spread less quickly; so that the number of oscillations effected in the unit of time being, for movements of this order, half of what it is for light, the speed of propagation would also be reduced to half; an analogous conclusion should be applied to the movements which engender chemical action. It will be agreed that strong reasons are needed to admit that this is so.

We are therefore led to conclude that a source of heat and light emits, in reality, heat alone, which can be distinguished into dark heat or heat properly so called, and luminous heat or simply light. Although the reasons we have just given for the existence of luminous heat seem to us peremptory, it is nevertheless not without interest to indicate and discuss two objections which can be raised, and which must be cleared up if we wants the identity of light and heat to be admitted without difficulty.

How is it, in the first place, that luminous heat does not always influence our thermometers, as should always happen on the hypothesis of the identity of the two agents?

How is it, secondly, that there is dark heat? Why does it not impress the retina, especially when it is capable of exerting an energetic action on inert substances?
It would seem, in fact, first, if light is nothing but heat, that a sensible thermometric effect must always accompany the production of a somewhat intense luminous image, a consequence contradicted by observation. The light of the moon, even concentrated at the focus of lenses or mirrors, influences our thermometers in such a small quantity that its action has been denied for a long time.

Likewise, by interposing on the path of the luminous rays emitted by our artificial sources suitably chosen substances, we can remove, as we have seen, almost all the heat, without notably diminishing the light transmitted. But it must first be remarked that the proportion of luminous heat contained in the radiation of known sources, is always an inconsiderable fraction of the whole, the tenth in the most favorable circumstances; so that we can attribute this apparent nullity of action to the lack of sensitivity of our instruments, which show the presence of a source of heat only in cases where the latter possesses a certain degree of intensity.

Moreover, our apparatuses and our organs, our thermometers and the retina, are so different from each other in respect to sensibility, that one cannot really deduce from the silence of the first, when they are subjected to action of quantities of light clearly marked by the latter, a serious objection to the identity of the two agents luminous and calorific.

Of all the sources of heat tried, the electric lamp is that which undergoes the greatest diminution by the absorption of the iodine dissolved in the disulphide of carbon: the loss is one-tenth of the total heat; so that the non-luminous heat is worth at least nine times the heat and light which are absorbed. On the other hand, the light emanating from the battery is, under ordinary conditions, equivalent to that of 560 candles; the non-luminous heat contained in the beam is therefore equal to that of 9 times 560, or 5000 candles at least. This heat, received at a distance of one meter on the thermoelectric pile, the most sensitive of our thermometers, would only produce a very slight effect if its intensity were reduced to 1/10000 of its value, that is to say if it were only equivalent to half the light energy of a candle. But the organ of sight is not only very impressed by such light, it is also sensitive to that of a single candle located 100 meters away, assuming that the air interposed exerts no appreciable absorption. , that is to say, at an intensity equal at most to five thousandth part of that which is barely capable of impressing our most delicate thermometric instruments.

The second difficulty, that of the insensitivity of the retina to dark heat, would be very serious if the sensation of vision depended solely on the quantity of vis viva communicated to the retina; but here it is necessary to question the properties of the organ. Now, everything leads us to believe that this sensation depends not on the quantity of movement communicated, but on the duration of the oscillations or on the number of vibrations perceived in a determined time.

According to the experiments of M. Janssen, out of a hundred calorific rays emanating from a moderator lamp which fall on the transparent cornea, four are reflected, eighty-eight are absorbed by the media of the eye, and eight penetrate to the retina. According to those of M. Frantz, the maximum temperature of the rays of a solar spectrum transmitted through a layer of water whose diathermy, as again results from the very precise measurements of M. Janssen, is the same as that of the environments of the eye, found in orange; and, moreover, rays coming from the dark part situated beyond the red are transmitted in very appreciable quantity. We must conclude from this that the ultra-red rays of a calorific spectrum furnished by a dark or luminous source pass in sensible numbers through the media of the eye, and reach the retina. This, in spite of its sensitivity, does not denote the sensation of light, although other neighboring rays, of a little different intensity, as indicated by the galvanometric deviations, but of a shorter wavelength, give birth to it without difficulty. Whatever the cause of this singular property, whether it must be attributed to a tension constantly maintained between determined limits or to a special structure of the nerves which compose it, the retina seems to behave like a body capable of vibrating at unison of undulatory movements of a certain duration, and which does not obey periods of a longer duration; it yields, in the first case, to the action of an infinitely small living force, and resists, in the second, to much greater quantities of movement. whether it must be attributed to a tension maintained constantly between determined limits or to a special structure of the nerves which compose it, the retina seems to behave like a body capable of vibrating in unison with undulatory movements of a certain duration , and which does not obey periods of longer duration; it yields, in the first case, to the action of an infinitely small living force, and resists, in the second, to much greater quantities of movement. whether it must be attributed to a tension maintained constantly between determined limits or to a special structure of the nerves which compose it, the retina seems to behave like a body capable of vibrating in unison with undulatory movements of a certain duration , and which does not obey periods of longer duration; it yields, in the first case, to the action of an infinitely small living force, and resists, in the second, to much greater quantities of movement.

It even appears, according to the latest experiments of Mr. Tyndall, that if we receive in the eye the bundle of rays emanating from an electric lamp, stripped of the luminous part by the aid of an opaque solution of iodine, and made convergent by a lens, one feels no impression, one in no way notices the presence of the calorific beam, provided however that it falls on the retina alone and not on the neighboring parts, for otherwise the sensation of heat becomes intolerable, which is not at all surprising in view of the energetic actions which the caloric concentrated in the hearth can develop. But these experiences are known to me only through the translation, in the last January 19 issue of the journal Les Mondes, of an article from the Philosophical Magazine. They raise a serious objection: that of the non-alteration of the surroundings of the eye by a beam of heat capable of burning paper, of melting zinc, of making platinum and carbon blades incandescent. It seemed appropriate to me to wait for the publication of the original Memoir of the famous English physicist.

It was therefore necessary to have regard, in this question, as one could foresee a priori, to two very distinct things: it was necessary to study, in the first place, the mode of production of the agent, either heat or light; and, in the second place, the state of the organ which receives the imparted movement and transmits it, whence results the perceived sensation. Of these two questions, the first has been deep enough for us to be able, in the present state of science, to make some very probable assertions on the immediate cause of these two agents, on the state in which the atoms of the hot or luminous bodies. The second is of an entirely different order; to be elucidated, it will require the assistance of experimental physiology. But according to the curious results which I have just recalled,

We therefore come to the conclusion that there is no essential difference between the states of bodies which emit heat alone, or else heat and light at the same time; in both cases, there is oscillatory movement, or rather a set of such movements, of very diverse durations and amplitudes, which probably pass through all the orders of magnitude between two determined limits. Of these movements, some can act on inorganic bodies, cause the mutual distances of their molecules to vary, cause electric currents to arise in them; they can also impress our organs, and give us the sensation of heat. The others, less intense, enjoy the same properties as the first, but can, in addition, determine the sensation of light in the eye.

IV.
The researches made up to now on the vibratory movement which gives rise to light and heat do not only establish the reality of this movement. We can, in certain cases, assign with a fairly high probability the duration of the oscillations when they are unique, and, when they are multiple, the duration of those which have the greatest amplitude, or, which comes to the same thing, the greatest intensity; one can fix, in a word, the period of those which are predominant, which give to the body considered as source of heat and light its distinctive character.

Almost constantly the bodies execute a set of oscillations of various durations; their atoms probably dividing into groups, each of which vibrates with a determined duration and amplitude. For some, the various periods marked by observation are few. Thus, the sodium in vapor executes oscillations whose wavelength corresponds to the double line D of the solar spectrum, and is found to have, consequently, 588 millionths of a millimeter. For potassium, there are three lines: the first red, the second blue, the third violet; for thallium, a green line; for rubidium, a red line; all characteristics and corresponding, consequently, to well-determined movements. the second blue, the third violet; for thallium, a green line; for rubidium, a red line; all characteristics and corresponding, consequently, to well-determined movements. the second blue, the third violet; for thallium, a green line; for rubidium, a red line; all characteristics and corresponding, consequently, to well-determined movements.

But a very large number of bodies, brought to a high temperature, are the seat of a multitude of movements whose durations seem to vary by imperceptible degrees between certain limits, and whose amplitudes are not the same for all, judging from it. at least according to the intensities of the. light rays they emit. What happens as body temperature rises? Experiment shows first that the movements of which we can ascertain the existence below the red heat increase in intensity with the heat itself. The limits of the increase in amplitude depend on the sensitivity of the measuring instruments. With a platinum wire brought gradually to incandescence using an electric current, Mr. Tyndall was able to clearly recognize that the oscillations corresponding to the dark red rays vary in amplitude in the ratio of eleven to unity. For the light rays, they are certainly more extended, although we lack, on this point, precise measurements, if one thinks of the sensitivity of the retina and the facility with which it is impressed by light rays.

These movements seem to depend above all on the nature of the bodies; they are probably influenced, but within rather restricted limits, by molecular forces: the state in which the substance under test presents itself to us brings only a slight modification to the duration of the atomic oscillations which constitute heat.
This curious consequence results from a relation which exists between the absorbing and radiating powers of a body for the same kind of heat. It has long been known that these two powers are equal. But if we examine this law in its relation to vibratory motion, we are led to regard as very probable that the vibrations which are extinguished in a substance when they reach its surface, and which, consequently, are not not transmitted, must be isochronous with those of the substance itself; the amplitude of the latter must then increase in a ratio ordinarily too weak for our apparatus to show it clearly. Longer or shorter period vibrations propagate through the substance, and constitute the emergent beam.
On the other hand, observation shows that the calorific rays emitted by the vapor of strongly heated water are absorbed in a very large proportion by the vapor of water at the ordinary temperature. Hence the consequence that in these two bodies whose molecules are of the same nature and differ only in the state of heat, the duration of the vibrations is the same. In the same way, carbonic acid energetically absorbs the rays emitted by the flame of carbon monoxide; the sulphurous acid allows itself to be traversed only by an inconsiderable fraction of the calorific rays emitted in the combustion of the bi-sulphide of carbon. The atomic vibrations of carbonic acid and sulfurous acid are therefore of the same period for each of these substances respectively,

If these theoretical views are confirmed, the study of absorptive powers will furnish the means, perhaps unique, of determining the duration of the vibratory movements of bodies, unless we encounter thermoelectronic combinations much more sensitive than those which serve basic to current devices.

The rise in temperature also causes new and shorter waves to appear, which correspond to the birth of increasingly rapid movements. Did these new movements preexist in the substance and were they only insensible to our organs and our apparatuses, or did they really appear only at a certain degree of heat? This question, which may be of interest from the point of view of the constitution of bodies, matters less when we only propose to seek the modifications experienced by the atoms of the substance. Be that as it may, they still seem to depend on the nature of the heated body. The water vapor obtained by the combustion of pure hydrogen emits long-period vibrations, only a few of which are capable of acting on the organ of sight. When a solid body is plunged into this flame, the molecular movements of the latter are communicated to its mass, but they take on another character. The oscillations of the solid body thus brought to white appear to be the same as those which it would possess if it had been heated in any other way, either with the aid of another flame, or with the aid of a Electric power; they are specific to it, have special durations and amplitudes which very probably depend on the nature of its molecules. We see here movements of long duration giving rise to others of a shorter duration, just as by immersing certain substances, a dissolution of a salt of quinine, for example,

Finally, the rays emitted by bodies brought to a high temperature still enjoy the property of determining certain chemical combinations, and this action is even one of the most powerful causes of vegetation, and is thus found to contribute to the maintenance of life on the globe. It has been verified for a long time that these chemical rays offer the same properties as their luminous and calorific congeners, and must, consequently, be attributed like them to a wave motion in the source from which they emanate. But I do not propose here to go into the consequences that result from this.

We therefore come to this general conclusion, that our natural and artificial sources of heat and light are the seat of very rapid movements, of variable durations and amplitudes, which coexist without being confused, are propagated by waves, produce on our organs impressions which depend on the state of these organs themselves, and, on the bodies, effects modified from one to the other by the vibrations peculiar to these bodies, vibrations whose duration is determined above all by the species of the bodies. molecules or atoms that make them up. The study of these movements, considered either in the sources of heat, or in the bodies which receive it, must be the object of future research. There is every reason to hope that the new direction in which science is currently entering,