248 PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 



not so mucli perhaps because the generality of physicists had declared, in oppo- 

 sition to the two English philosophers, for the material theory, as that the scien^ 

 tific inquirers of that period scarcely occupied themselves at all with the question. 

 Still, however, the material in hand did not cease to accumulate, which, when 

 thrown at a later period into the scale, was destined to give an unquestionable 

 preponderance to the mechanical theory. 



The study of radiant heat had taught that every heated body in a colder 

 medium sends forth on all sides cah^rific ra3's in like manner as a luminous body 

 distributes rays of light. And as the luminous rays pass through air and other 

 transparent bodies, without communicating to them the property of luminosity, 

 so the rays of heat traverse the air and other diathermanous substances without 

 imparting to them any sensible warmth. The rays of heat are then only con- 

 verted into perceptible heat when they are absorbed by some body upon which 

 they strike, in the same way that certain bodies (phosphorus, for instance) become 

 themselves luminous under the influence of strong raj's of light. 



Like the rays of light, the rays of heat are propagated with a velocity which, 

 in relation to terrestrial distances, may be termed instantaneous. They follow 

 the same laws of reflection and refraction as the rays of light. In the rays of 

 heat just such differences appear as those wliich, in the case of the rays of light, 

 determine the diversity of colors. In a word, it is now full}' recognized that the 

 rays of light and heat are, in their nature, identical, and that if any modification 

 distinguishes them, it can only be of a quantitative nature ; whence it follows 

 that the phenomena of light and heat must be referred in principle to the same 

 explanation. Since, then, in regard to the phenomena of light, the theorjr of 

 vibration has triumphantly vindicated its claims against the theory of emanation, 

 no doubt can any longer be properly entertained that the phenomena of heat 

 also are to be rei'errcd to mechanical principles. 



A body is luminous when its several atoms oscillate with a sufficient degree 

 of intensity and velocity about their position of equilibrium. These atomic 

 vibrations call forth in the surrounding ether an undulatory movement, by which 

 the rays of light are propagated, and hence many analogies exist between sound 

 and light. While sound is generated through the vibratory motion of elastic 

 bodies, light arises from a far more rapid oscillatory motion of the minutest or 

 ethereal particles of matter. As sound is propagated through an nndulatory 

 movement of the air, so is light through an undulatory movement of the ether. 

 Like the diversity of tones, so the diversity of colors arises from a difference in 

 the duration of the oscillations of the conducting medium. But, seeing that the 

 rays of light and of heat emitted hy a'body in combustion are identical, can we 

 avoid the conclusion that the cause of its light and its heat is the same ; that the 

 heat of hodics proceeds^ also, only from an oscUlafonj movement of its atoms f 



Perhaps it may be objected that non luminous bodies also emit heat; that the 

 sun's light, as well as electrical light, is accompanied in large proportion by invisible 

 rays of heat. It might hence seem that a difference exists between the rays of 

 light and those of heat. But more exact investigation has shown that it is only 

 a qucmlitative difference which is here in question. The obscure rays of heat are 

 not diffbrent in their intrinsic nature from those which are at the same time lumi- 

 nous; there are rays which are endued with a greater amplitude of oscillation than 

 the red, and whose period of vibration, therefore, exceeds the limit to wliich the 

 organization of the eye restricts its visual perceptions. 



Thus the study of radiant heat has led to the same consequences which Rum- 

 ford and Davy had deduced from their experiments on the production of heat 

 by friction.* But, though the majority of physicists shared the views of the two 

 philosophers, and entertained the conviction that the emanation theory was 

 thenceforth untenable, for a long time nothing further was done to bring this 

 question to a decision until some 24 years ago it was again taken in hand with 

 great energy and prosecuted with ardor in various quarters. The first by whom 



