M. MELLONI ON LIGHT AND RADIANT HEAT. 389 



It is true that there appears a striking disparity in their manner of 

 propagating themselves when the calorific and the luminous radiations 

 fall upon the surface of diaphanous bodies, whether solid or liquid. In 

 this case the medium is immediately traversed by one portion only of 

 tlie radiant lieat, while the other portion is slowly transmitted from 

 layer to layer. But to a certain extent this phsenomenon may be ac- 

 counted for by supposing the ordinary conductible heat to be produced 

 by a vibratoiy motion communicated by the aethereal undulations of 

 every length to the anterior molecules of the medium, and then gra- 

 dually propagated to its further surface*. 



Considerations derived from the different lengths of the sethereal un- 

 didations will enable us to account for the two very distinct species of 

 transparence observable in diaphanous bodies relatively to the rays of 

 heat and those of light. Thus it will be easy to conceive why certain 

 substances possess but verj' little diathermaneity, though they are per- 

 fectly clear, if it be admitted that they intercept all the obscure waves, 

 the sum of whose intensities is greater than that of the luminous waves 

 even in the radiation of the most brilliant flame. On the other hand, 

 we shall perceive the cause of the diathermaneity of certain perfectly 

 opake media, in the supposition that they allow themselves to be pene- 

 trated by particular groups of obscure undulations. 



The hypothesis of identity is sufficient, no doubt, to explain a great 

 number of general facts. It would not, however, embrace all particular 

 cases, and, if we should proceed to a numerical examination of experi- 

 ments, would even give rise to some serious objections. But I deem it 

 useless to enter into detail on this subject, as the phaenomena which I 



* The propagation of ordinary conductible heat considered as the effect of 

 molecular vibration is essentially different from the vibratory motion pi-oduced 

 by sound in ponderable matter, or that produced by light in the aether ; for, in 

 the slow propagation of ordinai-y heat, the points first heated lose their tem- 

 perature only by little and little, and this temperature is always higher than 

 that which is gradually transmitted to the rest of the bod}', unless it be lowered 

 by other causes. But in the propagation of sound and light, the points first 

 agitated immediately communicate their motion to the adjacent points, and then 

 return to a state of repose, in which they remain until they are again set in 

 motion by a subsequent impulse. A wave is thus formed which propagates 

 itself with great velocity, and at any given instant there is no motion except in 

 the point which the sonorous or luminous wave has attained. The cause of 

 this difference, which seems inexplicable by the midulatory theory, has been 

 the object of M. Ampere's inquiry in a memoir published for the first time in 

 the Jiihliol.heque Universelle of Geneva (May, 1832), and subsequently re- 

 printed in the Aniiules de Chimie and in the London and Edinburgh Philoso- 

 phical Magazine. M. Ampere finds that the cause of the difference between the 

 flow propagation of heat and other undulatory motions is to be assigned to the 

 diKtinction which he establishes between the vibrations of the molecules of bo- 

 dies with respect to one another, and the vibrations of the (itoms which consti- 

 tute each molecule ; inasmncli as these two species of vibration may take place 

 nut only separately but simultaneously in the same pohits of a body. 



