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 
the radiant heat, while the other pertion is slowly transmitted from 
layer to layer. But to a certain extent this phenomenon may be ac- 
counted for by supposing the ordinary conductible heat to be produced 
by a vibratory motion communicated by the ethereal 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 ethereal un- 
dulations 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 very 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 
numper 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 phenomena which I 
* The propagation of ordinary conductible heat considered as the effect of 
molecular vibration is essentially different from the vibratory motion produced 
by sound in ponderable matter, or that produced by light in the ether ; for, in 
the slow propagation of ordinary heat, the points first heated lose their tem- 
perature only by little and little, andthis temperature is always higher than 
that which is gradually transmitted to the rest of the body, 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 isno motion except iu 
the point which the sonorous or luminous wave has attained. The cause of 
this difference, which seems inexplicable by the undulatory theory, has been 
the object of M. Ampére’s inquiry in a memoir published for the first time in 
the Bibliotheque Universelle of Geneva (May, 1832), and subsequently re- 
printed in the Annales de Chimie and in the London and Edinburgh Philoso- 
phical Magazine. M.Ampére finds that the cause of the difference between the 
slow propagation of heat and other undulatory motions is to be assigned to the 
distinction which he establishes between the vibrations of the molecules of bo- 
dies with respect to one another, and the vibrations of the atoms which consti- 
tute each molecule ; inasmuch as these two species of vibration may take place 
not only separately but simultaneously in the same points of a body. 
