34. M. MELLONI ON THE FREE TRANSMISSION 
stance which shows that the diminution of ‘effect was only about one 
seventh for an increase of thickness equal to thirty-five times that of 
the first piece. The experiment was still more interesting when I em- 
ployed rock salt, in which I was unable to discover that thickness had 
any influence whatever on the amount of the transmission: for pieces 
of 2™™ gave the same galvanometric deviation as pieces of 30™™ 
and 40™™. 
From these observations it follows that the numbers in the second 
column of the table of crystals, though they express the ratios of the 
calorific transmissions of those bodies reduced to the common thickness 
of 2™™-6, may be employed also to represent approximately the ratios of 
the transmissions, even when the common thickness is greater. I say 
approximately, because, in order to determine the true specific trans- 
missions, it would be necessary to know the exact law of the loss at the 
several points of the media. If the losses, as compared with the quan- 
tities of heat which arrive at each of the thin lamine into which we may 
imagine the medium to be divided, were constant, the intensity of the 
rays would decrease in a geometrical, while the layers increased in an 
arithmetical ratio; and in order to know how much one substance is 
more diathermanous than another, we should vary the relative degrees 
of thickness of the plates until we obtained the same transmission in the 
two cases. The ratio sought would be inversely as the degrees of thick- 
ness which produced an equality of action *. Now we have seen that 
this constancy in the loss does not exist. But in the particular case 
of crystallized bodies, the differences are so very small when the thick- 
ness is increased beyond 3™™, that the ratios obtained by operating on 
thicker screens would not differ materially from those which we have 
found. 
But even if we had succeeded in ascertaining the specific transmissive 
powers of the different substances, the question would not yet be solved 
in a general manner ; for we shall see in the second Memoir, that if, 
while we vary the temperature of the calorific source, we do not change 
the order of the transmissions also, the relations of these quantities are 
no longer the same. To perceive this we have only to recollect what 
has been already stated as to the action of rays emitted from a source of 
low temperature on certain substances; that is, that the heat of the 
human body instantly passes through a certain erystal, and that crystal 
is rock salt. 
It is known that the caloric rays of the hand are completely stopped 
* by glass. Hence, although the ratio of transmission between glass and 
rock salt, when the source is an Argand lamp, be 62 : 92, it becomes 
~ * For the demonstration of this proposition, see-Bouguer, Traité d'Optique 
sur la Gradation de la Lumiere, Paris, 1760, liv. m1. sect. 1'¢, art. 1, 2,3, 4. 
