34 M. MEI.LONI ON THE FREE TRANSMISSION 



stance wliich 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 tlic 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 tlie 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 

 approximateli/, 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 laminae 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 hoAV much one substance is 

 more diathermanous than another, we should vaiy 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 crj^staUized 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 crystal, and that crj^stal 

 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, TrailS d'Optiqnc 

 mil- la Gradation de la Lumiere, Paris, 1760, liv. iii. sect. !•'«, art. 1, 2, .3, 4. 



