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 employed rock salt, in which I was unable to discover that thickness had any influence whatever on the amount of the transmission: for pieces of 2ᵐᵐ gave the same galvanometric deviation as pieces of 30ᵐᵐ and 40ᵐᵐ.
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 2ᵐᵐ·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 transmissions, 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 quantities of heat which arrive at each of the thin laminæ 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 thickness which produced an equality of action[1]. 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 thickness is increased beyond 3ᵐᵐ, 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 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 Lumière, Paris, 1760, liv. iii. sect. 1ʳᵉ, art. 1, 2, 3, 4.
