Page:Scientific Memoirs, Vol. 1 (1837).djvu/70

58 of a millimetre of each layer, when referred to the quantities of incident heat, will have the values that is, 0.0328 0.0070 0.0030 0.0011 0.0005 0.0003 0.0002. By similar calculations the successive losses sustained by the radiations from the incandescent platina and the copper heated to 390° will be found to be Now the differences between every two terms of these series are for the As to the fourth source it is useless to speak of it, as its rays are completely extinguished at the distance of one millimetre. Thus, notwithstanding the inequalities of the increase of the distance from the second and the third layer to the surface of entrance, we observe in the three series the two principles we have laid down, namely, 1st, the decrease of the losses; 2nd, the tendency of this decrease towards a limit at which the loss becomes constant: but for each particular case the points of the medium at which the rays begin to suffer this constant action are evidently placed at a fixed distance from the origin. Therefore, if the glass be divided into equal layers, the limit of the decrease of the losses will be attained more slowly in proportion as the layers are more numerous, that is to say, thinner. It is for this reason that in each series the limit at which the losses become constant depends, as we have already said, on the thickness of the elementary layers. The only difference observable between the transmission through a continuous medium and the transmission through a series of detached screens is in the amount of the losses, which, for a given thickness, are found to be greater in the latter, because of the reflexions produced by each separate surface. These facts cannot surprise us after the idea we have formed to ourselves of the influence exercised by diaphanous substances on radiant heat. For the calorific sources always emit a certain portion of rays heterogeneous (if we may use the expression) to the calorific tint of the glass, which, through the absorbent action of the matter constituting the continuous medium or the detached screens, are successively extinguished until no rays remain but those that are homogeneous to this tint. Now these homogeneous rays must suffer a loss greater or less in its amount, but constant in respect to layers of equal thickness, as is the case, in the transmission of light, with red rays passing through a medium of the same colour, and with white rays passing through a medium diaphanous and colourless. What we have said of glass is equally true of every other partially diathermanous substance.

The calorific transmission through a series of homogeneous screens is then absolutely of the same nature as that which is effected through the