Page:Scientific Memoirs, Vol. 1 (1837).djvu/79

Rh from the source. Citric acid and tartrate of potash become more permeable to the rays issuing from the alum and sulphate of lime, and less permeable to those which proceed from black or green glass. With the opake mica and the tourmaline the case is directly the contrary. Some substances are equally permeable to the heat radiating from several screens. Others experience variations so great as to exhibit all the phases of the phænomenon, from an extremely abundant to an excessively feeble transmission This change in the faculty of ulterior transmission is not the only modification that radiant heat undergoes in passing through the diathermanous bodies. It becomes also more or less susceptible of being absorbed in different quantities by the black and the white surfaces. This fact can be thus proved by experiment:We take two thermometers of equal sensibility, and after having coloured one of the balls black and the other white we expose them simultaneously to the radiant heat, sometimes direct, sometimes transmitted through a plate of glass. The two thermometers are then observed to rise unequally, but the inequality is greater when the transmitted heat is employed. Mr. Powell, to whom we are indebted for this ingenious experiment, has performed it on calorific radiations from a bright red hot iron and from an Argand lamp. The means of several series of observations furnished, as the ratio of absorption of the thermometer with the black to that of the thermometer with the white ball, 100 : 78 when the red hot iron was employed, and 100 : 72 when the lamp was used. These ratios became 100 : 50 and 100 : 57 when he operated on the rays transmitted through glass. (Report of the First and Second Meetings of the British Association for the Advancement of Science, pp. 274, 275.) I have obtained numerical data perfectly analogous, by means of the thermomultiplier. The pile of the apparatus was well washed, afterwards whitened on one side and blackened on the other. The two colours were made from lamp black and Spanish white mingled with gumwater. Turning the pile on its stand I caused the direct or transmitted rays of a Locatelli lamp to fall successively on the two coloured surfaces, and observed the corresponding indications of the galvanometer. This experiment is promptly and easily executed. It has moreover the advantage of requiring no more than one thermoscopic body, a. Through all these vicissitudes the action of the rock salt continues the same and uniformly transmits 92 rays out of 100. Hence follows the inverse proportion that if the series of plates be exposed to one hundred rays emerging from a plate of rock salt, the ratios of the quantities of heat transmitted would be the same as those obtained through the action of the immediate radiation; a proposition which I have besides verified by direct experiments.

After V hat we have so often repeated respecting the action of universal and partial diathermanous bodies, it would be superfluous again to point out the perfect similarity between these facts and the analogous phænomena presented by the transmission of light through diaphanous media, colourless and coloured. We shall therefore confine ourselves to a single observation on the nature of the rays which traverse certain screens.

The heat emerging from alum is almost totally absorbed by the opake