OF RADIANT HEAT THROUGH DIFFERENT BODIES. | 67 
rectly 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 ra- 
diating from several screens. Others experience variations so great as 
to exhibit all the phases of the phenomenon, from an extremely abun- 
dant to an excessively feeble transmission *. 
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 these obtained through the action 
of the immediate radiation; a proposition which I have besides verified 
by direct experiments. 
After what we have so often repeated respecting the action of uni- 
versal and partial diathermanous bodies, it would be superfluous again 
to point out the perfect similarity between these facts and the ana- 
logous phenomena presented by the transmission of light through dia- 
phanous media, colourless and coloured. We shall therefore confine 
ourselves to a single observation on the nature of the rays which tra- 
verse certain screens. 
The heat emerging from alum is almost totally absorbed by the opake 
* This change in the faculty of ulterior transmission is not the only modifi- 
cation 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 expe- 
riment: 
We take two thermometers of equal sensibility, and after haying 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 
reater when the transmitted heat is employed. Mr. Powell, to whom we 
are indebted for this ingenious experiment, has performed it on calorific radia- 
tions from a bright red hot iron and from an Argand lamp. ‘The means of se- 
veral series of observations furnished, as the ratio of absorption of the thermo- 
meter 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 trans- 
mitted through glass. (Report of the First and Second Meetings of the British 
Association for the Advancement of Science, pp. 274, 275.) 
I haye obtained numerical data perfectly analogous, by means of the thermo- 
multiplier. 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 succes- 
sively 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 
FQ 
