upon Radiant Heat. 357 
leaf rolled out as thin as possible and stretched over a frame. 
As no terrestrial source of heat would be sufficiently intense, the 
leaf was placed in the track of the sun’s rays, which were sent 
into a darkened room by a Silbermann’s heliostat ; a sensible 
action on a thermo-electric pile was the consequence. In order 
to augment the action, a glass lens was placed in the window- 
shutter. By this means a deflection of the astatic needle of a 
multiplying galvanometer connected with the thermo-electric 
pile was obtained, which amounted to 6 degrees. 
A second experiment was made with a thin layer of mosaic 
gold, which was spread upon a smooth glass plate. The greater 
thickness of this layer reduced the deflection to 3 degrees. 
Three gold precipitates, prepared chemically by Prof. Béttger 
of Frankfort, were peculiarly fitted for these experiments*. By 
transmitted light they showed a beautiful green colour, which 
deepened as the thickness of the layer augmented, while the leaf 
made use of in the experiment just recorded transmitted light of 
rather a bluish tint. The thicknesses of the layers were in the 
proportion 1: 2:38. When the rays of heat, concentrated in the 
manner described, passed through these precipitates, which lay 
upon watch-glasses, the consequent deflections were,—for the 
thinnest, 33°-64; for the middle one, 4°:41 ; and for the thick- 
est, 19-42. 
The same experiments were made with silver. Rolled leaves 
of this metal proved, however, too thick ; and those only which 
were chemically prepared showed themselves capable of trans- 
mitting the radiant heat. The precipitates were transparent for 
blue light, the intensity of which indicated the thickness through 
which it had passed. Of four precipitates, the thickness of the 
strongest was about double that of the weakest. The deflections 
produced by the heat passing through these four layers, com- 
mencing with the thinnest, were 10°, 8°, 5°, and 1° respectively. 
A similar result was obtained with platinum, which was also 
made use of as a thin coating upon the surface of a watch-glass. 
This permitted of the passage of the sun’s rays to sucha degree, 
that with a thin layer a deflection of 34°-7 was obtained, and 
with a thicker one a deflection of 8°'5. 
Other metals than the above were either not thin enough, or 
not uniform enough to permit of similar experiments. The 
glass on which the precipitates rested could of course only have 
the effect of rendering the action more feeble. 
But it might be asked whether the calorific rays pass really 
through the metal as they do through glass, crystals, or other 
diathermanous bodies; or whether they are not permitted to 
|* Faraday’s method of preparing gold films might be turned to valuable 
account in such experiments as the above. See Phil. Trans. February 1857.) 
