358 Prof. Knoblauch on the Influence of Metals 
pass through fine apertures in the leaves. Or the idea may 
suggest itself, that the thermo-electric pile was simply affected 
by the heat radiated by the exposed body itself, after it had been 
heated by the sun. At the end of this section a special proof 
shall be given that the action observed is not due to either of the 
causes referred to. 
There is therefore no doubt that metals, such, for example, as 
gold, silver, platinum, when in thin layers, are to be regarded as 
diathermanous bodies, which transmit calorific rays ; the quantity 
transmitted naturally diminishing as the thickness of the metallic 
layer increases. 
It was of interest to ascertain whether the heat which had 
passed through the metallic leaves was of the same quality as 
the heat before its passage. To determine this, the heat was 
first examined before it had passed through the metal. The 
rays reflected from the heliostat were allowed to fall directly 
upon the thermo-electric pile, and the consequent deflection was 
observed. A diathermanous plate, for example a plate of yellow 
glass, was then interposed in the path of the rays, and the po- 
sition of the needle was read off afresh. Supposing the first 
deflection to be expressed by 100, the latter, according to the 
observation, would be 59; a result which proved that for a 
direct quantity of heat equal to 100, a portion of it equal to 59 
could pass through the yellow glass. In this way the heat was 
examined with different glasses: the following Table expresses 
the ratio of the incident to the transmitted heat in the respect- 
ive cases :— 
Blue glass. 
Yellow glass. Red glass. Green glass. 
100 : 59 100: 41 
100: 46 
100 : 20 
The solar rays were next sent through a gold-leaf stretched 
over a frame, and afterwards allowed to fall upon the above 
glasses in succession: the object, it will be remembered, was to 
ascertain the ratio of the heat imcident upon each glass to that 
which passed through it: the result is given in the following 
Table :-— 
Yellow glass. 
100 : 20 
Blue glass. . 
100 :32 
Red glass. 
100: 14 
Green glass, 
100 ; 24 
If these results be compared with the foregoing, remarkable 
differences appear. Before passing through the gold-leaf, the 
heat was capable of passing through the yellow glass in the 
ratio of 59: 100; while, after crossmg the metal, it was only 
able to penetrate the glass in the ratio of 20: 100. A similar 
