upon Radiant Heat. 359 
diminution of the ratio is observed in the case of the blue and 
red glasses, whereas in the case of the green glass the reverse is 
observed to be the case: the ratio here is augmented from one- 
fifth to nearly one-fourth. The experiments therefore show that 
the radiant heat, after having passed through the gold-leaf, is 
different in quality from what it was before. 
The leaf of mosaic gold already referred to, when stretched 
upon a frame and tested in the same manner, gave a result very 
nearly coincident with the above. A precipitate of gold upon 
the surface of a watch-glass was also examined, and similar 
results were obtained with it. These experiments prove beyond 
doubt, that calorific rays which have passed through a layer of 
gold, possess the property of passing through diathermanous 
bodies in a totally different degree from those which have not 
passed through the metal. 
The influence of the thickness of the metallic layer was also 
investigated. Four layers were made use of, which were suc- 
cessively placed in the path of the rays, and bebind each of them 
the test with the coloured glasses was repeated. In the follow- 
ing Table the results are recorded; the second column of which 
shows the deflection produced when the rays, after having passed 
through the layer of gold, and without passing through the glass, 
were allowed to fall upon the pile. The last four columns show 
the ratios of the quantities of heat which passed through the 
gold alone, to those which passed through both gold and glass, 
in the respective cases :— 
Deflection after | Yellow Blue Red Green 
passing through glass. glass. glass. glass. 
gold. 
Thinnest layer... 33-64 100: 46] 100:30| 100:32) 100: 21 
Thicker layer ... 31:28 100:38}| 100:30| 100:25| 100; 24 
Still thicker ... 4°41 100:26) 100:32)| 160:18) 100: 24 
Thickest layer... 1-42 100: 22) 100:34| 100:14| 100:54 
It is here shown, that, as regards the yellow and red glasses, 
the transmissibility of the heat diminished as the thickness of 
the metallic layer increased. But the thicker the layer, the 
greater is the power of the heat transmitted to penetrate the 
green and blue glasses. For example: of the heat which passed 
through the thinnest layer, we find that 46 parts out of 100 can 
pass through the yellow; 21 out of 100 through the green glass. 
Of the heat, however, which has passed through the thickest 
layer, a quantity represented by 24 passes through the yellow 
glass, while 54 passes through the green. Hence the longer the 
path which the calorific ray describes through the diathermanous 
