upon Radiant Heat. 369 
the thicker deposits was most copiously transmitted through the 
yellow glass. The result is stated in the following Table :— 
Yellow glass. | Red glass. 
Thick layer......+6. 100: 70 100: 55 
Thin layer ........ 100:67 ~ 100 : 55 
Unreflected heat... 100 : 63 100: 52 
Three surfaces of silver were next made use of. One was 
a dull plate of a whitish colour obtained by electrolysis ; another 
was obtained by the so-called cold silvering process, and had 
more of a gray aspect; the third was a highly polished daguerre- 
otype plate, which appeared almost black. The experiments 
with these three plates gave the following results :— 
Yellow glass. Red glass. 
Dull white surface...... 100: 72 100 : 56 
Gray layer’ .as:3..252 0060s 100 : 68 100 :51 
Daguerreotype plate ... 100 : 65 100 : 50 
Unreflected heat ...... 100 : 66 100 : 50 
In this case the heat reflected from the dull white surface 
passed most copiously through the yellow glass; that from the 
gray surface less copiously; while that reflected from the po- 
lished plate was transmitted least copiously. The latter heat 
was not to be distinguished from the unreflected calorific rays. 
Similar effects are observed in the case of the red glass. 
A similar result was obtained with mercury rubbed upon a 
plate of copper until a dull whitish layer was obtained, when the 
heat reflected from this surface was compared with that reflected 
from the ordinary shining surface of the metal. The following 
Table gives the result of the experiment :— 
Yellow glass. Red glass. 
Dull whitish surface ...... 100: 73 100: 55 
Shining metallic surface .. 100 : 67 100:51 
Unreflected heat ......... 100 : 67 100: 51 
In experiments made with dull and polished surfaces of cop- 
per, and with rough and polished surfaces of brass, there was 
no difference observed. “It must be confessed,” remarks M. 
Knoblauch, “that in the case of copper and brass, as in the case 
Phil, Mag. 8, 4. Vol. 14, No, 94, Nov. 1857. 2B 
