360 Prof. Knoblauch on the Influence of Metals 
gold, the more decided are the changes produced in them by the 
metal. ' 
That the results hitherto recorded were not produced by any 
modification of the heat in its passage through the glasses im 
which the gold precipitates spread, M. Knoblauch shows both 
by reasoning and experiment. We will give the results of the 
latter. The heat was first permitted to pass through the yellow, 
red, and blue glasses, direct to the pile, and the action was 
observed. An uncoated portion of the watch-glass which held 
the gold, or a second uncoated watch-glass, was then introduced 
in the path of the rays, before they reached the coloured glass. 
The ratios of the heat incident upon the latter to that trans- 
mitted in the respective cases are recorded in the following 
Table :— 
Yellow glass. Blue glass. Red glass. Green glass. 
Watch-glass absent..| 100: 59 100: 41 100 : 46 100 : 20 
Watch-glass present.| 100:59 100 : 39 100 : 46 100 : 22 
This experiment shows clearly that the introduction of the 
watch-glass had no sensible influence upon the transmissibility 
of the heat through the coloured glasses. It is, however, proved 
by the experiments of Delaroche and Melloni, that after passing 
through a transparent body, the heat acquires a power of pass- 
ing more freely through a second body of the same kind: even 
this action is absent in the experiments of M. Knoblauch, and 
for the simple reason, that, before reaching the watch-glass, the 
heat had already passed through, and been sifted by, a glass lens 
of much greater thickness. 
Experiments similar to those made with the gold were made 
with silver. The ratios of the heat mcident upon the coloured 
glass to that transmitted, were first determined before the silver 
layer was introduced, and are stated in the following Table :— 
Yellow glass. 
100 : 63 
Blue glass. Red glass. 
100: 49 
Green glass. 
100: 19 
100: 38 
It was necessary to repeat this experiment in each case, as the 
same ratios could only be obtained when precisely the same ex- 
perimental conditions were adhered to. It was, however, prac- 
tically impossible to secure this in experiments conducted on the 
fine days of several successive summers. 
Six different layers of silver, prepared chemically by Professor 
Béttger, were at M. Knoblauch’s disposal. They were placed 
in succession in the path of the solar rays. The following Table 
shows the ratios of the heat incident upon the glasses to that which 
