362 Prof. Knoblauch on the Influence of Metals 
In this case the numbers in the two series are hardly to be 
distinguished from each other. Similar results were obtained 
with a layer of platinum so thick as to permit less than one- 
fourth of the above quantity of heat to pass through it. Hence 
it follows, that in the case of platinum, the incident and trans- 
mitted rays do not sensibly differ in their power of passing through 
coloured glasses. 
Certain metals, therefore, such as gold and silver, do not in- 
tercept all kinds of calorific rays in the same proportion: like 
coloured bodies in the case of light, they exert an elective absorp- 
tion on the rays ; while others, such as platinum, like colourless 
transparent bodies in the case of light, intercept or transmit all 
kinds of calorific rays with the same facility. 
If it be desired to regard the deportment of bodies like pla- 
tinum through an optical analogy, it would be necessary to call 
them gray; for to merit the name of white, they must be capable 
of transmitting a greater quantity of heat. Such substances 
would, according to this, show the same deportment to both 
luminous and calorific rays. M. Knoblauch remarks here, that 
he does not know a single diathermanous body which behaves in 
the same manner towards heat as a colourless transparent body 
towards light. Even in the case of rock-salt, which Melloni 
regarded as such a body, despite its incomparable diathermaneity, 
M. Knoblauch has always been able to perceive (when the plate 
was thick enough) an elective absorption. 
In order to show that the heat transmitted by metals does 
not pass by means of the little holes and fissures in the metallic 
leaves, M. Knoblauch submits the following experiments. 
In place of the thin layer of silver, a thicker one, in which 
fissures and holes were visible, was made use of; and the heat 
which had passed through such fissures and through the coloured 
glasses to the thermo-electric pile, was compared with that which 
fell upon the instrument when the silver layer was removed. 
Such rays showed no difference in quality whatever, as the fol- 
lowing numbers prove: the first line gives the ratios of the inci- 
dent to the transmitted heat when the metallic layer was absent ; 
the second the same ratios when the silver was present :— 
Yellow glass.} Blue glass. | Red glass. | Green glass. 
Silver absent ............... 100 : 56 100 : 39 100: 42 100: 21 
Fissured silver interposed.| 100:57 100: 39 100: 43 100 : 22° 
This experiment proves that if the heat transmitted in the ex- 
periments recorded in the preceding pages had been due to its 
passage through the fissures and holes of the metal, the quali- 
