PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 363 
while with the small flame the absorptions of the same two substances stand to each 
other in the ratio of 
100 : 76. 
Numerous experiments were subsequently made, with a view of testing this result, but 
in all cases the bisulphide was found more opaque than the chloroform to the radiation 
of the small gas-flame. The same result was obtained when a very small oil-flame was 
employed ; and it came out in a very decided manner when the source of heat was a 
flame of bisulphide of carbon. It was found moreover that, whenever two liquids under- 
went a change of position of this kind, the vapours of the liquids underwent a similar 
change ; in its finest gradations, the deportment of the liquid was imitated by that of 
its vapour. 
§ 12 . 
And here we find ourselves in a position to offer solutions of various facts which have 
hitherto stood as enigmas in researches upon radiant heat. It was for a time generally 
supposed that the power of heat to penetrate diathermic substances augmented as the 
temperature of the source of heat became more elevated. Knoblauch contended against 
this notion, showing that the heat emitted by a platinum wire plunged into an alcohol 
flame was less absorbed by certain diathermic screens than the heat of the flame itself, 
and justly arguing that the temperature of the spiral could not be higher than that of 
the body from which it derived its heat. A plate of glass being introduced between his 
source and his thermo-electric pile, the deflection of his needle fell from 35° to 19° 
when the source was the platinum spiral ; while, when the source was the flame of 
alcohol, when the glass was introduced the deflection fell from 35° to 16°, proving 
that the radiation from the flame was intercepted more powerfully than that from the 
spiral — showing, in other words, that the heat emanating from the body of highest 
temperature possessed the least penetrative power. Melloni afterwards corroborated 
this experiment. 
Transparent glass allows the rays of the visible spectrum to pass freely through it ; 
but it is well known to be highly opaque to the radiation from obscure sources — in other 
words, to waves of long period. A plate 2-6 millimetres thick intercepts all the rays 
from a source of 100° C., and allows only 6 per cent, of the heat emitted by copper 
raised to 400° C. to pass through it*. Now the products of the combustion of alcohol 
are aqueous vapour and carbonic acid, whose waves have been proved to be of slow 
period, and hence of that particular character which are most powerfully intercepted by 
glass ; but by plunging a platinum wire into such a flame, we virtually convert its heat 
into heat of higher refrangibility ; we break up the long periods into shorter ones, and 
thus establish the discord between the periods of the source and the periods of the dia- 
thermic glass, which, as before defined, is the physical cause of the transparency. On 
purely a priori grounds, therefore, we might infer that the introduction of the platinum 
Meeloni. 
