Prof. TyndalPs Contributions to Molecular Physics. 529 



when the source of heat was a flame of bisulphide of carbon. It 

 was found moreover that, whenever two liquids underwent 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. 



§ XII. Explanation of certain results of Melloni and 

 M. Knoblauch. 



And here we find ourselves in a position to offer solutions 

 of various facts which have hitherto stood as enigmas in re- 

 searches upon radiant heat. It was for a time generally sup- 

 posed 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, show- 

 ing 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 inter- 

 cepted 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 opake 

 to the radiation from obscure sources — in other words, to waves 

 of long period. A plate 26 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 plati- 

 num 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 diathermic 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 spiral would augment the penetra- 



* Melloni. 



