RADIATION. 



MELLONI'S EXPKRIMENT. 



185 



You will observe that with non-luminous heat, alum 

 refuses to allow any heat to pass through it at all, although 

 it is a substance which is perfectly transparent. Plate- 

 glass is a substance which lies half-way between rock-salt 

 and alum, the loss being only about 10 per cent. ; so that 

 he got 90 per cent, of the heat through the second piece of 

 alum, clearly showing that there are certain rays which 

 are absorbed by the alum, and all the other rays are allowed 

 to pass quite freely through it. In fact, he saw that every 

 substance had the property of absorbing certain colours of 

 the spectrum, and that heat differed in no degree from 

 light ; that there was a heat spectrum as well as a light 

 spectrum, but that the great intensity of heat was as a 

 general rule in parts of the spectrum which were invisible ; 

 and the reason of this is simply that the substance of which 

 the eye is composed does not allow these radiations of heat 

 to pass through. The general results of Melloni's work 

 may be represented by a diagram something of this sort in 

 which the upper part shows the curves representing the 

 intensity of radiation at different temperatures with the 

 curve of limiting visibility, whilst in the diagram beneath 

 is given the resulting curve of apparent visibility. The 

 thing wanting to show the absolute identity of light and 

 heat was some experiment on the phenomena of heat 

 analogous to those which had been performed on the 

 theory of light which acquired the name of polarised. 

 You know that if we employ several pieces of this sub- 

 stance called tourmaline, which is tolerably transparent, 

 if we first pass the light of a candle through one piece and 

 then through another, we can turn this piece of tourmaline 

 about in a certain direction until we get total darkness ; 

 that is to say, when the two pieces of tourmaline are 



