92 FEAGMENTS OF SCIENCE. 



it is also a powerful absorber. While, therefore, at the 

 present moment, it is copiously pouring forth radiant 

 heat itself, it does not allow a single ray from the metal 

 behind to pass through it. The varnish then, and not 

 the metal, is the real radiator. 



Now Melloni, and Masson, and Courtepee experi- 

 mented thus: they mixed their powders and precipi- 

 tates with gum-water, and laid them, by means of a 

 brush, upon the surfaces of a cube like this. True, 

 they saw their red powders red, their white ones white, 

 and their black ones black, but they saw these colours 

 through the coat of varnish which surrounded every par- 

 ticle. When, therefore, it was concluded that colour 

 had no influence on radiation, no chance had been 

 given to it of asserting its influence; when it was found 

 that all chemical precipitates radiated alike, it was the 

 radiation from a varnish, common to them all, which 

 showed the observed constancy. Hundreds, perhaps 

 thousands, of experiments on radiant heat have been 

 performed in this way, by various enquirers, but the 

 work will, I fear, have to be done over again. I am not, 

 indeed, acquainted with an instance in which an over- 

 sight of so trivial a character has been committed by so 

 many able men in succession, vitiating so large an 

 amount of otherwise excellent work. 



Basing our reasonings thus on demonstrated facts, 

 we arrive at the extremely probable conclusion that 

 the envelope of the particles, and not the particles 

 themselves, was the real radiator in the experiments 

 just referred to. To reason thus, and deduce their 

 more or less probable consequences from experimental 

 facts, is an incessant exercise of the student of physical 

 science. But having thus followed, for a time, the 

 light of reason alone throTigh a series of phenomena, 

 and emerged from them with a purely intellectual con- 



