92 FRAGMENTS OF SCIENCE. 



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 

 particle. 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 oversight 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 refer red to. To reason thus, and deduce their 

 more or less probable consequences from experimenta 

 facts, is an incessant exercise of the student of physica 

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

 light of reason alone through a series of phenomena 

 and emerged from them with a purely intellectua 



