RADIANT HEAT AND ITS RELATIONS 99 



black, but tliey saw these colors through the coat of varnish 

 which surrounded every particle. When, therefore, it was 

 concluded that color 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, per- 

 haps thousands, of experiments on radiant heat have been 

 performed in this way, by various inquirers, but the work 

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

 deed, 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 en- 

 velope 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 ex- 

 ercise of the student of physical 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 

 intellectual conclusion, our duty is to bring that conclu- 

 sion to an experimental test. In this way we fortify our 

 science. 



For the purpose of testing our conclusion regarding the 

 influence of the gum, I take two powders presenting the 

 same physical appearance; one of them is a compound of 

 mercury, and the other a compound of lead. On two sur- 

 faces of a cube are spread these bright red powders, with- 

 out varnish of any kind. Filling the cube with boiling 



