ON RADIANT HEAT. 69 



Basing our reasonings thus on demonstrated facts, we. 

 arrive at the extremely probable conclusion that the envel- 

 ope 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 conse- 

 quences 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 through a 

 series of phenomena, and emerged from them with a purely 

 intellectual conclusion, our duty is to bring that conclusion 

 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 

 water, and determining the radiation from the two sur- 

 faces, one of them is found to emit thirty-nine units of 

 heat, while the other emits seventy-four. This, surely, is a 

 great difference. Here, however, is a second cube, having 

 two of its surfaces coated with the same powders, the only 

 difference being that the powders are laid on by means of 

 a transparent gum. Both surfaces are now absolutely 

 alike in radiative power. Both of them emit somewhat 

 more than was emitted by either of the unvarnished pow- 

 ders, simply because the gum employed is a better radiator 

 than either of them. Excluding all varnish, and compar- 

 ing white with white, vast differences are found; compar- 

 ing black with black, they are also different; and when 

 black and white are compared, in some cases the black 

 radiates far more than the white, while in other cases the 

 white radiates far more than the black. Determining, more- 

 over,the absorptive power of those powders, it is found to go 

 hand-in-hand with their radiative power. The good 

 radiator is a good absorber, and the bad radiator is a bad 

 absorber. From all this it is evident that as regards the 

 radiation and absorption of non-luminous heat, color 

 teaches us nothing; and that even as regards the radiation 

 of the sun, consisting as it does mainly of non-luminous 

 rays, conclusions as to the influence of color may be alto- 

 gether delusive. This is the strict scientific upshot of our 



