RADIANT HEAT AND ITS RELATIONS. 231 



sparing no pains, shirking no toil to secure sound materials 

 for the edifice which it is our privilege to raise. 



For the purpose of testing our conclusion regarding the 

 influence of the gum I take two powders of the same physi- 

 cal appearance ; one of them is a compound of mercury and 

 the other a compound of lead. On two surfaces of this 

 cube are spread these bright-red poAvders without varnish 

 of any kind. Filling the tube with boiling water, and de- 

 termining the radiation from the two surfaces, one of them 

 is found to emit thirty-nine rays, 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 now 

 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 powders, simply because the gum 

 employed is a better radiator than either of them. Exclud- 

 ing all varnish, and comparing white with white, I find 

 vast differences ; comparing black with black, I find them 

 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, moreover, 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 evi- 

 dent 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 main- 

 ly of non-luminous rays, conclusions as to the influence of 

 color may be altogether delusive. This is the strict scien- 

 tific upshot of our researches. But it is not the less true 

 that in the case of wearing apparel — and this for reasons 

 which I have given in analyzing the experiment of Frank- 



