On the Influence of Color on Radiation. 17 



different times with Prussian blue, red lead, and white lead, and in 

 a room at 50° Fah., the fall, from 120° through 25 degrees, was in 

 seventeen minutes, eighteen minutes and nineteen minutes. 

 . I am constrained to differ from Professor Powell in his remarks up- 

 on the method just referred to, and, with great deference to so high 

 authority would state why I consider them inconclusive. Professor 

 Powell deems it necessary, or at least highly important to the deter- 

 mination of the question, that the radiating coatings of the globe should 

 be equalized in respect to thickness, conducting power, density, &c, 

 and refers to the experiments of Prof. Leslie, in which equal quanti- 

 ties of different radiating substances were dissolved and spread upon 

 a surface, for comparison. That equal thicknesses of substances 

 possessing different radiating powers should be compared together, 

 seems to me to be disproved by the law established by Sir John Leslie's 

 own experiments, namely, that radiation takes place not only from the 

 surface, but in a thickness which is appreciable in good radiators, 

 Thus when different coatings of jelly were applied, in succession, up- 

 on one of the sides of the cube in Prof. Leslie's experiments, the ra- 

 diation increased with the thickness, up to a certain point. The ef- 

 fect of conducting power appears by this same experiment to be so 

 small that an increase of the thickness in the bad conductor was actu- 

 ally more than compensated by the increased radiating power. The 

 influence of density on conducting power is well known, but the effect 

 of either as controlling the radiating power of a substance, or as mod- 

 ifying it, is, I apprehend, yet to be appreciated. If these views be^ 

 correct, and they are, I believe, founded upon the authorities so ably 

 illustrated by Professor Powell in his report on radiant heat, to the 

 British Association, the radiating powers of substances would not be 

 rightly compared by equalizing their thicknesses upon a given surface, 

 nor by equalizing their weights ; but by ascertaining, for each sub- 

 stance, that thickness beyond which radiation does not take place* 

 This will be placed in a clearer point of view in the sequel. 



I do not however, consider the question at issue as the less difficult 

 to determine, " no substance can be made to assume different colors 

 without at the same time changing its internal structure,"* and I be- 

 lieve with Professor Powell that " a very extensive induction is per- 

 haps the only means open to us of ascertaining this, (the circumstances 

 and properties wherein the coatings differ) considering how totally 

 ignorant we are of the peculiarities on which their color depends." 



* Prof. Leslie's Essay on Heat 



Vol. XXX.— No. 1. 3 



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