250 Influence of Colour on the 



with different pigments, and of a glass globe filled with warm 

 water and variously coated. I gave the preference to a modifi- 

 cation of this latter method, from the greater extent of radiating 

 surface which may, without inconvenience, be commanded by 

 it. The glass globe used by Dr Stark was one inch and a quar- 

 ter in diameter ; it was coated at different times with Prussian 

 blue, red lead, and white lead, and in a room at 50° Fahrenheit, 

 the fall from 120° through 25° was in seventeen minutes, eighteen 

 minutes, and nineteen minutes. 



I am constrained to differ from Professor Powell in his re- 

 marks upon the method just referred to, and, with great defer- 

 ence to so high authority, would state why I consider them in- 

 conclusive. Professor Powell deems it necessary, or at least 

 highly important, to the determination 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 ex- 

 periments of Professor Leshe, in which equal quantities of differ- 

 ent radiating substances were dissolved and spread upon a sur- 

 face for comparison. That equal thickness of substances pos- 

 sessing 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 the different coatings of jelly were 

 applied in succession upon one of the sides of the cube in Pro- 

 fessor Leslie's experiments, the radiation increased with the 

 thickness up to a certain point. The effect of conducting power 

 appears by this same experiment to be so small that an increase 

 of the thickness in the bad conductor was actually 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 

 modifying it, is, T apprehend, yet to be appreciated. If these 

 views be correct, and they are, I believe, founded upon the au- 

 thorities 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 

 thickness upon a given surface, nor by equalizing their weights, 



