xix.j EXPERIMENT. 425 



number of little square pieces of broadcloth from a tailor's 

 pattern card, of various colours. They were black, deep 

 blue, lighter blue, green, purple, red, yellow, white, and 

 other colours and shades of colour. I laid them all out 

 upon the snow on a bright sunshiny morning. In a few 

 hours the black, being most warmed by the sun, was sunk 

 so low as to be below the stroke of the sun's rays ; the 

 dark blue was almost as low ; the lighter blue not quite 

 so much as the dark ; the other colours less as they were 

 lighter. The white remained on the surface of the snow, 

 not having entered it at all." This is a very elegant and 

 apparently simple experiment ; but when Leslie had com- 

 pleted his series of researches upon the nature of heat, he 

 came to the conclusion that the colour of a surface has 

 very little effect upon the radiating power, the mechanical 

 nature of the surface appearing to be more influential. 

 He remarks * that " the question is incapable of being posi- 

 tively resolved, since no substance can be made to assume 

 different colours without at the same time changing its 

 internal structure." Eecent investigation has shown that 

 the subject is one of considerable complication, because 

 the absorptive power of a surface may be different accord- 

 ing to the character of the rays which fall upon it ; 

 but there can be no doubt as to the acuteness with which 

 Leslie points out the difficulty. In Well's investigations 

 concerning the nature of dew, we have, again, very 

 complicated conditions. If we expose plates of various 

 material, such as rough iron, glass, polished metal, to the 

 midnight sky, they will be dewed in various degrees; 

 but since these plates differ both in the nature of the 

 surface and the conducting power of the material, it would 

 not be plain whether one or both circumstances were of 

 importance. We avoid this difficulty by exposing the 

 same material polished or varnished, so as to present dif- 

 ferent conditions of surface ; 2 and again by exposing 

 different substances with the same kind of surface. 



When we are quite unable to isolate circumstances we 

 must resort to the procedure described by Mill under the 

 name of the Joint Method of Agreement and Difference. 



Inquiry into the Nature of Heat, p. 95. 

 2 Herschel, Preliminary Discourse, p. 161. 



