EXPERIMENT. 33 



Failure in the Simplification of Experiments. 



In some cases it seems to be impossible to carry out the 

 rule of varying one circumstance at a time. When we 

 attempt to obtain two instances or two forms of experi 

 ment in which a single circumstance shall be present or 

 absent, it may be found that this single circumstance 

 entails one or more others. Benjamin Franklin s experi 

 ment concerning the comparative absorbing powers of 

 different colours is well known. I took, he says, a 

 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 

 sank 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 

 completed 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 1 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. More recent investigation has shown 

 that the subject is one of considerable complication, be 

 cause the absorptive power of a surface may be different 

 according to the character of the rays whicJi fall upon it ; 



1 Inquiry into the Nature of Heat, p. 95. 

 VOL. II: D 



