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 

 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 

 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 c 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 taay be different 

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



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

 VOL. II. D 



