8 INDUCTION. 



the phenomenon itself, that is no reason against attempting to 

 discover the conditions. The interdict against endeavouring 

 to reduce distinctions of colour to any common principle, would 

 have held equally good against a like attempt on the subject 

 of distinctions of sound ; which nevertheless have been found 

 to be immediately preceded and caused by distinguishable 

 varieties in the vibrations of elastic bodies : though a sound, 

 no doubt, is quite as different as a colour is from any motion 

 of particles, vibratory or otherwise. We might add, that, in 

 the case of colours, there are strong positive indications that 

 they are not ultimate properties of the different kinds of sub- 

 stances, but depend on conditions capable of being super- 

 induced upon all substances ; since there is no substance which 

 cannot, according to the kind of light thrown upon it, be made 

 to assume almost any colour ; and since almost every change 

 in the mode of aggregation of the particles of the same sub- 

 stance, is attended with alterations in its colour, and in its 

 optical properties generally. 



The real defe'ct in the attempts which have been made to 

 account for colours by the vibrations of a fluid, is not that the 

 attempt itself is unphilosophical, but that the existence of 

 the fluid, and the fact of its vibratory motion, are not proved ; 

 but are assumed, on no other ground than the facility they 

 are supposed to afford of explaining the phenomena. And this 

 consideration leads to the important question of the proper 

 use of scientific hypotheses ; the connexion of which with the 

 subject of the explanation of the phenomena of nature, and 

 of the necessary limits to that explanation, needs not be 

 pointed out. 



4. An hypothesis is any supposition which we make 

 (either without actual evidence, or on evidence avowedly 

 insufficient) in order to endeavour to deduce from it conclusions 

 in accordance with facts which are known to be real ; under 

 the idea that if the conclusions to which the hypothesis leads 

 are known truths, the hypothesis itself either must be, or at 

 least is likely to be, true. If the hypothesis relates to the 

 cause, or mode of production of a phenomenon, it will serve, 



