426 THE PRINCIPLES OF SCIENCE. [CHAF. 



We must collect as many instances as possible in which 

 a given circumstance produces a given result, and as many 

 as possible in which the absence of the circumstance is 

 followed by the absence of the result. To adduce his 

 example, we cannot experiment upon the cause of double 

 refraction in Iceland spar, because we cannot alter its 

 trystalline condition without altering it altogether, nor can 

 we find substances exactly like calc spar in every circum- 

 stance except one. We resort therefore to the method of 

 comparing together all known substances which have the 

 property of doubly-refracting light, and we find that they 

 agree in being crystalline. 1 This indeed is nothing but an 

 ordinary process of perfect or probable induction, already 

 partially described, and to be further discussed under 

 Classification. It may be added that the subject does 

 admit of perfect experimental treatment, since glass, when 

 compressed in one direction, becomes capable of doubly- 

 refracting light, and as there is probably no alteration in 

 the glass but change of elasticity, we learn that the power 

 of double refraction is probably due to a difference of 

 elasticity in different directions. 



Removal of Usual Conditions. 



One of the great objects of experiment is to enable us 

 to judge of the behaviour of substances under conditions 

 widely different from those which prevail upon the surface 

 of the earth. We live in an atmosphere which does not 

 vary beyond certain narrow limits in temperature or 

 pressure. Many of the powers of nature, such as gravity, 

 which constantly act upon us, are of almost fixed amount. 

 Now it will afterwards be shown that we cannot apply a 

 quantitative law to circumstances much differing from 

 those in which it was observed. In the other planets, the 

 sun, the stars, or remote parts of the Universe, the con- 

 ditions of existence must often be widely different from 

 what we commonly experience here. Hence our know- 

 ledge of nature must remain restricted and hypothetical, 

 unless we can subject substances to unusual conditions by 

 suitable experiments. 



1 System of Logic, bk. iii. chap. viii. 4, 5th ed. vol. i. p. 433. 



