34 THE PRINCIPLES OF SCIENCE. 



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 m ; 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 Mr. J. S. Mill 

 under the name of the Joint Method of Agreement and 

 Difference. 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 

 crystalline condition without altering it altogether, nor can 

 we find substances exactly like calc spar in every circum 

 stance except one. We can only 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 11 . This in 

 deed is nothing but an ordinary process of perfect or 

 probable induction, already partially described, and to be 

 further discussed under the subject of Classification. It 

 may be added, however, that the subject does admit of 



m Herschel, Preliminary Discourse on the Study of Natural Philo 

 sophy/ p. 16 1. 



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



