142 DISCOURSE ON THE STUDY 



opposite of the former, is only its extreme lowest 

 degree. Again, in the arrangement of natural objects 

 under the head of weight or specific gravity, the 

 scale extends through all nature, and we know of no 

 natural body in which the opposite of gravity, or 

 positive levity, subsists. On the other hand, the 

 opposite electricities ; the north and south mag- 

 netic polarities ; the alkaline and acid qualities of 

 chemical agents ; the positive and negative rotations 

 impressed by plates of rock crystal on the planes of 

 polarization of the rays of light, and many other 

 cases, exemplify not merely a negation, but an active 

 opposition of quality. Both these modes of classifi- 

 cation have their peculiar importance in the induc- 

 tive process : the one, as affording an opportunity of 

 tracing a relation between phenomena by the observ- 

 ation of a correspondence in their scales of intensity ; 

 the other, by that of contrast, as we shall show more 

 at large in the next section. 



(136.) There is a very wide distinction, too, to be 

 taken between such classes as turn upon a single 

 head of resemblance among individuals otherwise 

 very different, and such as bind together in natural 

 groups, by a great variety of analogies, objects 

 which yet differ in many remarkable particulars. 

 For example : if we make colourless transparency 

 a head of classification, the list of the class will 

 comprise objects differing most widely in their 

 nature, such as water, air, diamond, spirit of wine, 

 glass, &c. On the other hand, the chemical families 

 of alkalies, metals, &c. are instances of groups of 

 the other kind; which, with properties in many 

 respects different, still agree in a general resem- 



