OF NATURAL PHILOSOPHY. 141 



mediately and ipso facto constitute themselves into 

 a group or class, which may become enlarged to any 

 extent by the accession of such new objects, pheno- 

 mena, or laws, agreeing in the same point, as may 

 come to be subsequently ascertained. It is thus that 

 the materials of the world become grouped in natural 

 families, such as chemistry furnishes examples of, 

 in its various groups of acids, alkalies, sulphurets, &c.; 

 or botany, in its euphorbiaceae, umbelliferae, &c. 

 It is thus, too, that phenomena assume their places 

 under general points of resemblance ; as, in optics, 

 those which refer themselves to the class of periodic 

 colours, double refraction, &c. ; and that resem- 

 blances themselves become traced, which it is the 

 business of induction to generalize and include in 

 abstract propositions. 



(135.) But every class formed on a positive resem- 

 blance of characters, or on a distinct analogy, draws 

 with it the consideration of a negative class, in which 

 that resemblance either does not subsist at all, or the 

 contrary takes place ; and again, there are classes in 

 which a given quality is possessed by the differ- 

 ent individuals in a descending scale of intensity. 

 Now, it is of consequence to distinguish between 

 cases in which there is a real opposition of quality, 

 or a mere diminution of intensity, in some quality 

 susceptible of degrees, till it becomes impercep- 

 tible. For example, between transparency and 

 opacity there would at first sight appear a direct 

 opposition ; but, on nearer consideration, when we 

 consider the gradations by which transparency di- 

 minishes in natural substances, we shall see reason 

 to admit that the latter quality, instead of being the 



