CLASSIFICATION. 347 



metals, and have been selected by the ballot. But when 

 chemists carefully selected from the list the five metals, 

 Potassium, Sodium, Caesium, Eubidium, and Lithium, 

 and called them the Alkaline metals, a great deal was 

 implied in this classification. On comparing the qualities 

 of these substances, they are all found to combine very 

 energetically with oxygen, to decompose water at all 

 temperatures, and to form strongly basic oxides, which 

 are very soluble in water, yielding powerfully caustic and 

 alkaline hydrates from which water cannot be expelled 

 by heat. Their carbonates are also soluble in water, and 

 each metal forms only one chloride. It may also be ex- 

 pected as a general rule that each salt into which one of 

 the five metals enters will correspond to salts into which 

 the other metals enter, there being a general analogy 

 between the properties and compounds of these metals. 



Now in forming this class of alkaline metals, we have 

 done more than merely select a convenient order of 

 statement. We have arrived at a discovery of certain 

 empirical laws of nature, the probability being very con- 

 siderable that a metal which exhibits some of these pro- 

 perties will also possess the others. If we discovered 

 another metal whose carbonate was soluble in water, 

 and which energetically combined with water at all tem- 

 peratures, producing a strongly basic oxide, we should 

 infer that it would form only a single chloride, and 

 that, generally speaking, it would enter into a series of 

 compounds corresponding to the salts of the other 

 alkaline metals. The formation of this class of alkaline 

 metals, then, is no mere matter of convenience ; it is an 

 important and highly successful act of inductive dis- 

 covery, enabling us to register many undoubted propo- 

 sitions as results of perfect induction, and to make an 

 almost indefinite series of inferences depending upon the 

 principles of imperfect induction. 



