676 THE PRINCIPLES OF SCIENCE. [chap. 



generally speaking, it would enter into a series of com- 

 pounds 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 successful act of inductive discovery, enabling us to 

 register many undoubted propositions as results of perfect 

 induction, and to make a great number of inferences 

 depending upon the principles of imperfect induction 



An excellent instance as to what classification can do, is 

 found in Mr. Lockyer's researches on the sun.^ Wanting 

 some guide as to what more elements to look for in the 

 sun's photosphere, he prepared a classification of the ele- 

 ments according as they had or had not been traced in 

 the sun, together with a detailed statement of the chief 

 chemical characters of each element. He was then able 

 to observe that the elements found in the sun were for tlie 

 most part those Ibrming stable compounds with oxygen. 

 He then inferred that other elements forming stable 

 oxides would probably exist in the sun, and he was 

 rewarded by the discovery of five such metals. Here 

 we have empirical and tentative classification leading to 

 the detection of the correlation between existence in the 

 sun, and tlie power of forming stable oxides and then 

 leading by imperfect induction to the discovery of more 

 coincidences between these properties. 



Professor Huxley has defined the process of classifica- 

 tion in the following terms.^ " By the classification of any 

 series of objects, is meant the actual or ideal arrangement 

 together of those which are like and the separation of 

 those which are unlike ; the purpose of this arrangement 

 being to facilitate tlie operations of the mind in clearly 

 conceiving and retaining in the memory the characters of 

 the objects in question." 



This statement is doubtless correct, so far as it goes, but it 

 does not include all that Professor Huxley himself implicitly 

 treats under classification. He is fully aware that deep 

 correlations, or in other terms deep unitornuties or laws of 

 nature, will be disclosed by any well chosen and profound 

 system of classification. 1 should therefore propose to 



' Proceedings of tlie Ttoyal Society, November, 1873, vol. xxi. \). 512. 

 ^ Lectures on the Elements of Coinparalive Anatomy, 1864, p. i. 



