ANALOGY. 285 



if we said that there was an analogy between iron, nickel, 

 and cobalt, manifested in the strength of their magnetic 

 powers. There is a still more perfect analogy between 

 iodine and chlorine ; not that every property of iodine is 

 identical with the corresponding property of chlorine ; 

 for then they would be one and the same kind of sub- 

 stance, and not two substances ; but every property of 

 iodine resembles in all but degree some property of chlo- 

 rine. For almost every substance in which iodine forms 

 a component, a corresponding substance may be dis- 

 covered containing chlorine, so that we may confidently 

 infer from the compounds of the one to the compounds 

 of the other substance. Potassium iodide crystallizes in 

 cubes ; therefore it is to be expected that potassium chlo- 

 ride will also crystallize in cubes. The science of chemistry, 

 as now developed, rests almost entirely upon a careful and 

 most extensive comparison of the properties of substances, 

 bringing to light deep-lying analogies. When any ap- 

 parently exceptional or new substance is encountered, the 

 chemist is guided in his treatment of it entirely by the 

 analogies which it seems to present with previously known 

 substances. 



In this chapter I cannot hope to illustrate the all- 

 pervading influence of analogy in human thought and 

 science. All science, it has been said, at the outset, arises 

 from the discovery of identity, and analogy is but one 

 name by which we denote the deeper-lying cases of re- 

 semblance. I shall only try to point out at present how 

 analogy between apparently diverse classes of phenomena 

 often serves as an all-important guide in discovery. We 

 thus commonly gain the first insight into the nature of an 

 apparently unique object, and we thus, in the progress of 

 a science, often discover that we are treating over again, ' 

 in a new form, phenomena which were well known to us 

 in another form. 



