GENERALIZATION. 259 



that qualities variable in degree will be found in every 

 substance in a greater or less degree. 



It is highly remarkable that Newton, whose method of 

 investigation was logically perfect, seemed incapable of 

 generalizing and describing his own procedure. His 

 celebrated ' Rules of reasoning in Philosophy,' described 

 at the commencement of the third book of the * Principia/ 

 are of very questionable truth, and still more questionable 

 value. 



Extreme Instances of Properties. 



Although, as we have seen, substances usually differ 

 only in degree as regards their physical properties, great 

 interest may attach to particular substances which mani- 

 fest a property in a conspicuous and intense manner. 

 Every branch of physical science has usually been de- 

 veloped from the attention forcibly drawn to some sin- 

 gular substance. .Just as the loadstone disclosed mag- 

 netism and amber frictional electricity, so did Iceland 

 spar point out the existence of double refraction, and 

 sulphate of quinine the phenomenon of fluorescence. 

 When one such startling instance has drawn the attention 

 of the scientific world, numerous less remarkable cases of 

 the phenomenon will soon be detected, and it will pro- 

 bably prove that the property in question is actually 

 universal to all matter. Nevertheless, the extreme in- 

 stances retain their interest, partly in a historical point of 

 view, partly because they furnish the most convenient 

 substances for experiment. 



Francis Bacon was fully aware of the value of such 

 examples, which he called Ostensive Instances or Light- 

 giving, Free or Predominant Instances. ' They are those/ 

 he says, ' which show the nature under investigation 

 naked, in an exalted condition, or in the highest degree 



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