GENERALISATION. 607 



qualities vary in degree ; substances are more or less 

 dense, more or less transparent, more or less compressible, 

 more or less magnetic, and so on. One common result of 

 the progress of science is to show that qualities supposed 

 to be entirely absent from many substances are present 

 only in so low a degree of intensity that the means of 

 detection were insufficient. Newton believed that most 

 bodies were quite unaffected by the magnet ; Faraday and 

 Tyndall have rendered it very doubtful whether any sub- 

 stance whatever is wholly devoid of magnetism, including 

 under that term diamaguetism. We are rapidly learning 

 to believe that there are no substances absolutely opaque, 

 or non-conducting, non-electric, non-elastic, non-viscous, 

 non-compressible, insoluble, infusible, or non-volatile. All 

 tends to become a matter of degree, or sometimes of direc- 

 tion. There may be some substances oppositely affected 

 to others, as ferro-magnetic substances are oppositely 

 affected to diamaguetics, or as substances which contract 

 by heat are opposed to those which expand ; but the 

 tendency is certainly for every affection of one kind of 

 matter to be represented by something similar in other 

 kinds. On this account one of Newton's rules of philo- 

 sophising seems to lose all validity ; he said, " Those 

 qualities of bodies which are not capable of being 

 heightened, and remitted, and which are found in all 

 bodies on which experiment can be made, must be con- 

 sidered as universal qualities of all bodies." As far as I 

 can see, the contrary is more probable, namely, that 

 qualities variable in degree will be found in every sub- 

 stance in a greater or less degree. 



It is remarkable that Newton whose method of investi- 

 gation was logically perfect, seemed incapable of generalis- 

 ing 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 questionable truth, and still more questionable value. 



Extreme Instances of Properties. 



Although substances usually differ only in degree, great 

 interest may attach to particular substances which manifest 

 a property in a conspicuous and intense manner. Every 



