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NOTES BY THE EDITOR 



{JAMES CLERK MAXWELL} 



i See Table of Contents at the beginning of this volume. } 



NOTE i, ARTS. 5 AND 67. 

 On the theory of the Electric Fluid. 



The theory of One Electric Fluid is here stated very completely by 

 Cavendish*. The fluid, as imagined by him, is not a purely hypothetical sub- 

 stance, which has no properties except those which are attributed to it for the 

 purpose of explaining phenomena. He calls it an elastic fluid, and supposes 

 that its particles and those of other matter have certain properties of mutual 

 repulsion or of attraction, just as he supposes that the particles of air are 

 indued with a property of mutual repulsion, but according to a different law. 

 See Art. 97 and Note 6. But in addition to these properties, which are all that 

 are necessary for the theory, he supposes that the electric fluid possesses the 

 general properties of other kinds of matter. In Art. 5 he speaks of the weight 

 of the electric fluid, and of one grain of electric fluid, which implies that a 

 certain quantity of the electric fluid would be dynamically equivalent to one 

 grain, that is to say, in the language of Boscovich and modern writers, it would 

 be equal in mass to one grain. 



We must not suppose that the word weight is here used in the modern sense 

 of the force with which a body is attracted by the earth, for in the case of the 

 electric fluid this force depends entirely on the electrical condition of the earth, 

 and would act upward if the earth were overcharged and downward if the 

 earth were undercharged. 



Cavendish also supposes that there is a limit to the quantity of the electric 

 fluid which can be collected in a given space. He speaks (Art. 20) of the electric 

 fluid being pressed close together so that its particles shall touch each other. 

 This implies that when the centres of the particles approach to within a certain 

 distance, the repulsion, which up to that point varied as the th power of the 

 distance, now varies much more rapidly, so that for an exceedingly small 

 diminution of distance the mutual repulsion increases to such a degree that 

 no force which we can bring to bear on the particles is able to overcome it. 



We may consider this departure from the simplicity of the law of force as 

 introduced in order to extend the property of "impenetrability " to the particles 

 of the electric fluid. It leads to the conclusion that there is a certain maximum 



* For an earlier form of Cavendish's theory of electricity, see "Thoughts con- 

 cerning electricity" (Arts. 195-216), and Note 18. 



