Note 1 8 : Cavendislis early views on electricity 397 



NOTE 18, ART. 216. 



On the " Thoughts Concerning Electricity," and on an early draft of 

 the Propositions in Electricity. 



The theory of electricity sketched in the "Thoughts" is evidently an earlier 

 form of that developed in the published paper of 1771. We must therefore 

 consider the "Thoughts" as the first recorded form of Cavendish's theory, and 

 this for the following reasons. 



(1) Nothing is said in the "Thoughts" of the forces exerted by ordinary 

 matter on itself and on the electric fluid. The only agent considered is the 

 electric fluid itself, the particles of which are supposed to repel each other. This 

 fluid is supposed to exist in all bodies whether apparently electrified or not, but 

 when the quantity of the fluid in any body is greater than a certain value, 

 called the natural quantity for the body, the body is said to be overcharged, 

 and when the quantity is less than the natural quantity the body is said to be 

 undercharged. 



The forces exerted by undercharged bodies are ascribed, not, as in the later 

 theory, to the redundant matter in the body, but to the repulsion of the fluid 

 in other parts of space. 



The theory is therefore simpler than in its final form, but it tacitly assumes 

 that the fluid could exist in stable equilibrium if spread with uniform density 

 over all space, whereas it appears from the investigations of Cavendish himself 

 that a fluid whose particles repel each other with a force inversely as any power 

 of the distance less than the cube would be in unstable equilibrium if its density 

 were uniform. 



This objection does not apply to the later form of the theory, for in it the 

 equilibrium of the electric fluid in a saturated body is rendered stable by the 

 attraction exerted by the fixed particles of ordinary matter on those of the 

 electric fluid. 



(2) The hypotheses are reduced in the later theory to one, and the third 

 and fourth hypotheses of the "Thoughts" are deduced from this. 



(3) In the "Thoughts" Cavendish appears to be acquainted only with those 

 phenomena of electricity which can be observed without quantitative experi- 

 ments. Some of his remarks, especially those on the spark, he repeats in the 

 paper of 1771, but in that paper (Art. 95) he refers to certain quantitative 

 experiments, the particulars of which are now first published [Art. 265]. 



The "Thoughts," however, though Cavendish himself would have con- 

 sidered them entirely superseded by the paper of 1771, have a scientific interest 

 of their own, as showing the path by which Cavendish arrived at his final 

 theory. 



He begins by getting rid of the electric atmospheres which were still clinging 

 to electrified bodies, and he appears to have done this so completely that he 

 does not think it worth while even to mention them in the paper of 1771. 



