206 HISTORY OP ELECTRICITY. 



The theory of ^Epinns, however, still required to have the law of 

 action of the particles of the fluid determined. If we were to call to 

 mind how momentous an event in physical astronomy was the deter- 

 mination of the law of the cosmical forces, the inver.se square of tht 

 distance, and were to suppose the importance and difficulty of the 

 analogous step in this case to he of the same kind, this would he to 

 mistake the condition of science at that time. The leading idea, the 

 conception of the possibility of explaining natural phenomena by 

 means of the action of forces, on rigorously mechanical principles, had 

 already been promulgated by Newton, and was, from the first, seen to 

 be peculiarly applicable to electrical phenomena ; so that the very 

 material step of clearly proposing the problem, often more important 

 than the solution of it, had already been made. Moreover the con- 

 firmation of the truth of the assumed cause in the astronomical cape 

 depended on taking the right law ; but the electrical theory could bo 

 confirmed, in a general manner at least, without this restriction. Still 

 it was an important discovery that the law of the inverse square pre- 

 vailed in these as well as in cosmical attractions. 



It was impossible not to conjecture beforehand that it would be so. 

 Cavendish had professed in his calculations not to take the exponent 

 of the inverse power, on which the force depended, to be strictly 2, 

 but to leave it indeterminate between 1 and 3 ; but in his applications 

 of his results, he obviously inclines to the assumption that it is 2. 

 Experimenters tried to establish this in various ways. liobison, 9 in 

 1769, had already proved that the law of force is very nearly or 

 exactly the inverse square ; and Meyer 10 had discovered, but not pub- 

 lished, the same result. The clear and satisfactory establishment of 

 this truth is due to Coulomb, and was one of the first steps in his 

 important series of researches on this subject. In his first paper " in 

 the Memoirs of the Academy for 1*785, he proves this law for small 

 globes ; in his second Memoir he shows it to be true for globes one 

 and two feet in diameter. His invention of the torsion-balance, which 

 measures very small forces with great certainty and exactness, enabled 

 him to set this question at rest for ever. 



The law of force being determined for the particles of the electric 

 fluid, it now came to be the business of the experimenter and the 



8 Works, iv. p. OS. 10 Blog. Univ. art Coulomb, by Biot. 



11 Me>n. A. P. 1TS5, pp. 509, 578. 



