prior to the Introduction of the Potentials. 51 



this experiment that the attraction of electricity is subject to 

 the same laws with that of gravitation, and is therefore according 

 to the squares of the distances ; since it is easily demonstrated 

 that were the earth in the form of a shell, a body in the inside 

 of it would not be attracted to one side more than another ? " 



This brilliant inference seems to have been insufficiently 

 studied by the scientific men of the day ; and, indeed, its author 

 appears to have hesitated to claim for it the authority of a com- 

 plete and rigorous proof. Accordingly we find that the question 

 of the law of force was not regarded as finally settled for eighteen 

 years afterwards.* 



By Franklin's law of the conservation of electric charge, and 

 Priestley's law of attraction between charged bodies, electricity 

 was raised to the position of an exact science. It is impossible 

 to mention the names of these two friends in such a connexion 

 without reflecting on the curious parallelism of their lives. In 

 both men there was the same combination of intellectual bold- 

 ness and power with moral earnestness and public spirit. Both 

 .of them carried on a long and tenacious struggle with the reac- 

 tionary influences which dominated the English Government in 

 .the reign of George III ; and both at last, when overpowered in 

 the conflict, reluctantly exchanged their native flag for that of 

 the United States of America. The names of both have been 

 held in honour by later generations, not more for their 

 scientific discoveries than for their services to the cause of 

 religious, intellectual, and political freedom. 



The most celebrated electrician of Priestley's contemporaries 

 in London was the Hon. Henry Cavendish (b. 1731, d. 1810), 

 whose interest in the subject was indeed hereditary, for his 

 father, Lord Charles Cavendish, had assisted in Watson's experi- 

 ments of 1747.f In 1771 Cavendish} presented to the Koyal 

 Society an " Attempt to explain some of the principal phenomena 

 of Electricity, by means of an elastic fluid." The hypothesis j 



* In 1769 Dr. John Robison (b. 1739, d. 1805), of Edinburgh, endeavoured to 

 determine the law of force by direct experiment, and found it to be tbat of the 

 inverse 2'06 th power of the distance. 



t Phil. Trans, xlv, p. 67 (1750). J Phil. Trans. Ixi, p. 584 (1771). 



E 2 



