John Canton, Henry Cavendish 81 



the defects of Bennett's doubler and converted it into an in- 

 strument which ought to have come into more extensive use. 



William Watson (1715-1787), who started life as an 

 apothecary, but reached sufficient distinction as a medical 

 man to obtain the honour of knighthood, improved the 

 Leyden jar by substituting tin-foil for the liquid which till 

 then had formed the inner coating. In his experiments with 

 these jars he was much assisted by Dr. John Bevis (1695- 

 1771), another medical man, who was, however, mainly 

 interested in astronomical work, and also deserves to be men- 

 tioned as being the first to make a glass containing borax, 

 and to note that its refractive power was thereby increased. 

 Dr. Ingenhouse, a Dutch doctor settled in England, conducted 

 many electrical experiments, and claimed to have been the 

 first to replace the glass cylinder used in electrical machines 

 by a disc. The same claim is, however, made by others both 

 in France and Germany, and, among Englishmen, by Jesse 

 Ramsden, the optician and instrument maker, of whom more 

 will have to be said presently, and who certainly first brought 

 glass-plate machines into general use. 



On a higher plane stand the researches of Henry Cavendish 

 which now demand our consideration. A paper published 

 in the " Philosophical Transactions " contains the foundation 

 of the mathematical theory of electrostatics. There were 

 probably but few mathematicians at the time interested in the 

 subject, and the experimental part of the enquiry, which 

 might have directed more general attention to the importance 

 of the work, was not published until a century later. The 

 mathematical investigation showed that if the whole of the 

 electricity communicated to a body collects at its surface, 

 none entering the interior, it necessarily follows that the 

 repulsion between two quantities of electricity must diminish 

 with increasing distance according to the same law as that 

 of gravitation. No other law would lead to the same result. 

 Robison appreciated the importance of this investigation 

 (see p. 69), but, like others, he was ignorant of the unpublished 

 experiments which Cavendish had actually made on the 

 subject. These verified with a sufficient degree of accuracy 

 that the charge of a body in electrostatic equilibrium resides 

 at the surface, and that if any part of it penetrates into the 



P 



