80 Britain's Heritage of Science 



electricity without committing ourselves to any definite 

 theory as to their ultimate nature. 



When the primary phenomena of static electricity had 

 been established, the further progress took its natural and 

 regular course. Experimental appliances had to be improved, 

 and instruments constructed suitable for quantitative measure- 

 ments. In this work John Canton (1718-1772), a private 

 schoolmaster, took an active and successful part. He 

 increased the efficiency of electrical machines by coating 

 the friction cushion, which was pressed against the glass 

 cylinder, with an amalgam of mercury. For the coarser 

 indicators of electricity, such as that which Gray had used, 

 Canton substituted two small spheres of pith or cork, 

 suspended from threads, which diverged when the spheres 

 became electrified. 



Canton was also successful in other fields of science; we 

 owe to him the first experimental demonstration that water 

 is compressible, and the discovery of a new phosphorescent 

 body which he prepared by the action of sulphur on oyster 

 shells. William Henley, a linen-draper residing in London, 

 who reached sufficient distinction to be admitted to the 

 fellowship of the Royal Society, also constructed an electro- 

 scope intended for quantitative measurements. He was 

 chiefly interested in thunderstorms and atmospheric elec- 

 tricity generally, and noted the positive electrification of 

 the air in a dry fog. Greater importance is to be attached 

 to Abraham Bennett (1756-1799), a clergyman residing in 

 the Midland counties, who introduced the gold-leaf electro- 

 scope, the most sensitive instrument invented up to that 

 time for the detection of small quantities of electricity. 

 Simultaneously with Volta, he showed how the electric 

 condensers could be used in conjunction with electrometers 

 so as to increase their effectiveness. This led him to invent 

 an instrument called a duplicator which in principle is 

 identical with Lord Kelvin's replenisher ; but as it contained 

 conductors covered with shellac for purposes of insulation, 

 irregularities in its action interfered with the experiments. 

 In spite of these defects it was the embryo of our modern 

 " influence " machine. William Nicholson (1753-1815), to 

 whom further reference will be made (p. 107), cured most of 



