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Proceedings of the Royal Society 
possible, however, that the state of the air as regards moisture may 
have some influence. If the surface of the metal is moist, or the 
rubber moist, the amount of electricity produced by a rub is not so 
great as when both are dry. This difference is due, in part at least, 
and it may be entirely, to the worse insulating power of the moist 
or moistened rubber. 
Cavallo,'^ with sealing-wax as the rubbing material, found all the 
metals he tried negative ; but Singerf found iron, steel, graphite, 
lead, and bismuth positive, the others negative. The iron, how- 
ever, was positive only when rubbed with smooth wax; it was 
negative when rubbed v/ith tarnished (soft '?) wax. WilsonJ found 
Avith a silver plate, that when he rubbed the Avax on the surface the 
plate became positive, but Avhen he rubbed it on the edge the plate 
became negative. Some experiments which I have made throw 
light on these anomalies. 
With sulphur as the rubbing material, Davy§ found that lead 
having a fresh surface became negative, but when it had a tarnished 
surface became positive. While Wilcke found all the metals nega- 
tive excepting lead, Faraday found iron, copper, brass, tin, silver, 
and platinum positive. The contradictory results in the case of 
sulphur appear to be due to the presence or absence of abrasion in 
the rubbing, and to the presence or absence of a charge on the 
sulphur. 
De la Five, II using a variety of rubbers — the hand, ivory, horn, 
cork, caoutchouc, resin — found the following metals always nega- 
tive — 
Fhodium, platinum, palladium, gold, tellurium, cobalt, nickel; 
the following mostly negative — 
Silver, copper, brass ; 
and the folloAving negative or positive — 
Antimony, bismuth, lead, zinc, tin, iron. 
He found great difflculty with his mode of friction in getting either 
* Treat, of El., i. 21. f Elem. d. Elekt., 21 
X Vrieatley'’& Hist, of EL, 144. § Gilbert’s 28 168. 
11 Bihliothique universetle, 59, 13. 
