72 HISTORY OF CHEMISTRY. [LECTURE V. 



tries to refer all chemical combination and decomposition to 

 similar causes. 



Volta had assumed 13 that simple contact of two hetero- 

 geneous bodies was sufficient to place them in opposite electri- 

 cal conditions, and this hypothesis explained the generation of 

 the electric current to himself and to his numerous followers. 

 Davy identified himself with this opinion, and tried to prove 

 its accuracy by direct experiments. 14 He brought dry, in- 

 sulated acids into contact with metals, and showed by means 

 of the gold-leaf electroscope that the acids thereby become 

 negatively, and the metals positively electrified. He observed 

 similar phenomena on rubbing sulphur upon copper, when 

 the former became negative and the latter positive, Davy 

 found further that these electrical energies which, in the last 

 case, for instance, are only slight at ordinary temperatures, 

 increase considerably on heating, and are very great at the 

 melting-point of sulphur. On still further elevating the 

 temperature, the two substances unite with evolution of light, 

 and the compound obtained is non-electric. Davy con- 

 cludes from this that the combination consists in a mutual 

 discharge of the opposite electricities, and that heat and 

 light, which appear simultaneously, are consequences of this 

 discharge. According to him, chemical affinity is produced by 

 difference in electrical condition, and the affinity increases or 

 diminishes the greater or less this difference is. In cases 

 where there is considerable difference of energy, equalisation 

 is accompanied by phenomena of fire ; with feebly electrified 

 substances, only small quantities of heat are evolved ; but if 

 combination is to take place at all, the electrical energy must 

 always be able to overcome the cohesion of the substances. 

 Davy tries to prove directly the dependence of chemical 

 affinity upon electrical condition, since he says : 15 



" As the chemical attraction between two bodies seems to 

 be destroyed by giving one of them an electrical state different 



13 Brugnatelli, Ann. di. Chim. 13 and 14 ; compare also Ann. Chim. 

 40, 225. 14 Phil. Trans. 1807, 32. 15 Ibid. 1807, 39. 



