DISCOVERY OF CONTACT ELECTRIFICATION 445 



lead ore, gold, silver, copper, brass, regulus of antimony, bismuth, 

 tutenag, merciir}', various kinds of wood and stone. Zinc and tin gave 

 negative charges to his plate. 



Here is apparently the beginning of that arrangement of substances 

 which has since come to be known as Volta's Contact Series. It is well 

 to bear in mind that these experiments were published in 1789, two years 

 before Galvani made his celebrated observation on the twitching of frogs' 

 legs which finally led up to the controversy through which A^olta dis- 

 covered the electric current. 



The next experimenter to investigate the subject of contact electri- 

 fication was apparently Tiberius Cavallo. Cavallo was an Italian by 

 birth, but was a resident of London and a prominent member of the 

 Eoyal Society. Cavallo published " A Complete Treatise on Electricity,'' 

 which went through a number of editions. In the fourth edition, pub- 

 lished in 1795, he adds a new volume containing tlie important discover- 

 ies in the subject since tlie publication of the third edition. Among these 

 he gives first place to the investigations of Galvani and Volta on animal 

 electricity, and mentions the fact that Volta suspected the phenomena 

 might be caused by the contact of two dissimilar metals. He refers to 

 Bennett's experiments, but says that others who have repeated them 

 have obtained inconstant results. Finally he hit upon a different method 

 of experimentation which enabled him to detect with certainty the elec- 

 trification due to metallic contact. In his section devoted to experiments 

 on metallic substances he says : 



After many fruitless attempts, and after having sent to the press the pre- 

 ceding part of this volume, I at last hit upon a method of producing electricity 

 by the action of metallic substances upon one another, and apparently without 

 the interference of electric bodies. I say apparently so, because the air seems to 

 be in a great measure concerned in those experiments, and perhaps the whole 

 effect may be produced by that surrounding medium. But though the irregular, 

 contradictory, and unaccountable effects observed in these experiments do not as 

 yet furnish any satisfactory theory, and though much is to be attributed to the 

 circumambient air, yet the metallic substances themselves seem to be endowed 

 with properties peculiar to each of them, and it is principally in consequence of 

 those properties that the produced electricity is sometimes positive, at other 

 times negative, and various in its intensity. 



The discovery of those properties of metallic bodies opens a new field of 

 useful investigation, and renders more manifest the general or extensive influ- 

 ence of a fluid wonderful in its nature and action. But how far they will enable 

 us to explain the phenomena of animal electricity, and of other operations of 

 nature, are considerations which will be noticed after the recital of the experi- 

 ments. — In this account I shall endeavor to select and methodise the experiments, 

 in the best manner that the irregularity of their results seems to admit of. 



Exp. I. A piece of zinc, which weighed little more than half an ounce, was 

 dropped ten times successively upon an insulated tin plate. This plate was then 

 brought in contact with the plate A of the multiplier: the lever was worked, and 



