lead ore, gold, silver, copper, brass, regulus of antimony, bismuth, tutenag, mercury, 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 Volta discovered the electric current.
The next experimenter to investigate the subject of contact electrification was apparently Tiberius Cavallo. Cavallo was an Italian by birth, but was a resident of London and a prominent member of the Royal Society. Cavallo published "A Complete Treatise on Electricity," which went through a number of editions. In the fourth edition, published in 1795, he adds a new volume containing the important discoveries in the subject since the 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 electrification 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 preceding 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 influence 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 experiments.—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 of the multiplier: the lever was worked, and
