680 LECTURE LIV. 



extended his researches, and the application of his discoveries, to a vari- 

 ety of natural as well as artificial phenomena, and there can be no doubt but 

 that he will still make such additions to his experiments, as will be of the 

 greatest importance to this branch of science. 



The operation of the most usual electrical machines depends first on the 

 excitation of electricity by the friction of glass on a cushion of leather, 

 covered with a metallic amalgam, usually made of mercury, zinc, and tin, 

 ■which probably, besides being of use in supplying electricity readily to 

 different parts of the glass, undergoes hi general a chemical change, by means of 

 which some electricity is extricated. The fluid, thus excited, is received into an 

 insulated conductor by means of points, placed at a small distance from 

 the surface which has lately undergone the effects of friction, and from this 

 conductor it is conveyed by wires or chains to any other parts at pleasure. 

 Sometimes also the cushion, instead of being connected with the earth, is itself 

 fixed to a second conductor, which becomes negatively electrified; and either 

 conductor may contain within it ajar, which may be charged at once by the 

 operation of the machine, when its internal surface is connected cither with the 

 earth, or with that of the jar contained in the opposite conductor. The 

 glass may be either in the form of a circular plate or of a cylinder, and it is 

 uncertain which of the arrangements affords the greatest quantity of electri- 

 city from the same surface; but the cylinder is cheaper than the plate, and 

 less liable to accidents, and appears to be at least equally powerful. (Plate 

 XL. Fig. 558, 559.) 



The plate machine in the Teylerian museum, employed by Van Marum, 

 when worked by two men, excited an electricity, of which the attraction was 

 sensible at the distance of 38 feet, and which made a point luminous at 27 

 feet, and afforded sparks nearly 24 inches long. A battery charged by it, 

 melted at once twenty five feet of fine iron wire. Mr. Wilson had also a few 

 years ago, in the Pantheon in London, an apparatus of singular extent; the 

 principal conductor was 150 feet long, and 16 inches in diameter, and he 

 employed a circuit of 4800 feet of wire. 



The electrophorus derives its operation from the properties of induced elec- 

 tricity. A cake of a nonconducting substance, commonly of resin or of 



