ON ELECTRICITY IN MOTION. 525 



while the third is destroyed, so that the first overcomes the second, and the 

 circulation is determined, although very feebly, in such a direction, that 

 the fluid passes from the acid to the copper. When, in the fourth place, 

 the combination consists of copper, sulfuret, and water, the tendencies are, 

 first, from the copper to the sulfuret, and from the water to the copper ; 

 and secondly, from the water to the sulfuret : in this instance a chemical 

 action must be supposed between the oxygen of the water and the sulfuret, 

 which lessens the electromotive tendency, more than the action that takes 

 place between the sulfuret and the copper, so that the fluid passes from the 

 copper to the sulfuret ; and the current has even force enough to prevent 

 any chemical action between the water and the copper, which would tend 

 to counteract that force, if it took place. 



Mr. Davy has observed that the decomposition of the substances 

 employed in the battery of Volta, is of much more consequence to their 

 activity than either their conducting power, or their simple action on the 

 other elements of the series : thus, the sulfuric acid, which conducts elec- 

 tricity better, and dissolves the metals more readily, than a neutral 

 solution, is, notwithstanding, less active in the battery, because it is not 

 easily decomposed. Mr. Davy has also extended his researches, and the 

 application of his discoveries, to a variety 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 in 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 nega- 

 tively electrified ; and either conductor may contain within it a jar, which 

 may be charged at once by the operation of the machine, when its 

 internal surface is connected either 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 electricity 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 



