38 ELECTRICITY IN MOTION. 



force to be only a little weakened by the chemical action, while 

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

 the circulation is determined, although very feebly, in such a direc. 

 tion that the fluid passes from the acid to the copper. When, in 

 the fourth place, the combination consists of copper, sulphuret, 

 and water, the tendencies are, first, from the copper to the sul- 

 phuret, and from the water to tlie copper ; and secondly from the 

 water to the sulphuret : in this instance a chemical action must be 

 supposed between the oxygen of the water and the sulphuret, 

 wl.ich lessens the electromotive tendency more than the action that 

 takes place between the sulphuret and tiie copper, so that the fluid 

 passes from the cupper to the sulphurel ; 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 element of the series : thus, the sulphuric acid, 

 which conducts electricity 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 phaenomena, and there 

 can be no doubt but that he will still make such additions to bis 

 experiments, as will be of the greatest importance to this branch 

 of science. 



The operation of the most useful 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, un- 

 dergoes in general a chemical change, by means of which some 

 electricity is ercited. 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 con. 

 ductor, which becomes negatively electrified ; and either conductor 



