ON ELECTRICITY IN MOTION. 519 



lengthened, accordingly as it is loose or stretched during the experiment. 

 The more readily a metal conducts, the shorter is the portion of it which 

 the same shock can destroy ; and it has sometimes been found that a 

 double charge of a battery has been capable of melting a quadruple length 

 of wire of the same kind.* 



The mechanical effects of electricity are probably in many cases the 

 consequences of the rarefaction produced by the heat which is excited ; 

 thus, the explosion attending the transmission of a shock or spark through 

 the air, may easily be supposed to be derived from the expansion caused 

 by heat ; and the destruction of a glass tube, which contains a fluid in a 

 capillary bore, when a spark is caused to pass through it, is the natural 

 consequence of the conversion of some particles of the fluid into vapour. 

 But when a glass jar is perforated, this rarefaction cannot be supposed to 

 be adequate to the effect. It is remarkable that such a perforation may be 

 made by a very moderate discharge, when the glass is in contact with oil 

 or with sealing wax ; and no sufficient explanation of this circumstance 

 has yet been given. 



A strong current of electricity, or a succession of shocks or sparks, 

 transmitted through a substance, by means of fine wires, is capable of 

 producing many chemical combinations and decompositions, some of which 

 may be attributed merely to the heat which it occasions, but others are 

 wholly different. Of these the most remarkable is the production of 

 oxygen and hydrogen gas from common water, which are usually extri- 

 cated at once, in such quantities, as, when again combined, will reproduce 

 the water which has disappeared ; but in some cases the oxygen appears to 

 be disengaged most copiously at the positive wire, and the hydrogen at the 

 negative.t 



When the spark is received by the tongue, it has generally a subacid 

 taste ; and an explosion of any kind is usually accompanied by a smell 

 somewhat like that of sulfur, or rather of fired gunpowder. The peculiar 

 sensation, which the electric fluid occasions in the human frame, appears 

 in general to be derived from the spasmodic contractions of the muscles 

 through which it passes ; although in some cases it produces pain of a dif- 

 ferent kind ; thus, the spark of a conductor occasions a disagreeable 

 sensation in the skin, and when an excoriated surface is placed in the 

 galvanic current, a sense of smarting, mixed with burning, is experienced. 

 Sometimes the effect of a shock is felt most powerfully at the joints, on 

 account of the difficulty which the fluid finds in passing the articulating 

 surfaces which form the cavity of the joints. The sudden death of an 

 animal, in consequence of a violent shock, is probably owing to the im- 

 mediate exhaustion of the whole energy of the nervous system. It is 

 remarkable that a very minute tremor, communicated to the most elastic 



* Kinnersley on an Electrical Air Thermometer, and on the Extension of Wire, 

 Ph. Tr. 1763, p. 84. Nairne on Shortening Wires, ibid. 1780, p. 334. Riess, 

 Fogg. Ann. xl. 321 ; xliii. 47 ; xlv. 1. 



t Consult Cavendish, Ph. Tr. 1788, p. 261. Pearson, ibid. 1797, p. 142. Wol- 

 laston, ibid. 1801, p. 417. Davy, ibid. 1807, p. 1. Van Trostwyk, Gren's Jour, 

 i*. 130. Schonbein, Pogg. Ann. 1. 616. 



