556 



HISTORY OF SCIENCE. 



alkaline earths. He obtained an amalgam of the metal by forming 

 the negative pole of the battery of a drop of mercury resting on the 

 oxide to be decomposed, which was supported on a plate of platinum 

 (See Fig. 266). 



Here it will be convenient to mention the principal modifications of 

 Volta's pile, which have been in use since Wollaston's, whose improve- 

 ment of Cruikshank's battery consisted in folding the plates of copper 

 so that each of them had a plate of zinc within it without contact, of 

 course. Thus a copper surface was presented to the two sides of the 

 zinc plate instead of one, as in Cruikshank's arrangement. A plan by 

 which large surfaces of metal are exposed to the action of the exciting 



FIG. 266. 



FIG. 267. A VOLTAIC ELEMENT. 



liquid was devised by Dr. Hare, of Philadelphia. A long sheet of 

 copper is separated from a similar sheet of zinc by narrow strips of 

 cloth, and the two are wound round a wooden cylinder. This com- 

 pound roll plunged into a vessel of dilute acid forms an extremely 

 powerful single couple, and by combinations of a few such couples 

 extraordinary effects have been produced. 



. It is desirable, in order that the reader may understand certain 

 terms which must now be frequently employed, and that he may have 

 a clear idea of the theories of the pile which have been proposed, that 

 he should know the facts which are presented by a voltaic element. 

 Let Fig. 267 represent a vessel which contains diluted sulphuric acid, 

 z is a plate of pure zinc, c a plate of copper, and to each a wire is 

 attached. We suppose, in the first instance, that these wires do not 

 touch, but are insulated. If the wire attached to c were examined by 

 appropriate and delicate tests, it would be found to exhibit feeble indi- 

 cations of a charge of vitreous or positive electricity. Similarly the 

 wire attached to z would exhibit indication of a very feeble negative 

 (page 328) charge. So long as the wires are not in contact with each 



