PHYSICS OF NINETEENTH CENT. ELECTRICITY. 557 



other no action occurs in the vessel the zinc does not dissolve in the 

 acid. The instant, however, that a metallic communication is esta- 

 blished between the two plates the zinc begins to dissolve, but no visible 

 action is perceived on its surface ; whereas the copper plate, on the con- 

 trary, which is not acted upon, is immediately covered with minute 

 bubbles of hydrogen gas, and these rise up through the liquid. While 

 this action is proceeding, the wire which connects the metals, and 

 which may be of any length, is possessed of very remarkable properties, 

 and it will be the business of several of our subsequent pages to relate 

 the discovery of these properties. Further, although the liquid inter- 

 vening between the plates remains unchanged to the eye, it also pos- 

 sesses certain properties in common with the wire. For example, there 

 is a particular effect on a compass-needle (of which more hereafter) ; but 

 the thing now to be noted is that the liquid, or any part of the wire, 

 equally possesses this power, and that it is immediately lost if the 

 smallest break occurs anywhere that is, there must be a perfect con- 

 tinuity of metallic and liquid connection throughout. It will hereafter 

 be seen, also, that the action is such that we must recognize the in- 

 fluence of the wire or circuit as having a definite direction. These cir- 

 cumstances at once suggest to the mind the idea of something flowing 

 through the whole, and thus the phenomena of voltaic element appear 

 in harmony with the previous conception of the nature of electricity. 

 It must, however, be once more repeated that the " electrical fluid " 

 has no objective existence. It is only to facilitate our records and 

 reasonings that we picture the unknown influence in our minds under 

 the image of something flowing in one direction like water through a 

 pipe. It is assumed, by convention, that the current flows always from 

 the positively electrified body to the negatively electrified. This is 

 the direction shown by the arrows in Figs. 265 and 267. It will be 

 observed that there is, and must be, a complete circuit through which the 

 same current of electricity may flow equally at the same time across every 

 section. At what part of the circuit does the force originate by which 

 we imagine the current to be driven on ? This is a point which has 

 been the subject of great controversies and of many researches. Two 

 contending hypotheses, each of which has in turn prevailed, are already 

 before the reader, and the views entertained by several eminent physi- 

 cists of the present day are something like a fusion of the contact theory 

 of Volta (page 553) and the chemical theory advanced by Davy (page 

 555). Supposing the wires represented in Fig. 267 to be of copper, 

 the origin of the electrical force is, according to the contact theory, 

 chiefly at the junction of the copper wire with the zinc plate, because 

 there are at that point two different metals in contact. The hetero- 

 geneity of the liquid and metal contacts would also be admitted as 

 contributing to the result, and the chemical action would be considered 

 as the effect of the current. The chemical theory, on the other hand, 

 places the origin of the current at the junction of the liquid and the 



