524 LECTURE LIV. 



bodies concerned ; since such bodies are always found, by delicate tests, to 

 exhibit, either during their contact or after separation, marks of different 

 species of electricity ; and their mutual actions may be either augmented 

 or destroyed, by increasing their natural charges of electricity, or by 

 electrifying them in a contrary way. Thus, an acid and a metal are found 

 to be negatively and positively electrical with respect to each other ; and 

 by further electrifying the acid negatively, and the metal positively, their 

 combination is accelerated ; but when the acid is positively electrified, or 

 the metal negatively, they have no effect whatever on each other. The 

 acid is also attracted, as a negative body, by another positively electrified, 

 and the metal by a body negatively electrified, so that a metallic salt may 

 be decomposed in the circuit of Volta, the positive point attracting the 

 acid, and the negative point the metal ; and these attractions are so strong 

 as to carry the particles of the respective bodies through any intervening 

 medium, which is in a fluid state, or even through a moist solid ; nor are 

 they intercepted in their passage, by substances which, in other cases, have 

 the strongest elective attractions for them. Alkali, sulfur, and alkaline 

 sulfurets, are positive with respect to the metals, and much more with 

 respect to the acids : hence they have a very strong natural tendency to 

 combine with the acids and with oxygen : and hydrogen must also be con- 

 sidered as belonging to the same class with the alkalis. 



Supposing now a plate of zinc to decompose a portion of water ; the 

 oxygen, which has a negative property, unites with the zinc, and probably 

 tends to neutralise it, and to weaken its attractive force ; the hydrogen is 

 repelled by the zinc, and carries to the opposite plate of silver its natural 

 positive electricity ; and if the two plates be made to touch, the energy of 

 the plate of zinc is restored, by the electricity which it receives from the 

 silver : and it receives it the more readily, as the two metals, in any case 

 of their contact, have a tendency to become electrical, the zinc positively, 

 and the silver negatively. Mr. Davy therefore considers this chemical 

 action as destroying, or at least counteracting, the natural tendency of the 

 electric fluid to pass from the water to the zinc, and from modifications of 

 this counteraction he explains the effects of galvanic combinations in all 

 cases. Thus, in a circle composed of copper, sulfuret, and iron, the fluid 

 tends to pass from the iron towards the sulfuret, and from the copper to 

 the iron, in one direction, and in the opposite direction from the copper to 

 the sulfuret, with a force which must be equal to both the others, since 

 there would otherwise be a continual motion without any mechanical 

 cause, and without any chemical change ; but the action of the sulfuret on 

 the copper tends to destroy its electromotive, or rather electrophoric, power, 

 of directing the current towards the sulfuret, and its combination with the 

 sulfur makes it either positively electrical, or negatively electrical in a less 

 considerable degree ; consequently the fluid passes, according to its natural 

 tendency, from the copper to the iron, and from the iron to the sulfuret. 

 In a third case, when copper, an acid, and water, form a circle, the natural 

 tendency is from the acid to the copper on one side, and from the acid to 

 the water, and from the water to the copper on the other ; here we niust 

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



