1S13.] Memoirs of the Manchesfer Sociely. 465 



which have never been proved to exist, and assumes the invisible 

 passage of a current of hydrogen gas through a liquid, which it 

 is difficult to understand. 3. Sir Humphry Davy has given a 

 theory of the galvanic energy, in which he unites the two pre- 

 ceding explanations. He conceives that when the battery, for 

 example, is composed of copper^ zinc, and solution of common 

 salt, the zinc becomes positive, and the copper negative. There- 

 fore the zinc attracts the oxygen and acid, which are negative; 

 and the copper, the hydrogen and alkali, which are positive. 

 But this equilibrium is immediately destroyed by the formation 

 of muriate of zinc, and the evolution of hydrogen gas. Hence 

 the action of the zinc and copper is again repeated, and this 

 goes on as long as the chemical action continues. 



Dr. Henry is of opinion that the primary excitement of elec- 

 tricity is owing to the chemical changes ; but he conceives it to 

 be essential to the activity of the battery that one set of elements 

 of the fluid should have no affinity for one of the metals. Thus 

 in the preceding example the oxygen and the acid combine with 

 the zinc ; but the hydrogen and alkali having no affinity for the 

 copper, deposite a portion of their electricity on it, and thus the 

 accumulation proceeds. He accounts for the evolution of the 

 two constituents of a substance decomposed by the battery at the 

 two poles, though at a distance from each other, by supposing a 

 series of intermediate decompositions to go on. Suppose water 

 to be the substance decomposed ; we may conceive a series of 

 particles of water arranged between the two poles. An atom of 

 oxygen gas escapes at the positive pole. The hydrogen pre- 

 viously combined with this atom unites with the oxygen of the 

 next particle of water ; and this successive decomposition goes 

 on till it reaches the negative pole, when the atom of hydrogen 

 remaining makes its escape in the form of gas. This is a very 

 ingenious explanation, and as far as water is concerned, appears 

 satisfactory. But it will not apply to other bodies, as Dr. Henry 

 thinks it will. Suppose a weak solution of common salt : we 

 know that the action of the battery may be continued till every 

 particle of the salt is decomposed, and all the acid accumulated 

 round one pole, and the base round the other. Now what must 

 happen when only a single particle of common salt remains to 

 be decomposed ? It is dear that no series of decompositions take 

 place ; the acid must move to one pole^ and the base to the 

 other, through the water. 



( To he coniimied.) 



II. Tcxhleau des Especes Minerales', par I. A. H. Lucas. 



Beconde partie. Paris, 1813.] We shall give an account of , 

 this book, if possible, in our next Number. 



Vol. L N° YL 2 G- 



