46 Dr Davy's Observations on 



iron, is on the contrary preserved by the latter metal, and the 

 destruction of the latter accelerated by it. I shall merely remark, 

 that though my experiments are not in accordance with his, I 

 can conceive circumstances in which the result may be as M. 

 Van Beek maintains, such for instance, as water capable of de- 

 positing matter in the tin, so as to cover it with a closely adher- 

 ing crust. In illustration of my meaning, I may offer the in- 

 stance of the action of very dilute nitric acid, on these two me- 

 tals in contact, in which the iron though negative compared 

 with the tin, yet dissolves first, in consequence being positive in 

 relation to the insoluble oxide that speedily envelopes the tin. 



I shall conclude with the mention of a few particulars which 

 bear on the theory of voltaic electricity, and which have come 

 under observation in the course of this inquiry. 



In the Bakerian lecture already referred to, my brother has 

 adduced an instance of decided electrical effect, indicated by 

 the galvanometer, under circumstances excluding chemical ac- 

 tion, and in which the sole cause appears to have been the con- 

 tact of the connecting platina wires with an acid and alkali, 

 separated by a neutral, imperfect, conducting fluid. This 

 he brought forward with many other facts in support of his 

 views relative to the primary production of voltaic electricity, 

 views nearly accordant with the original theory of Volta, and 

 which he adopted in preference to the purely chemical hypothe- 

 sis after careful consideration of the leading facts, a large num- 

 ber of which were the fruits of his labours. 



Latterly, other experiments have been published in favour of 

 the same theory, two of which in particular have been considered 

 demonstrative by their authors ; I allude to those of M. Pfaff, 

 and of Matteuci, both given in Annales de Chimie et de Phy- 

 sique,* one, in which electricity is excited by a pile of zinc and 

 copper with cloth, moistened with a solution of sulphate of zinc 

 deprived of air ; the other, in which it is produced, as indicated 

 by the most delicate of all galvanometers, a prepared frog, by 

 zinc and copper and distilled water purged of air, circumstan- 

 ces in which it is supposed that no chemical action can take 

 place. 



The objection to these instances appears to me to be, that proof 



• Tom. xli. p. 246, and xlv. p. lOG. 



