Actions of Liquids on Metals. 403 



especially in the experiments in which the development of 

 electricity is feeble. But this is not the case; for the pre- 

 cautions we take secure us from such effects. Before the com- 

 mencement of an experiment, we assure ourselves by several 

 trials that the plates retain no electricity: when we find 

 any, we have them heated sufficiently to drive it away, or else 

 we place, as M. Pouillet does, a disk of tin between the two 

 plates ; then the small quantities of electricity which remain 

 on the surface of the layers of varnish become combined by 

 its intervention. Afterwards, we employ successively, as a 

 collector, each of these two plates, both being adapted to 

 receive a capsule. If a different electricity is then obtained, it 

 is a proof that the experiment has succeeded, the disturbing 

 causes being removed. Besides, the laws to which the greater 

 part of the results are submitted, come still further in aid of 

 the exactness of the experiments. 



We have also another objection to obviate. When a capsule 

 of earthenware, glass, &c, is placed on one of the plates of the 

 condenser, and is touched with the finger, — which, as is well 

 known, is not a good conductor, — the condenser does not be- 

 come charged. It may be thought, that in touching the water 

 which it contains with any metal which is an excellent con- 

 ductor, the discharge of the electricity supposed to be ac- 

 quired by the capsule, in its contact with the plate, ought to be 

 more rapid, and that the condenser should become charged : 

 but this does not take place: for all the metals ought to give the 

 same electricity in their contact with the water, which is not 

 the case. Finally, by holding the metal between the fingers, 

 the conductibility is only momentarily augmented; since the 

 electricity, after having traversed the metal, is obliged to pass 

 afresh through the hand, where it finds the same obstacles as 

 when the finger touched the liquid. It is, therefore, now well 

 proved, that we have really observed the electro-motive actions 

 of water on metals. 



These actions being found, we then inquired if their in- 

 tensity was not susceptible of augmentation or diminution, 

 according to the modifications which the surfaces of me- 

 tals might experience in certain circumstances. A plate of 

 gold was plunged into nitric acid for some moments, and 

 then washed several times in water; it was then put in contact 

 with the water of the wooden capsule : the result was a de- 

 velopment of electricity much more considerable than before; 

 the water again took the negative electricity. 



The same plate plunged afresh into asolution of potash, lost 

 in great part the property of acquiring electricity by the contact 



3 E 2 of 



