402 M. Becquerel on the Electro-motive 



M. Rousseau has of late fully illustrated, by showing, with an 

 electrometer of his invention, that all badly conducting bo- 

 dies, covered with a thin stratum of hygrometric water, con- 

 ducted with facility very small quantities of electricity. We 

 therefore took a little capsule of wood or porcelain, filled with 

 distilled water ; and after having slightly moistened its sides, 

 we placed it on the upper plate of the condenser. But as 

 this box sometimes exercises an electro-motive action, very 

 feeble indeed, on the plate, that is destroyed by touching 

 the lower plate with a capsule formed of the same matter. 

 The precaution may even be pushed so far as to fill it with 

 distilled water ; then, touching this water with the finger, 

 account is kept on each side of the action of the water on 

 the wood. Laminae of different metals held between the 

 fingers were plunged in one of the two capsules, taking the 

 precaution of not touching the sides ; the other plate was in 

 communication with the earth. Zinc, iron, lead, tin, copper, 

 &c, communicated to it positive electricity ; whilst platinum, 

 gold, silver, &c, gave it negative electricity. Water then is 

 positive with the metals which are the most positive, and ne- 

 gative with those which are the least so. It thus exhibits the 

 same effects with the oxidable metals as alkalies do in their 

 contact with acids, when there is no chemical action. 



These phenomena also take place when the water contains 

 a small quantity of sulphuric acid. It may be remarked, that 

 the water is then decomposed by the iron and the zinc, and 

 that these metals are attacked. The chemical action in this 

 case has not therefore hindered the production of the electric 

 phaenomena which result from the contact of the metals with 

 the water. 



These experiments require great precautions. It is neces- 

 sary that the surface of each metal be perfectly freed from rust, 

 or well polished, not with emery paper, but with pounded glass, 

 in order to remove any grains of emery from their surfaces, 

 which might there exercise electro-motive effects. It is also 

 preferable to use a wooden capsule slightly moistened, be- 

 cause its fibres, being impregnated with moisture, conduct the 

 electric fluid with more facility than glass or porcelain. Not- 

 withstanding these precautions, it sometimes occurs that no re- 

 sult is obtained; but when these phaenomena are produced, 

 it is always in the order we have just mentioned. 



Perhaps we might be led to think that the small quan- 

 tities of electricity, which often remain attached to the light 

 coat of varnish with which each plate of the condenser is co- 

 vered, disturb the results by their presence, and lead to error, 



especially 



