HISTORY, &c., OF INDUCTION INSTEUMENTS. 317 



cliloride of silver liermetic batteiy of M. Gaiffe would originally 

 furnish continuous action of constant intensity for about ten hours, 

 the circuit being completed by an induction coil ; but the manu- 

 facturer now informs me that his battery will work for twenty- 

 four hours, at least, in consequence of a modilieation that he has 

 made in it.^ 



Wlien this battery is exhausted, it is necessary to recharge it 

 with a new piece of chloride of silver (see description, fig. 87). 

 Tills is easy of execution. 



One of the principal merits of these batteries in boxes is, 

 according to their makers, that they close herinetically, so that 

 there is no fear that any liquid will escape from them to injure 

 the instrument, or the clothing, when carried upon the person. 

 This is an error ; for the hydrogen — a greater or less development 

 of which is the inevitable result of the chemical action — never fails 

 to make its way by a little fissure, which generally exists around 

 the metallic wires, which pass through the lid. Even if the 

 mastic, which surrounds the metallic wires, resists the pressure of 

 the gas, so that it cannot escape, there will still be a risk of ex- 

 plosion if, under any circumstances, there should be a rapid pro- 

 duction of gas in any large quantity. 



We are chiefly exposed to the same accident with the bisul- 

 phate of mercury battery, the chemical action of which producps 

 hydrogen by the decomposition of water. The chemical action of 

 the chloride of silver battery neither disengages hydrogen nor any 

 other gas.* But if its chloride of silver is exhausted, that is, if it 

 loses its proper action, and forms part of a circuit of a battery, 

 composed of a sufficient number of elements, it fulfils the office of 

 a voltameter, and then the current, in traversing it, decomposes 

 its water, and produces a great quantity of hydrogen. It is under 

 such circumstances that a chloride of silver element, inclosed in 

 an impermeable case, may explode.^ 



^ " The chloride of sodium," M. Gaiffe ■* There is no production of gas in the 



writes to me, " being only employed to chloride of silver battery, because there is 



render the liquid a conductor, and the only a displacement of chlorine, which 



product of the battery being chloride of leaves the silver to combine with the 



zinc, I have replaced the former by an zinc ; so that before the battery begins 



equal quantity of the latter, in order to > working we have : AgCl, Zn, HO, NaCl, 



have one salt only, and to avoid the I and, after its action : Ag, ZnHO, NaCl. 



action which the two chlorides exert The chlorine only, therefore, has changed 



upon each other ; an action which Avas ' its j^lace ; the water and the salt have 



injurious to the battery, since, charged ! only acted by their presence, 



in the new way, it will work regularly J ^ This has happened to myself, when 



for a much longer time. Another ad- holding in my hand one of these small 



vantage of the chloride of zinc is that, by closed batteries of hard caoutchouc. The 



its employment, we avoid the production case burst, and some of the contained 



of gas, even when the charge of chloride liquid was thrown in the face of a jierson 



is exhausted.'' standing opposite to me. 



