Chemical Science. 375 



with zinc filings, the two bodies immediately act, and a hea 

 above that of boiling water is produced. 



Zinc is not the only common metal which thus rapidly decom- 

 poses chloride of silver, in the dry way. Tin acts even more 

 powerfully when triturated with it : and copper and iron have 

 both of them affinities for chlorine strong enough to produce the 

 same effect. 



There is therefore no occasion to assume hydrogen as the de- 

 composing agent, when chloride of silver is reduced in contact 

 with zinc or iron (iron acts as zinc does in all these experiments, 

 though not so powerfully); for the metals, by their attraction for 

 chlorine, are sufficiently energetic to produce the effect. Yet, 

 as I had supposed, from general opinion, that hydrogen could, 

 by its attraction for chlorine, separate that element from silver, 

 I endeavoured to ascertain in what circumstances it had the 

 power of doing so. If a stream of hydrogen, rapidly generated 

 from iron or zinc, be sent against moist chloride of silver, in a 

 dark place or by candle-light, it appears to alter it ; but this 

 effect must be due to metals or impurities held in solution for 

 when purified, it has no power of changing it out of day-light; 

 nor have I been able, even in the sun-shine of this month, to 

 make hydrogen act on chloride of silver in several hours. 



Still, however, hydrogen may be allowed in certain circum- 

 stances to have the power of decomposing chloride of silver, 

 but the circumstances are not such as were, I believe, generally 

 supposed to have place in the experiment first referred to. When 

 zinc, iron, tin, ^c, are thrown into moist chloride of silver, the 

 first decomposition is occasioned by the action of the zinc on the 

 chloride, afterwards a voltaic circle is formed by the zinc, the re- 

 duced silver and the water; water is decomposed, the zinc 

 takes oxygen, the hydrogen liberated at the surface of the sil- 

 ver takes the chlorine from the chloride in its immediate 

 neighbourhood, and thus the reduction will go on to the distance 

 of an inch or more from the piece of zinc, and the consequent 

 products are silver and solution of muriate of zinc. But as 

 this is a case of decomposition entirely different to the supposed 

 one of the reduction of chloride of silver, by hydroi^en, any 



