of the Galvanic Battery . 45 7 



for every pound of chlorine taken up ; and the less intensity 

 of the combustion which you witness is an indication of that 

 smaller evolution of heat. 



Now iron can take chlorine away from any feebler metal, 

 forming chloride of iron, and turning the feebler metal out of its 

 compounds. For instance, if I take a solution of chloride of 

 copper (this deep-green liquid) and break a bulb containing 

 finely-divided iron inside it, two things will happen at the same 

 time : chloride of iron will be made, and chloride of copper will 

 be unmade ; we shall have a yellow solution of iron, and a fine 

 red powder consisting of precipitated metallic copper. Before I 

 begin the experiment, I want you to notice that the chloride of 

 copper does not act on this air-thermometer, as both are at the 

 temperature of this room. You see how rapidly the powder of 

 metallic iron which I throw into the copper solution acts upon 

 it, destroying the green colour, and precipitating the copper, but 

 at the same time the liquid is getting hot. The air-thermo- 

 meter now shows a considerable rise of temperature. If you 

 measured carefully all the heat which is given off when chloride 

 of iron is formed in this indirect manner, you would find it less 

 than when direct combination takes place. Every pound of 

 chlorine which goes to form protochloride of iron in this expe- 

 riment gives off only 530 degrees of heat instead of 1492. 

 Whence this difference of 962 degrees of heat ? The reason is 

 obvious : we have not only made chloride of iron, evolving 

 thereby 1 492 degrees of heat, but we have at the same time, 

 and in the same liquid, unmade chloride of copper ; and the chlo- 

 ride of copper absorbs when unmaking just as much heat as it 

 evolved when making, viz. 962 degrees ; so that 962 degrees of 

 heat are consumed in the decomposition of the chloride of cop- 

 per: 1492 — 962 = 530 only are obtained. In this experiment 

 the iron was used in the form of a fine powder for the sake of 

 rapidity of action, and the copper remains in a similar form, 

 each little fragment of copper being precipitated beside the little 

 fragment of iron which was taking its chlorine. In fact the two 

 chemical processes (of combination and decomposition) are mixed 

 up with one another throughout the liquid. But I can effect 

 the same decomposition and the same combination in such a way 

 as to keep them separate, the combination in one part of the 

 liquid, the decomposition at some distance from it. For this 

 purpose I put a piece of hard carbon into this porous cylinder, 

 and pour some chloride of copper in with it. The porous cylinder 

 thus filled is placed in a jar containing hydrochloric acid, and a 

 plate of amalgamated zinc is immersed in this acid. I must 

 then attach the zinc outside the porous cell to the carbon inside 

 it by means of a metallic wire. The zinc will then dissolve, 



Phil. Mag. S. 4. Vol. 26. No. 177. Bee. 1863. 2 H 



