292 Mr. Grove on a new Voltaic Battery of great enei'gy. 



forming a sort of honey-comb of square or hexagonal cells, 

 each containing a tube and zinc rod; but this would be too 

 difficult of manipulation. Another excellent method of ceco- 

 nomizing space, and which is applicable to all these forms, is 

 that proposed by Mr. Spencer of Birmingham, viz. to plait 

 or crimp the negative metal : by this means in a given space 

 the surface may be doubled without increasing the mean di- 

 stance between the metals*. 



The principal difficulty in arrangements of great magni- 

 tude would be to prevent the interference of the heating power 

 of the battery. With mine, although my decomposing appa- 

 ratus contains 8 ounces of liquid, I do not like to continue 

 the decomposition for more than half an hour, as at that time 

 the liquid is much heated. In one case a capsule containing 

 half an ounce of dilute acid was, by the joint effect of gaseous 

 and vaporous evolution, reduced in the space of an hour to 

 a i^vi drops of concentrated acid : the heat was such that the 

 capsule charred to some depth the wood upon which it stood, 

 and water allowed to trickle down the outside, hissed as though 

 from the surface of heated iron. 



The electrodes also should bear a certain superficial pro- 

 portion to the battery or they do not yield the full amount of 

 gas, and are in most cases melted offi 



I will conclude this letter with an account of a case of me- 

 tallic inactivity which I believe (I say it doubtingly) to be new. 

 It is well known that nitro-sulphuric acid acts very sluggishly 

 upon copper, and advantage is taken of this circumstance in 

 the arts, to part silver from copper. If concentrated nitro- 

 sulphuric acid be decomposed by the pile, the negative elec- 

 trode being of copper and the positive of platina, no particu- 

 lar effect is observable ; oxygen is evolved from the platinum, 

 the current passes, and the copper is slightly attacked ; but if 

 the positive electrode be of copper, decomposition is entirely 

 arrested, the copper is not attacked and evolves no oxygen : if 

 a galvanometer be included in the circuit, it indicates, after 

 the first instant, only a very feeble current f- 



I kept for six hours in nitro-sulphuric acid a very thin strip 

 of copper, which formed the positive electrode of a constant 

 battery ; at the end of that period it was nearly divided at 

 the portion just above the surface of the acid. The immersed 

 portion had a thin milky coat, which I believe to have trickled 

 down from the corroded portion, as when this was wiped off, 



* In large batteries, the best way would be to zigzag the platina foil round 

 a number of porous vessels ; by that means both surfaces of both metals may 

 be rendered available. 



f I use the word current in its ordinary sense, without at all intending 

 to attach to it its own son)ewhat too specific meaning. 



