302: APPENDIX. [No. XXII. 



manner in which Mr. Skirving has executed his part of the work has met 

 with the highest approbation. It is the province of supply, however, 

 always to be equal to the demand, and therefore, if surface-cutting 

 increases, we have no right to suppose there will be dearth of labour 

 or talent in that department of art. 



For the duplication of the original designs, we have recourse to the 

 power afforded to us by the processes of electro-metallurgy. For the 

 purposes of the Bank of England, we have had recourse to the various 

 forms of battery apparatus described by myself in the ' Philosophical 

 Magazine ' for June 1840, and subsequently in my ' Elements of Electro- 

 Metallurgy.' "We employ, as a source of power, the platinized silver 

 voltaic battery, which many of the mem.bers of this Society may remember 

 was brought under their notice some years ago. It was devised when I 

 was a student of medicine at King's CoUege, and resided in my father's 

 house in the Bank of England. With friends entirely devoted to other 

 than scientific pursuits, I was placed in an awkward position by the 

 discovery of the principles on which it was founded. I brought it here 

 unknown to any member, and after a long investigation a gold medal was 

 awarded for its invention. For fourteen years it has stood the test of 

 experience : and when we see that by its agency the plates of the maps of 

 the Ordnance Survey have for years been deposited ; when we see at the 

 present time that by it the types of the Bank of England notes and cheques 

 are formed ; and lastly, when amongst other purposes we find that it daily 

 transmits the power from the Observatory at Greenwich to indicate the 

 correct time in London, I trust the Society, in consideration of its applica- 

 tion to these truly national objects, will not consider that their medal has 

 been altogether bestowed in vain. 



At the Bank we employ large batteries, in vessels holding several 

 gallons of the acid charge. The platinized silver plate is of fair thickness, 

 and the zincs are so aiTanged that they can be readily changed. The 

 purer we can obtain thick-rolled zinc, the more economically can we con- 

 duct our process, for then we are not subjected to the inevitable loss which 

 arises if tin, a very frequent impurity, is present. "We are careful, for the 

 sake of economy, very thoroughly to amalgamate the zinc; in fact, we 

 prefer to repeat the process once or twice, that no local action may exist. 



For charging the battery we use dilute sulphuric acid, and generally 

 mix the fluids in the proportion of one-eighth acid to seven-eighths water. 

 It is convenient to adjust the mixture to a specific gravity of 1130, which 

 gives a strength suitable for battery purposes. A battery charged with 

 this liquid will last in action nearly three weeks before it is completely 

 exhausted; but practically, after it has done efficient duty from 7 to 14 

 days, it has become feeble, it exhibits the natural decay of old age, and we 

 generally respite it from further work and substitute a new charge, to 

 resuscitate its former life and vigour. "When the battery is thoroughly 

 exhausted, the solution has a specific gravity of 1360, and contains 144 

 grains of zinc for every 1000 grains of bulk, if evaporation and conden- 

 sation have been compensated for by the daily addition of sufficient 

 water to make up the original bulk. 



To ascertain the changes which are occurring in the battery we 

 commonly employ an hydrometer; but I have specially constructed an 

 instrument which I call a battery-metei'. The point corresponding to 



