﻿Sir W. Thomson on a Constant Form o/ r Daniell , s Battery. 539 



superior to his own " gravity-battery." The comparative failure of 

 the latter, therefore, must have arisen from mixing by currents of the 

 liquids. All that seems necessary, therefore, to make the gravity-bat- 

 tery much superior instead of somewhat inferior to the porous-cell 

 battery, is to secure that the lower part of the liquid shall always remain 

 denser than the upper part. In seeking howjto realize this condition, it 

 first occurred to me to take advantage of the fact that saturated solu- 

 tion of sulphate of zinc is much denser than saturated solution of sul- 

 phate of copper. It seems* that, at 15° temperature, saturated aqueous 

 solution of sulphate of copper is of 1 ■ 186 sp. gr., and contains in every 

 100 parts of water 33*1 parts of the crystalline salt, and that at 15° the 

 saturated solution of sulphate of zinc is of sp. gr. 1*44 and contains 

 in every 100 parts of water 140*5 parts of sulphate of zinc— both 

 results being from Michel and Krafft's experiments f. Hence I 

 made an element with the zinc below, next it saturated solution of 

 sulphate of zinc, gradually diminishing to half strength through a 

 few centimetres upwards, saturated sulphate of copper resting on 

 this, and the copper plate fixed above in the sulphate-of-copper 

 solution. In the beginning, and for some time after, it is clear that 

 the sulphate of copper can have no access to the zinc otherwise than 

 by true diffusion. I have found this anticipation thoroughly realized 

 in trials continued for several weeks ; but the ultimate fate of such 

 a battery is that the sulphate of zinc must penetrate through the 

 whole liquid ; and then it will be impossible to keep sulphate of cop- 

 per separate in the upper part, because saturated solution of sulphate 

 of zinc certainly becomes denser on the introduction of sulphate of 

 copper to it. To escape this chaotic termination I have introduced 

 a siphon of glass with a piece of cotton-wick along its length inside 

 it, so placed as to draw off liquor very gradually from a level some- 

 what nearer the copper than the zinc — and a glass funnel, also pro- 

 vided with a core of cotton wick, by which water semisaturated with 

 sulphate of zinc may be continually introduced at a somewhat lower 

 level. A galvanic element thus arranged will undoubtedly continue 

 remarkably constant for many months ; but it has one defect, which 

 prevents me from expecting permanence for years. The zinc being 

 below, must sooner or later, according to the less or greater vertical 

 dimensions of the cell, become covered with precipitated copper from 

 the sulphate of copper, which finds its way (however slowly) to the 

 zinc. On the other hand, if the zinc be above, the greater part of 

 the deposited copper falls off incoherently from the zinc through the 

 liquid to the copper below, where it does no mischief, provided always 

 that the zinc be not amalgamated — a most important condition for 

 permanent batteries, pointed out to me many years ago by Mr. Var- 

 ley. Placing the zinc above has also the great practical advantage 

 that, even when after a very long time it becomes so much coated 

 with metallic copper as to seriously injure the electrical effect, it 

 may be removed, cleaned, and replaced without otherwise disturbing 



* Storer's Dictionary of Solubilities of Chemical Substances. Cambridge, 

 Massachusetts : Sever and Francis, 1864. 



t Ann. Ch. et Phys. (3) vol. xli. pp. 478, 482: 1854. 



