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Royal Society : — 



unless such as may come from the ordinary commercial sulphate-of- 

 copper and commercial sulphate-of-zinc crystals which were used. 



Postscript. 



(Eeceived February 2, 1871.) 

 The principle which I have adopted for keeping the sulphate of 

 copper from the zinc is to allow it no access to the zinc except by 

 true diffusion. This principle would be violated if the whole mass 

 of the liquid contiguous to the zinc were moved toward the zinc. Such 

 a motion actually takes place in the second form of element (that 

 which is represented in the drawing, and which is undoubtedly the 

 better form of the two) every time crystals of sulphate of copper are 

 dropped into the charging-tube. As the crystals dissolve, the liquid 

 again sinks, but not through the whole range through which it rose 

 when the crystals were immersed. It sinks further as the sulphate 

 of copper is electrically precipitated on the copper plate below in 

 course of working the battery. Neglecting the volume of the me- 

 tallic copper, we may say, with little error, that the whole residual 

 rise is that corresponding to the volume of water of crystallization of 

 the crystals which have been introduced and used. It becomes, there- 

 fore, a question whether it may not become a valuable economy to use 

 anhydrous sulphate of copper instead of the crystals ; but at present 

 we are practically confined to the "blue vitriol" crystals of com- 

 merce ; and therefore the quantity of water added at the top of the 

 cell from time to time must be, on the whole, at least equal to the 

 quantity of water of crystallization introduced below by the crystals. 

 Unless a cover is added to prevent evaporation, the quantity of water 

 added above must exceed the water of crystallization introduced 

 below by at least enough to supply what has evaporated. There 

 ought to be a further excess, because a downward movement of the 

 liquid from the zinc to the level from which the siphon draws is 

 very desirable to retard the diffusion of sulphate of copper upwards 

 to the zinc. Lastly, this downward movement is also of great value 

 to carry away the sulphate of zinc as it is generated in the use of 

 the battery. The quantity of water added above ought to be regu- 

 lated so as to keep the liquid in contact with the zinc a little less 

 than half saturated with sulphate of zinc, as it seems, from the 

 observations of various experimenters, that the resistance of water 



