258 



On a Constant Form of DanielVs Battery, [Jan. 19, 



have been introduced and used. It becomes, therefore, 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 commerce, 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 down- 

 ward 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 regulated 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 

 semisaturated with sulphate of zinc is considerably less than that of a sa- 

 turated solution. A still more serious inconvenience than a somewhat in- 

 creased resistance has been pointed out to me by Mr. Varley as a consequence 

 of allowing sulphate of zinc to accumulate in the battery. Sulphate of zinc 

 crystallizes over the lip of the jar, and forms pendents like icicles outside, 

 which act as capillary siphons, and carry off liquid. Mr. Varley tells me 

 that this curious phenomenon is not unfrequently observed in telegraph- 

 batteries, and sometimes goes so far as to empty a cell and throw it alto- 

 gether out of action. Even without this extreme result, the crystallization 

 of zinc about the mouth of the jar is very inconvenient and deleterious. 

 It is of course altogether avoided by the plan I now propose. 



In conclusion, then, the siphon-extractor must be arranged to carry off 

 all the water of crystallization of the sulphate of copper decomposed in the 

 use of the cell, and enough of water besides to carry away as much sulphate 

 of zinc as is formed in the use of the battery. Probably the most conve- 

 nient mode of working the system in practice will be to use a glass capillary 

 siphon, drawing quickly enough to carry off in a few hours as much water 

 as is poured in each time at the top ; and to place, as shown in the draw- 

 ing, the discharging end of the siphon so as to limit the discharge to a 

 level somewhat above the upper level of the zinc grating. It will no doubt 

 be found convenient in practice to add measured amounts of sulphate of 

 copper by the charging-tube each time, and at the same time to pour in a 

 measured amount of water, with or without a small quantity of sulphate of 

 zinc in solution. 



As 100 parts by weight of sulphate of copper crystals contain, as nearly 

 as may be, 36 parts of water, it may probably answer very well to put in, for 

 every kilogramme of sulphate of copper, half a kilogramme of water. Expe- 

 rience (with the aid of specific-gravity beads) will no doubt render it very 



