﻿Sir W. Thomson on a Constant Form of Daniell's Battery. 543 



semisaturated with sulphate of zinc is considerably less than that of 

 a saturated solution. A still more serious inconvenience than a 

 somewhat increased 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 pheno- 

 menon is not unfrequently observed in telegraph-batteries, and some- 

 times goes so far as to empty a cell and throw it altogether 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 de- 

 composed 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 convenient 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 drawing, 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 con- 

 venient 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 sul- 

 phate 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 kilo- 

 gramme of water. Experience (with the aid of specific-gravity beads) 

 will no doubt render it very easy, by a perfectly methodical action in- 

 volving very little labour, to keep the battery in good and constant 

 action, according to the circumstances of each case. 



When, as in laboratory work, or in arrangements for lecture-illus- 

 trations, there may be long intervals of time during which the battery 

 is not used, it will be convenient to cease adding sulphate of copper 

 When there is no immediate prospect of action being required, and to 

 cease pouring in water when little or no colour of sulphate of copper 

 is seen in the solution below. The battery is then in a state in which 

 it may be left untouched for months or years. All that will be 

 necessary to set it in action again will be to fill it up with water to 

 replace what has evaporated in the interval, and stir the liquid in the 

 upper part of the jar slightly, until the upper specific-gravity bead 

 is floated to near the top; by sulphate of zinc, and then to place a 

 measured amount of sulphate of copper in the funnel at the top of 

 the charging-tube. 



Feb. 2.— General Sir Edward Sabine, K.C.B., President, in the Chair. 

 The following communication was read : — 

 " Measurements of Specific Inductive Capacity of Dielectrics, in 



