as a Standard of Electromotive Force. Hi 



bladder. This form of cell is undoubtedly convenient for 

 some purposes, but not for others, as when the cell has to be 

 heated or cooled for comparison at different temperatures. 

 Moreover, the siphon becomes filled in a very short time with 

 a mixture of solutions by diffusion ; and if the levels of the 

 liquids are not exactly equal, a siphoning action is added to 

 that of diffusion in carrying liquid over from one side to the 

 other. After some numerous trials of all existing forms which 

 have been proposed for Daniell's cell, the following was de- 

 vised, which, though more bulky than others, has yet given 

 great satisfaction, and has the great recommendation of always 

 standing ready for use. 



A large U-tube, about § inch diameter and 8 inches long in 

 the limb, has four side tubes (see PL V. fig. 2). The two top 

 ones, A and B, lead to two reservoirs Z and 0, and the bottom 

 ones C and D are drainage-tubes. These side tubes are closed 

 by glass taps. The whole is mounted on a vertical board, with a 

 pair of test-tubes between the limbs. Suppose, now, a Daniell's 

 cell is to be formed with solutions of zinc sulphate and copper 

 sulphate, and that the zinc-sulphate solution is the denser. 

 The left-hand reservoir S Z is filled with the zinc solution, 

 and the right-hand reservoir SC with the copper solution. 

 The electrodes are zinc and copper rods, Zn and Cu, passed 

 through vulcanized-rubber corks, P and Q, fitting air-tight 

 into the ends of the U-tube. 



The operation of filling is as follows: — Open the tap A and 

 fill the whole U-tube with the denser zinc-sulphate solution ; 

 then insert the zinc rod and fit it tightly by the rubber cork P. 

 Now, on opening the tap C the level of the liquid will begin to 

 fall in the right-hand limb but be retained in the closed one. 

 As the level commences to sink in the right-hand limb, by 

 opening the tap B copper-sulphate solution can be allowed 

 to flow in gently to replace it ; and this operation can be so 

 conducted that the level of demarcation of the two liquids 

 remains quite sharp, and gradually sinks to the level of the 

 tap 0. When this is the case, all taps are closed and the 

 copper rod inserted in the right-hand limb. 



Now it is impossible to stop diffusion from gradually 

 mixing the liquids at the surface of contact ; but whenever 

 the surface of contact ceases to be sharply defined, the mixed 

 liquid at the level of the tap C can be drawn off, and fresh 

 solutions supplied from the reservoirs above. 



In this way it is possible to maintain the solution pure and 

 unmixed round the two electrodes with very little trouble; and 

 the electrodes, when not in use, can be kept in the idle cells or 

 test-tubes L and M, each in its own solution. In making 



Phil May. S. 5. Vol. 20. No. 123. August 1885. K 



