406 MR. F. E. SMITH ON THE NORMAL WESTON CADMIUM CELL. 



large drops; similarly the solubility of small particles is greater than that of large 

 ones, and therefore the saturated solution will show a greater E.M.F. than the saturated 

 solution of the large crystals. With fluctuating temperatures the tendency is for the 

 small crystals to disappear and the mean size of the crystals to increase ; this lowers 

 the concentration and with it the E.M.F. It is only for very small crystals that the 

 change in solubility is appreciable, and at present it is not possible to calculate the 

 change in E.M.F. produced by the variation in size. 



STEINWEHR* examined several samples of purchased mercurous sulphate ; two such 

 suits from KAHLBAUM and MERCK respectively gave a difference in E.M.F. of 5 parts 

 in 10,000, and it was found that the dimensions of the crystals of the salt K giving 

 the higher E.M.F. were of the order of 1 micron, while the crystalline structure could 

 be seen in a number of particles of the other salt (M). The crystals of M were about 

 10 to 20 microns long. By grinding the M salt the difference was reduced to 1 or 

 2 parts in 10,000. Very large crystals were prepared by a method very similar to 

 that afterwards used by ourselves (p. 404), and the E.M.F. of a cell containing these 

 very large crystals was lower than that of a cell containing the K salt by 0'7 to 

 0'8 millivolt. By grinding some of the large crystals and setting up a cell with the 

 small particles as a depolariser the E.M.F. was increased by 0'6 millivolt, and was 

 therefore comparable with the K cell. 



We also have set up cells with KAHLBAUM'S mercurous sulphate which was washed 

 with water and is therefore hydrolysed. The E.M.F. of these cells is at present 

 1'0186 volts and is constant ; indeed, from the point of view of constancy of E.M.F., 

 these cells are as good as any of those dealt with in Table I. ; fig. 1 1 (Plate 5) is a 

 microphotograph of the salt used. The average size of the crystals is from 2 to 

 10 microns. The salt from KAHLBAUM, used by STEINWEHR, was smaller than this, 

 the particles being from 1 to 2 microns in length at the most. The difference of 

 0'3 millivolt found by us between cells set up with KAHLBAUM'S salt and those set 

 up with mercurous sulphate prepared by ourselves appears to be due not to a difference 

 in size of grain, but to the hydrolysis of the former salt. 



HuLETrt has also measured the size of mercurous sulphate crystals prepared 

 electrolytically and found the particles to vary in length from 2 microns to 130 microns, 

 but has found no difference in the E.M.F. of cells set up with these crystals. 



It is, of course, always possible that in an occasional preparation a very large 

 number of exceedingly fine crystals may be produced, and in such a case the change 

 of E.M.F. described by STEINWEHR will result, but unless there are numerous small 

 crystals in all of our preparations and this is highly improbable the large crystals 

 of mercurous sulphate which are sufficiently soluble to act as an efficient depolariser 

 cannot give an E.M.F. appreciably lower than those which are from 5 to 30 microns 



* H. v. STEINWEHR, 'Zeitschr. f. Instrumentenk.,' 25, pp. 205-208, July, 1905; also 'Zeitschr. f. 

 Elektrochem.,' pp. 578-581, 1906. 



t G. A. HUI.ETT, 'Phys. Rev.,' 22, pp. 321-338, June, 1906. 



