Electromotive Force of different Forms of the Clark Cell. 14$ 



own observations at the higher points, as they were all carefnlly 

 verified. The dotted curve, which is practically a straight line,, 

 represents the formula of Bender and Yalson, which smooths out the 

 break. 



It is not theoretically improbable that a simple relation of this 

 type should be found to hold approximately in the case of salt solu- 

 tions. At the same time it is hardly to be expected that any such 

 simple expression should represent accurately the changes of density 

 at all temperatures. The expansion of water is anomalous in the 

 neighbourhood of the freezing point, and the coefficients of expan- 

 sion of solutions differ considerably from that of water at low tem- 

 peratures, and generally increase with increase of strength of solution. 

 These variations in the coefficients of expansion may well introduce 

 secondary effects of a corresponding order in the changes of density. 



In comparing the observations of Lannoy with our own, which 

 were taken at a slightly different temperature, it would appear not 

 improbable that systematic differences of this kind may exist, but 

 the point obviously requires much more careful investigation, as the 

 differences shown are so small, and might readily be explained by 

 errors of observation. For instance, at the two lowest points the 

 density according to Lannoy is greater than 0*9992 + w. Since he 

 apparently determined the composition of the solution by weighing 

 out quantities of the heptahydrate, the discrepancy might be explained 

 by a slight degree of efflorescence of the sample used for these deter- 

 minations. In the table of densities given by Carhart, the com- 

 position of the solution in each case is stated in terms of the per- 

 centage of ZnSO* by weight in 100 parts of solution. If we assume, 

 in the absence of any definite statement, that the symbol Zn$0 4 

 stands in this case also for the heptahydrate, we find that the den- 

 sities which he gives are much greater than those found by Lannoy,, 

 or by ourselves, the value of the density at 45 per cent, of the hepta- 

 hydrate, according to Carhart, being 1'343, instead of 1'318, as given 

 by our observations. It is possible that the sample used in this 

 case may have consisted largely of the hexahydrate, or the dis- 

 crepancy may be due to other causes. 



If we take older determinations of the density of zinc sulphate 

 solutions, such as those of Grerlach or of Schiff (1859), we find that 

 they show a general agreement with our observations rather than 

 with the formula of Bender, but that the characteristic point to 

 which we have drawn attention is neatly smoothed out in the tables 

 which they give as deduced from the results of their observations. 

 The point in question would not be noticed at all unless the obser- 

 vations were plotted by the method of differences, as shown in Fig. 3,. 

 and even in that case it might readily be mistaken for an error of 

 observation, unless the points were numerous, and had been inde- 



