Prof. H. L. Callendar and Mr. H. T. Barnes. 



temperatures between 0° and 30° C, and may therefore be considered 

 the normal hydrate at these temperatures. 



The peculiarities of the curve representing the temperature- varia- 

 tion of the E.M.F. of Clark cells are undoubtedly due in the main to 

 the very considerable changes of solubility of this hydrate with rise 

 of temperature, and are closely associated with the formation of 

 different hydrates at higher temperatures. We therefore thought 

 that it would be of interest to investigate this point more closely, at 

 least within the range of temperature over which accurate measure- 

 ments of the E.M.F. may be readily obtained. 



The solubility of zinc sulpbate has been studied by Etard,* who 

 discovered that tlie solubility of the sulphates decreased with rise of 

 temperature at higher points of the scale. His results are expressed 

 in terms of the percentage weight, p, dissolved in 100 parts of the 

 solution at a temperature t° C. Expressed in this manner, he finds 

 that the curve representing the solubility is a straight line, the 

 equation of which for zinc sulphate is given as being 



p - 27-6 + 0-2604* (Etard.) 



This equation is given as representing the solubility up to a tempera- 

 ture of about 80° C, above which the solubility decreases. 



Roscoe and Schorleminer,f on tbe other hand, give the following 

 table of solubility expressed in terms of the weight of ZnSO^ dis- 

 solved in 100 parts by weight of water. For the sake of comparison, 

 we have reduced their results to the corresponding percentage, £>, of 

 solution, and have added two lines of results calculated from the 

 formula of Etard, and from the observations of Poggiale. 



Table IV. — Solubility of Zinc Sulphate (Roscoe and Schorlemmer,. 

 • . Etard and Poggiale). 



ZnS0 4 in 100 of water (R. & 8.). 



0° 

 41-3 



20° 

 53'0 



50° 

 66-9 



75° 

 80-4 



Solution (?) [poggiale 



29- 2 

 27-6 



30- 



34-6 

 32-8 

 34 -7 



40-1 

 40-6 

 40-7 



44-5 

 47-1 

 45 -0 



It is evident from the discrepancies shown in the last three lines 

 that the matter requires further investigation. From our own 

 experiments we find that the rate of diifusion in these extremely 

 dense and viscous solutions is so slow that it is much more difficult 



* ' Compt. Bend.,' vol. 106 (1888), p. 206. 

 f Vol. 2, Part I, p. 262. 



