( 336 ) 
with one molecule of ZnSO, in the saturated solution of ZnSO,.6 HO 
at the temperature P'). The value of g in equation (2) is therefore 
to be taken as the heat of transformation of ZnSO,. 7 H,O. 
3. To determine a the solubility of Zn SO, 6 H.O (or Zn SO. 7 H,O) 
at the transition temperature must be known. We shall also find 
that the solubilities of the two salts at other temperatures (below 
the transition temperature) are required in order to calculate the 
E. M.F. of the transition-element at these temperatures. I therefore 
give at once the results of the determinations, which will be used 
in subsequent calculations. 
The zine sulphate employed was obtained from Merck; it was 
quite neutral to congo-red paper. Its purity was attested by the 
fact that CLARK-cells set up with it gave exactly the same E.M.F. 
as the standards of the Physikalisch-Technische Reichsanstalt at the 
same temperature. 
Determinations of the solubility of ZnSO,.7H,O (the stable 
system below 39°) and of ZnSO,.6 H:0 (the metastable system) 
were made in the usual way by means of the shaking apparatus 
of Noyes®). By taking special precautions which I have described 
elsewhere *) it was possible to deterinine the solubility of the meta- 
stable system down to — 5°. 
In the following table (I) the results obtained are given, along 
with the figures found by CALLENDAR and Barnes *) for tne solu- 
bility of the salt with seven molecules of water of crystallisation 
which agree very well with my own. 
That the determinations of POGGIALE, MUurLpeR, Roscoe and 
SCHORLEMMER, ErarpD and other authors are faulty is thus confirmed 5). 
The saturated solutions were evaporated in shallow platinum 
dishes on the water-bath. Zn SO,.1 HO is formed, its composition 
remaining unchanged even after prolonged heating. 
CALLENDAR and Barnes have also adopted this method. Since 
however they say in their paper: “They were then evaporated to 
dryness at 100°C., and the percentage of ZnSO, in each case was 
1) At the transition temperature the saturated solutions of the two salts Zn SO, . 6 H,O 
and Zn SO,.7 11,0 have the same concentration! 
2) Zeitschrift fiir phys. Chemie, 9 (1892) 606. 
3) Zeitschrift fiir phys. Chemie, 31 (1899). Jubelband S. 169. 
*) Proceedings Roya! Society, 62, 147. 
5) See Comey, A dictionnary of chemical solubilities (1896, London, Macmillan 
and Co.), p. 458. 
