981 
SrCl 2 , 6H 2 0 or CaF 2 cannot release Ihe supersaturation of'a solntion 
of calcium chloride. 
In Groth’s ‘Tntroduction to Chemical Crystallography” translated 
by Marshall (1903) the following statement occurs, “copper sulphate 
CuS0 4 . 5H 2 0 is isomorphous with sulphates of Magnesium, Man- 
ganese, Iron, Cobalt, Zinc and Cadmium”. 
Pure samples of all these hydrated sulphates were tried for the 
relief of supersaturation of the solution of copper sulphate but 
crystallization began only in the highly supersaturated Systems, which 
were rather unstable; in the Solutions containing as much as 60% 
of the salt in excess, there were no cases of occurrence of crystall- 
isation by the isomorphous bodies. lt was also observed tliat in the 
highly supersaturated Systems, crystallisation was sometimes induced 
by such substances as (NH 4 ) 2 S0 4 , K 2 S0 4 etc, whicÜ were not iso- 
morphous with copper sulphate; evidently these form unstable systems. 
The solubility data for cobalt sulphate are taken from the work 
of Wetzel [Zeit. Phys. Chem. 52, 395 (1905)]. In the case of 
cobalt sulphate crystals appeared with isomorphous substances only 
when the solution contains more than 70 °/ 0 of the dissolved salt in 
excess. : Evidently at this stage the supersaturated solution is unstable 
holding such enormous amount of cobalt sulphate in excess. As 
long as the supersaturated solution contained about 60 % of the 
dissolved salt in excess, there was no induction of crystallisation 
with the isomorphous substances. 
No crystals were obtained with the isomorphous substance sodium 
selenate (Na 2 Se 0 4 , 10 H 2 0) though the solution was highly super- 
saturated containing as much as 100 % of the dissolved substance 
in excess. The experiments were carried out in the month of 
February when the temperature was about 25°. 
In our climate, supersaturated Solutions of sodium sulphate cannot 
be obtained in the usual way in the summer, since the laboratory 
temperature is about 33° or 34°, i.e. above the transition temper- 
ature of Na 2 S0 4 , IOHjO; (the transition temperature being 32.383 at 
the International Hydrogen thermometer). Consequently supersatu- 
rated Solutions should be prepared at a temperature lower than 32°. 
Retgers (loc. cit.). has proved that sodium nitrate and silver nitrate 
are isodimorphous. Onder ordinary conditions, sodium nitrate is 
rhombohedral. and has the specific gravity of 2.265 ; silver nitrate 
is rhombic and has a specific gravity 4.35. 
Mixed crystals were obtained by Retgers, which were rhombo- 
hedral; these contained from 0 to 52 % of AgNO s and gave for 
the curve representing the relation between specific volume and 
63* 
