1096 
large amount of supersaturated solution. Osrwarp (loc. cit) has 
shown that the smallest particle capable of release has an approximate 
weight of 10 H gram. 
In all these cases we have the formation of a system the stability 
of which is not the greatest possible under the existing conditions 
of temperature and pressure. There are on the contrary other more 
stable conditions, which are characterised by the fact that in them 
a new phase, that is, a physically different component with other 
properties makes its appearance. In the case of a supersaturated 
solution of copper sulphate, it is the solid pentahydrate. As a rule 
such a new phase does not appear of its own accord and the system 
behaves as if it were in a state of equilibrium and hence stable 
supersaturated solutions can be kept unchanged for years; but if a 
small quantity of the absent phase comes in contact with the meta- 
stable system, the action at once sets in and the new phase increases 
until equilibrium is established. Thus we see that the essential con- 
dition is the presence of a metastable system which only passes 
from the more stable state by its inherent, forces, after the way 
has been opened to it by the external addition of a germ crystal. 
The germ crystal is not the cause of the reaction because it does 
not provide for the free energy necessary for the process, but is 
only the means of starting the process which goes to completion 
by its own forces after it has been once begun. 
Now in a saturated solution of a substance there is equilibrium 
between the solution and the solid phase of that substance only ; 
in a supersaturated solution there is an excess of the dissolved sub- 
stance. Hence it seems reasonable that only the same solid phase 
should release its state of supersaturation; consequently it seems 
improbable from theoretical considerations that an isomorphous sub- 
stance with which the solution is not in equilibrium would be able 
to release its state of supersaturation and this is corroborated by 
experimental observations. 
From this investigation it is clear that isomorphous substances or 
substances capable of forming solid solutions are not able to release 
the state of supersaturation of substances with which this work was 
undertaken and hence considerable doubt is thrown on this test of 
isomorphism long advocated by numerous workers. 
In order to test the hypothesis more thoroughly, works in non- 
aqueous solvents with various solutes are in progress. 
Presidency College, Calcutta. 
Imperial College of Science, London. 
