460 On Supersaturation of Salt-Solutions. 
simplification of these aggregations of molecules. It would 
be quite possible that the volume-changes attending this 
disgregation would escape detection by the methods I have 
employed ; but the changes in volume produced by hydration 
or nonhydration are far too large not to be detected by a 
comparison of the molecular volumes of the solutions, especially 
when dealing with salts containing a large number of mole- 
cules of water of crystallization. I do not at present wish to 
lay much stress on this explanation; it is, it seems to me, a 
point which has by no means received the attention which is 
its due ; but I am convinced that until this is conclusively 
settled one way or another, it is premature to bring forward 
colour-changes as the sole evidence in favour of the hydration 
of salts in solution—a conclusion directly negatived by the 
molecular volumes of the very salts on the colours of whose 
solutions the whole hypothesis is based. 
Further, such colour-changes are well known to occur in 
cases where no dehydration or other decomposition can occur, 
and where the only explanation is to be found in an alteration 
of the molecular constitution. Numerous instances are given 
in a paper by Carnelley on the colour of chemical compounds 
as a function of their atomic weights* ; but one or two may 
be given here with advantage. Thus, mercuric oxide and red 
lead are both red when cold, but darken when heated, ulti- 
mately becoming almost black though no decomposition has 
taken place, as is proved by their regaining their original 
colour on cooling. Again, the temperature of boiling water 
is sufficient to turn the scarlet cuprous mercuric iodide black, 
the scarlet colour returning when the salt is cold. The dif- 
ferences in colour of mercuric sulphide are also due to a 
difference in molecular constitution. But perhaps the most 
instructive instance of all is to be found in the effect of heat 
on the hexhydrated cobalt chloride itself: when the solid 
crystals are gently warmed they change from red to blue 
. without any loss of water, they do not lose their transparency, 
and the colour-change is found to have extended to the very 
centre of the crystal. 
The results of my experiments may be summarized as 
follows :— 
(1) The individual in solution is not the same as that in 
the solid state when hydrated salts are considered. 
(2) It is to this that supersaturation is due. 
(3) A supersaturated solution is a solution of the anhy- 
drous salt, which may or may not be saturated. So long as 
no disturbing cause operates to bring about combination of | 
* Phil. Mag. 1884, xvii. p. 130. 
