476 Proceedings of Royal Society of Edinburgh. [sess. 
Cliemie , published a few days ago, has compelled me to choose 
between the disadvantages attending too early publication, and the 
risk of finding my results published by another and independent 
observer. I shall therefore content myself with placing on record 
as succinctly as possible the leading experimental results on which 
the foregoing conclusions are based, reserving for a future com- 
munication the details and the discussion of them. 
1. Supersaturation caused by allotropy due to the presence of 
foreign molecules, generally water of crystallisation. 
{a.) A supersaturated solution of sodium thiosulphate evaporated 
at the ordinary temperature deposited crystals containing 15 to 20 
per cent, of water, indicating the presence of a hydrate containing 
not more than 2H. 2 0. All results of this nature obtained with 
supersaturated solutions are necessarily approximate and are always 
too high, as it is impossible to drain off the mother liquor or to dry 
the crystals. Under the microscope it is easy to distinguish the 
crystals of the one hydrate from those of the other, and on warm- 
ing the pentahydrate dissolves before the other. 
(A) A supersaturated solution of sodium sulphate deposits 
crystals of the heptahydrate, or of the anhydrous salt, according to 
the temperature, the former below, the latter above 20° C. The 
former was observed by Loewel, but, so far as I am aware, the 
latter had been obtained only by heating the decahydrate. 
(c.) A supersaturated solution of borax evaporated at the ordi- 
nary temperature deposited crystals containing five molecules of 
water, as already observed by Gernez. Under the microscope the 
two are easily distinguished, and the decahydrate dissolves much 
more easily on heating than the pentahydrate. 
(ft.) A warm saturated solution of barium chloride deposits under 
the microscope quadratic crystals, which become surrounded by 
eight-sided plates, the rectangular plates becoming rounded and 
finally disappearing, leaving, however, a mark on the new crystal 
roughly corresponding to the outline of the original crystal. This 
has been also observed by Lehmann, who states that the first 
crystals probably consist of a lower hydrate. A supersaturated 
solution of barium chloride can be prepared only with the greatest 
difficulty, and is extremely unstable. 
(e.) Lead acetate : the limit of temperature for concentrated 
