540 Scientific Proceedings, Royal Dublin Society. 
so that it easily passes into the other, and in so doing crystallizes 
completely, thus causing the whole of the solution to become 
solid. 
Cases of supersaturation and crystallization of the former 
category (I.) appear to be capable of receiving the same explana- 
tion of their cause as that which I have given, and by that which 
is advanced by Wyrouboff, while those described under II. are 
such as were accurately described by Page and Keightley, but 
were found difficult of explanation because the transitional 
crystalline forms were not recognised. The reason of their 
crystallization, as explained by Nicol, applies equally well to the 
anhydrous inorganic salts as to the organic substances he dealt 
with, and on that account is full of interest. 
I do not gather from Nicol’s paper that he has explained the 
cause of the crystallization in these cases, but it may be inferred 
from analogy with the hydrated compounds that the reason why 
the transitional or unstable form crystallizes only in small quantity 
is because it is the more soluble modification of the compound. 
We thus see how it is possible that certain salts can form, and do 
form, supersaturated solutions, even when they are anhydrous, as, 
for instance, calcium carbonate, an instance cited by Le Chatelier, 
the different crystalline forms of the dimorphous salts having in 
all probability different degrees of solubility in cold water. 
NoTE ADDED IN THE PREss. 
I have recently verified by a few simple experiments some 
observations on copper sulphate made by me many years ago, 
and referred toin a course of lectures! delivered at the Royal Insti- 
tution, but of which I find no details published. Three portions 
of the perfectly anhydrous salt, each weighing approximately 
0-46 gr., were dropped into different quantities of cold water, and 
of a saturated solution of CuSO,:5H,.O. 
(1). 0°46 gr. were placed in 0°5 c.c. of distilled water at 15° C. 
This is about twice as much as is necessary to form CuSO,'5H,0. 
The liquid became warm, the white anhydrous salt caked, and only 
1 ¢ Air and its Relations to Life’’: London, 1876. 
