248 
Proceedings of the Royal Irkli Academy. 
state. The salts experimented with were those of cobalt, copper, 
and nickel. 
As regards such bodies, it is a general characteristic that neutral 
solutions of any of their chlorides do not change colour on boiling 
at ordinary atmospheric pressure ; nor does the amount of dilution 
affect the tint any further than what would be due to attenuation. 
But in no case, where a colour change is evinced between the dry 
and hydrated salt, have I failed in obtaining a dehydration of 
it in solution by using extraordinary pressure. The laws of such a 
chromatic change differ from those produced by a basic condition of 
the salt, as will be seen further on. 
Let us take, for instance, the dehydration of the salts of cobalt, 
which is evinced by a well-known change of colour — the conversion of 
the light rose colour into a dark and pure blue. The following 
observations conclusively prove that water of crystallization, or hydra- 
tion, are easily held asunder in aqueous solutions by the dissociative 
influence of heat. That is to say, in a similar manner to the pheno- 
mena noticed as regards the basic and ultimate molecules in the trioxide 
group. 
No amount of boiling will, however, convert a pink solution of 
cobalt into a blue one, unless extremely concentrated- But it was 
observed by Prout, many years ago, that on evaporating a strongly acid 
solution of a cobalt salt to a concentrated point, he got a permanently 
blue solution, which, he considered, was due to the abstraction of the 
water of hydration by the acid. Prout also stated that he had obtained 
from such a solution blue crystals, supposed to be the anhydrous salt : 
this requires verification. 
Conceiving that the cobalt salts would, from their chromatic 
display, be well suited to illustrate the dissociation of water "of 
hydration by heat, I was rewarded by finding that it answered 
admirably for this purpose, and gave results which may be used as 
beautiful lecture experiments. 
The thermanalytic point of the cobalt salts being above 100° C, no 
results, as before stated, will be obtained on boiling a neutral solution ; 
but if any substance capable of exerting an affinity for the water of 
hydration be introduced, the thermanalytic point of the salt will be 
lowered, as shown by Prout's experiment. Sulphuric and other acids, 
chloride of calcium and other hygrosopic salts, and even sugar, act 
in a similar manner. 
However, as the quantity of these dehydrants required to affect 
the cobalt solution was considerable, I used alcohol to lower the 
thermanalytic point. Chloride of cobalt, dried at 100° C, and dissolved 
in pure and absolute alcohol, gives a magnificent pure blue, free from 
the slightest tinge of purple. Por this purpose it is necessary that 
* Tichborne on Molecular Dissociation. Read at the E.oyal Irish Academy, 24th 
April, 1871. 
