184 
Proceedings of the Royal Irhh Academy. 
heat upon its compounds in solution, which, so far, have escaped ob- 
servation, or have been misconstrued. 
It is well known that the ordinary chromic salts present a peach 
blossom colour when in solution, convertible by heat into green solu- 
tions. The most natural of the many explanations given of this phe- 
nomenon is one of dehydration. But there is something more than 
this. Theoretically it would seem that it was this — that the chromatic 
change was due to basicity of the solution as in the ferric salts, and 
that it was only necessary to increase the boiling point of the solution 
by pressure, or to increase the basicity by the volume of water, to 
obtain similar results to those obtained with iron. This was satis- 
factorily verified by many experiments, a few of which will suffice to 
illustrate this part of my subject. 
Experimeyits XYII. mid XYIII. : — 
Solutions of chromic sulphate and potassio-chromic alum, when sub- 
mitted in tubes to a temperature of two to three hours in the pressure 
apparatus, gave in each case basic precipitates — the temperature being 
177° C. The precipitate in the last-named substance being very volu- 
minous, I did not succeed in procuring anhydrous chromic oxide cor- 
responding to the ferric oxide, but look upon its consummation as merely 
a matter of pressure. 
Experiment XIX. : — 
A dilute solution of any chromic alum, if dropped into a flask of 
boiling water, is instantly dissociated. The contents of the flask be- 
come quite opaque, and the basic precipitate is palpable to the eye. 
This precipitate is redissolved on cooling, and even during boiling, if 
the solution of chromic alum is added in any quantity. 
If we accept the theory — of which there can be little doubt — that 
there can co-exist a basic and an acid salt in solution, this explains at 
once the change of violet salts of chromic oxide into green solutions on 
boiling. Any neutral salt of chromic oxide dissolved in the cold gives 
a violet colour contaminated with green (''peach blossom" ?). This, 
as in the iron salt, is due to the basylous action of the water. The cau- 
tious addition of dilute sulphuric acid converts it into nearly a pure 
violet. It is converted into the green solution either by the addition 
of an alkali, by the addition of water, or by heat. In other words, 
the green colour in chromic salts is due to the basylous condition. In 
the case of heat, the green solution regains, after some time, its 
original condition, but only slowly. Many explanations have been 
given of this phenomenon, and it seems almost strange that so simple 
a one should have escaped observation. 
The aluminic oxide also obeys the same law of dissociation, only 
in a still less marked manner. It was evident that from analogy this 
should be so, although at the first glance it would appear that such 
was not the case. But, on taking into consideration the relative 
atomic position of the elements concerned, it would be seen that disso- 
ciation could be only exerted at ordinary pressure to a very limited 
extent as regards this base ; thus there is no evidence of decomposi- 
