178 
Proceedings of the Royal IriaJi Acadony. 
properties in common. The first and last alnmina and ferric oxide 
are, perliaps/the most important substances of the mineral world, if we 
except silica and lime. There is a wonderful analogy between all their 
compounds, and they all possess in common a less degree of molecular 
stability, from complexity of construction, and, ergo, a corresponding 
susceptibility to the action of heat, when in solution. The minerals 
formed from these bases frequently take a form common to the col- 
loidal base. 
M. Debray has lately published some ingenious observations upon 
the action of heat upon one of these solutions — namely, the ferric 
salts. As my own experiments, however, have led to rather different 
results, I have simply given my own experience, and feel less delicacy 
in doing so, from the fact, that my investigations in this direction 
were well known, and have a prior date to those of M. Debray. I 
exhibited some experiments upon the dissociating influence of heat on 
these solutions before the Dublin Chemical Club in 1867.f 
This dissociation is well-marked in the ferric salts, by the colour of 
their solutions. All the ferric salts are nearly colourless, or have, 
perhaps, but a very faint lemon tinge. It is probable that if we could 
get the tri-salt in solution without decomposition, a perfectly colourless 
liquid would be the result. 
Experiment YI. : — 
Perric chloride, obtained by passing chlorine over iron filings heated 
in a porcelain tube, was dissolved in a moderate quantity of pure 
water (free from ammonia), a dilute solution of hydrochloric acid was 
then added, drop by drop, until the minimum point of coloration had 
been obtained. The iron was then estimated by precipitation with 
ammonia, and the chlorine by nitrate of silver. The following figures 
represent the result obtained : — 
Practice. Theory, per cent. 
Pe, . . . . 33-91 34-46 
CI, .... 66-09 65-54 
This slight increase in the amount of chlorine is evidently due to 
the basylous action of the water : and chromatic neutrality of such 
salts is dependent upon the amount of dilution, and is in a certain 
sense independent of the chemical neutrality. A chemically pure salt, 
when dissolved, gives a slightly basic solution, depending upon the re- 
lative volume of the water it is dissolved in. 
Experiment VII. : — 
The further addition of acid produced a darkening of the solution, 
which had previously been rendered as neutral as practical. There- 
fore, both acidity and basicity produce colour in these solutions. 
Heat applied to ferric solutions gives an intense darkening in ratio 
to the temperature employed. It is almost certain, as will be seen 
from the experiments detailed, that the first action of the heat is the 
* " Comptes Kendus," April. 1869. 
t Minutes of the Dublin Chemical and Philosophical Club, 1867. 
