METALLIC COMPOUNDS. 
121 
produce a phenomenon which indicates, that a fresh combina- Theoretical 
tion has taken place. Caloric is disengaged, and as the tempe- 
rature is already very high, this disengagement of the caloric 
is sufficient to produce intense ignition. On this occasion the 
mass neither gains nor loses in weight, but its colour and cha- 
racters are altered, and the constituents are found to have en- 
tered into another state of combination which may be called 
more intimate , because they cannot be destroyed by the affi- 
nities of bodies which had the property of decomposing these 
substances before this change had taken place. We know 
that a very elevated temperature produces, in several com- 
pound bodies, a degree of chemical indifference ; for example, 
the sulphate of alumine, the ferreous sulphate, the muriate of 
nickel, &c. on being heated to a certain degree, appear to be- 
come insoluble in water, and it requires the action of water to 
be continued a length of time, in order to restore their solubi- 
lity. Alumine, zircon, the oxides of titanium and of tautalite, 
&c. lose their solubility in acids, when heated to a red heat ; 
several combinations of the earths and metallic oxides, which 
are found in the interior of the earth, are not to be decomposed 
by the strongest acids, notwithstanding the affinity of these 
acids to the earths, under common circumstances, is infinitely 
stronger than that of the earths to each other. In all these 
cases it is necessary, in order to bring them to the ordinary 
state of combination, to expose these substances to the action 
of a strong chemical agent, and frequently at an extremely 
high temperature ; for example, to burn them with i.k.ibes, 
or alkaline earths, or with phosphoric acid, &c. I presuppose, 
in this instance, the electro-chemic indifference is destroyed 
in the same manner as the oxide of mercury is decomposed by 
heat, and that the constituents re-appear separated, and endowed 
with their original electro-chemic quantities. It may, then, 
be very probable, that all these phenomena arise from one 
same cause, and that they are not effective but at a degree of 
combination more intimate than that which belongs usually to 
bodies produced by chemical operations in aqueous solutions, 
or at least where water is present. 
