TiCHBORNE — Oyz Dissociation hy Heat of Compounds. Ill 
it will be seen further on that on the application of heat to such a 
compound as an alum it reacts upon all the more unstable parts exactly 
as if they were not grouped together in the first instance, and that 
the whole structure is if anything lowered as regards its thermanalytic 
point, and thus weakened by its complexity. As the components are 
removed the residue has more stability. 
The molecules known under the term ''water of crystallization" 
are but slightly removed from the water of ''hydration," and but 
slightly removed from the water of solution ; frequently the alteration 
of a few degrees of temperature is sufficient to convert one into the 
other. 
The water in the salt seems to be in a crystalloidal condition, but as 
a colloid in the other case. The crystalloidal water even in solution 
frequently is capable of dissociation, and of apparently changing its 
condition — such changes produce the phenomena of supersaturated 
solutions. The phenomenon of solution is a demonstration of force, 
and of actual combination between the molecules of water, and the 
substances dissolved. But if the more intimate unions be taken, it is 
evident that in such cases as hydration, or water of crystallization, 
the molecules that we recognize as II.2O are playing different func- 
tions, or are in a different condition to the water of solution. 
In an octahedral crystal of ammonia alum, water is almost essen- 
tially the geometrical solid, and is as important as the alumina, am- 
monia, or other elements. 
If this alum be dissolved by applying heat, it is split up into a 
substance — which is no longer alum geometrically or chemically, al- 
though we have got conventionally into the habit of saying it has 
"dissolved in water," or it has "dissolved in its water of crystalliza- 
tion," as the case may be. It is not only decomposed, but requires 
some little time for the molecules of water to regain their accustomed 
position or crystalloidal condition. So that they may be assimilated 
into the edifice — as illustrated by the following : — 
Experiment Y : — 
A saturated solution of the ammonio-ferric alum was poured upon 
a small quantity of the crystals of the same salt, and a few drops of 
acid added to prevent the precipitation of a basic salt. It was quickly 
brought to the boil, and then allowed to remain at rest until it had 
regained the original temperature, 15° C. Although the solution had 
a thermometer frequently inserted and withdrawn, there was no 
indication of crystallization, until some three to four hours had 
elapsed. I^ow under the ordinary rules of solubility, these crystals 
should begin to form immediately that the thermometer reached 15° C. 
This phenomenon is exhibited by all the hydrated soluble salts in 
some degree. 
A very interesting and remarkable group of molecules are the 
basylous trioxides, of which, however, we need only consider three. 
These three are Al ^"'0^, Cr /^^Og, and Pe ^'"0^ oxides, which per- 
form a very important part in our earth's crust, and possess well-knowu 
E. I. A. PROC. — TOL. I., SEU, II., SCIENCli. 2 A 
