TiCHBORNE — On Dissociation hy Heat of Compounds. 171 
vapour density of chloride of ammonium), whilst those dissociated be- 
tween a and ^ are less common, and are much less easy of observa-' 
tion. 
Of course the following broad rules may be accepted: — 1. The 
greater the temperature the greater the number of molecules capable 
of dissociation, or the greater the tendency to dissociation ; and on the 
contrary, the lower the temperature the greater the molecular stability. 
2. The more complicated the construction of the molecule, the lower 
the temperature at which dissociation commences ; and after disrup- 
tion, the higher the temperature necessary to carry on further dissO' 
elation of the residual molecule. 3. As a corollary, it follows that 
examples of dissociation by heat in liquids, or of solids in solution, are 
comparatively uncommon ; but that when such phenomena do occur, 
they materially affect ordinary reactions, and that, ceteris paribus, de- 
compositions at one temperature will not, of a necessity, correspond 
with those affected at another temperature. 
In aqueous solutions, and at ordinary pressure, all molecular disso- 
ciation must occur under 100"^ C, that is to say, within the boiling 
point of the fluid in which the substance is dissolved. Under pressure, 
however, from the increased range of temperature, extraordinary phe- 
nomena may be induced, and thus heating in sealed tubes offers facili- 
ties for producing decompositions that cannot be brought about at 
ordinary temperatures. 
Dissociation of compounds in solution, unless accompanied by some 
ocular demonstration, is apt to be overlooked, from the simple fact that 
the molecules rearrange themselves generally in the same form on 
cooling, and thus the change is transient. Frequently, however, in 
mixtures, a dissociation is accompanied, on cooling, by a re-arrangement 
of the two, or more groups of molecules, according to the altered affini- 
ties. The statical condition having been once broken, a different 
arrangement may take place. 
I may illustrate the phenomena to which I have referred in the in- 
troduction by the consideration of a few of the most simple cases. 
None are more common than dehydration. The molecules, H.O, in 
such cases, forming the crowning particles of complicated structures, 
are held thereto with but a feeble force, and are easily dissevered even 
in solution. Lime water is a familiar instance, and althoiigh it has 
not, that I am aware, been demonstrated, there can be little doubt that 
the precipitate got on boiling that fluid is due to dissociation of the 
molecule of water, or its dehydration, thus— the action of heat upon the 
hydrate of lime (Ca"0, H.O) is to produce Ca"0, and HoO. 
Ca"0, being insoluble in water, or very much less soluble than its 
hydrate. This is probably what takes place. The insolubility is, 
however, concurrent with the rise in temperature. 
As there do not seem to be any experiments extant, except the 
original ones of Dalton, the subject was considered sufficiently im- 
