990 REPORT—1885. 
10. On the Molecular Weights of Solids and Salts in Solution. 
By Professor W. A. TrtpEn, D.Sce., F.R.S. 
It seems to be generally agreed that bodies in the solid state consist of units or 
molecules which are very complicated and made up a of considerable number of such 
smalier aggregates as compose the molecules of gases. Accepting the conclusion, 
the author is disposed to go further, and to say that in any given case there appears 
to be no reason for limiting the number of small molecules which may thus be 
bound together to form the physical unit of a solid. The law of Dulong and Petit, 
and Neumann’s law, according to which every elemental atom, whether free or in 
combination, has the same, or nearly the same, specific heat, point to a conclusion of 
this kind. The law of Dulong and Petit states that, approximately at least, the specific 
heat in the solid element is inversely as the atomic weight, or as n times the atomic 
weight. This seems to indicate that, since n may be made as large as you please, 
there is in solids of this kind, and in salts, &c., no difference between molecule and 
mass, and that the physical unit is the atom, The same kind of argument may be 
deduced from the facts known concerning the specific volumes and refraction equiva- 
lents. 
Solid bodies according to this view are composed of atoms which are only dis- 
tributed into molecules capable of taking up an independent state of existence when 
the body becomes fluid. 
Such a hypothesis involves or implies another, viz., that chemical combination 
between atoms and the combination of molecules which ensues when a gas or a 
liquid returns to the state of solid are phenomena of the same nature. This agrees 
with the commonly recognised resemblance between the process of dissociation and 
the processes of fusion and of evaporation. In both the change is gradual and is 
dependent upon temperature, and both are directly measurable in terms of heat or 
some other form of energy. Another consequence of this view is that we must 
confine the idea of limited valency to gaseous substances. From the well known 
numerous double salts, and especially such compounds as the periodides of the 
organic bases, it appears, in the solid state, the elements and especially the non- 
metallic elemental radicles, are capable of developing an indefinitely large capacity 
for combination. None of these so-called molecular compounds exist in the gaseous 
state. 
With regard to solutions there are many facts which indicate that the molecules 
of dissolved substances are smaller than those of solids, Thus it appears that the 
alums, the numerous double iodides and chlorides, and such compounds as racemic 
acid, exist only in the solid state. When they pass into solution they are resolved 
into their proximate constituents. To determine whether common salt in solution 
is NaCl or n times NaCl seems almost an insoluble problem. The question of water 
of crystallisation is related to this subject, and here we have some evidence, though 
conflicting. On the one hand we have the evidence of such experiments as those of 
Dr. Nicol on the molecular volumes of salts in solution, from which it appears that 
the water of crystallisation of a salt dissolved in water is not distinguishable from 
the rest of the water with which the salt is mixed. But if this means that when 
a salt such as copper sulphate, for example, is dissolyed in water, the water com- 
bined in the crystal separates from it, leaving the salt molecule to wander free, the 
author can not assent to this view. By this hypothesis we can explain neither 
the colours of such solutions, nor the large evolution of heat which ensues on the 
introduction of such a salt in the anhydrous state into water, 
The composition of the salt molecule in solution appears to be dependent chiefly 
upon temperature. If we dissolve in water a salt like common salt, which habitu- 
ally crystallises without water of crystallisation, the salt molecule in solution at 
ordinary temperatures does not include the elements of water, in other words the 
salt dissolves in the anhydrous state. But if we take a salt such as sodium sul- 
phate, the dissolved molecule retains the same amount of water as the crystals 
formed at the same temperature, whilst if the temperature is raised these molecules 
are gradually broken down until at a certain temperature all the salt molecules 
