378 REPORT—1905. 
now being carried on, which promise to be most fruitful because their methods 
and aims are inspired by the discoveries and views of modern chemistry and 
modern physics. 
Van ’t Hoff’s Work on the Salt Deposits. 
Among such researches the most remarkable are those conducted by Professor 
van ’t Hoff and his pupils during the last eight years upon the Stassfurt salt 
deposits. These deposits are of enormous extent, more than 1,000 feet thick, and 
consist of fairly well-defined layers of various sulphates and chlorides of sodium, 
, Magnesium, and potassium, and their double salts and hydrates. It has long 
been supposed that the minerals have been derived from the evaporation of sea- 
water which contains in solution the chlorides of sodium, magnesium, and potas- 
sium, with sulphate of magnesium and small quantities of calcium salts; and the 
general sequence of the minerals is that of their solubility; the less soluble 
sodium chloride crystallised out first and is at the bottom, while the very soluble 
magnesium chlorides, having been the last to crystallise, occupy the top of the 
series. But the problem is by no means so simple as to be one of mere solubility 
in water; the rock salt itself persists through the whole series, and some of the 
associations are difficult to explain. 
As is well known, the modern theories of solution mainly rest upon the 
behaviour of dilute solutions from which the principles of electrolytic dissociation 
have been deduced; but in the case of the concentrated solutions from which 
dissolved substances actually crystallise, very little is really known about the 
liquid itself. A great deal is known, however, about its equilibrium with the 
solids that separate from it, and the general laws of this equilibrium are expressed 
by the phase-rule deduced from mathematical considerations by Willard Gibbs, 
which states how many mechanically separable constituents can coexist under 
varying conditions of equilibrium in a system containing a definite number of 
chemical components. 
A solution saturated with a given substance is one which is in equi- 
librium with that substance when the latter is in contact with it in the solid 
form ; the phase-rule indicates the number of solids which must be in contact 
with a given solution; the only difficulty in practice is to determine the nature 
of the double salts or distinct hydrates that may be formed. 
By means of a series of experiments upon the solubilities of these salts, either 
singly or in the presence of one another, in order to determine the composition of 
solutions saturated simultaneously with two or more substances, it is possible to 
obtain a graphic representation of all possible solutions containing the salts 
present in sea-water. From this the course of crystallisation of any particular 
solution, for example sea-water, can be predicted. 
The general sequence thus theoretically predicted is as follows: (1) Rock 
salt; (2) Rock salt with the magnesium sulphate, epsomite; (8) Rock salt 
with the double sulphate of potassium and magnesium, leonite; (4) Rock salt 
with leonite and the potassium cbloride, kainite; (5) Rock salt, the magnesium 
sulphate kieserite, and the double chloride of potassium and magnesium, carnal- 
lite ; (6) Rock salt, kieserite, carnallite, and the magnesium chloride, bischoffite. 
This last combination will persist until all the water is evaporated. This is found 
to be the general sequence, not only of the salts obtained on evaporating sea-water 
at 25°, but also of the Stassfurt deposits. 
Up to this point the results have been summarised by Dr. E. F. Armstrong in 
a report presented to the British Association in 1901. Since that date the re- 
search has been prosecuted actively by van ’t Hoff and his pupils, and now the 
conditions of equilibrium at 25° have been mapped out, not only for the above 
compounds, but also for the minerals thenardite, glaserite, astrakanite, and 
reichardtite, which occur in these deposits. The whole process of crystallisation 
of the solution, from which no fewer than twelve different salts have separated, 
can therefore be predicted, and their sequence and associations can be traced 
through numerous stages, beginning with the separation of rock salt and ending 
with a mixture of rock salt, kieserite, carnallite, and bischoffite, 
