388 REPORT—1905. 
For example, as I have already mentioned, doubt has been expressed in the 
case of silicate magmas, whether the substances in solution are the minerals about 
to crystallise or only their constituents; whether viscosity and supersaturation 
may not invert the theoretical order of their appearance ; whether we are to take 
into account possible dissociation of the molecules or not; whether the presence of 
a common ion in these minerals is « factor which determines their mutual solu- 
bility. In fact, very little is known about the actual condition of the materials 
in a strong solution, although I do not know that there is any evidence available 
which forbids us to regard a solution about to crystallise as a mixture of liquids 
one of which is about to pass into the solid state. 
But if little is known about the nature of strong and supersaturated solutions, 
a good deal may be learnt about their behaviour. Having complained that we 
need experiments in this field, 1 may perhaps be pardoned if I allude to some un- 
published experiments of my own which relate to the general behaviour of crystal- 
lising liquids, and appear to me to explain two difficult problems in petrography. 
To such experiments the objection of Doelter does not apply. 
The Metastable and Labile Conditions. 
When a solntion of any salt such as alum or sodium nitrate is allowed to 
crystallise at a uniform temperature the crystals will only grow so long as the 
solution is supersaturated; a crystal growing in the supersaturated solution will 
continue to do so until a condition of equilibrium is attained. If the solution be 
kept at rest and maintained at a constant temperature, the crystal will continue to 
concentrate the liquid around itself and to withdraw solid material, until by diffu- 
sion of the impoverished liquid the whole mass is ultimately reduced to saturation, 
equilibrium is established, and the crystal ceases to grow; but most saturated 
solutions are so viscous that a very long time is required before this point is 
reached, Prolonged and vigorous stirring is required if the supersaturation is to be 
completely relieved within, say, a day ; without stirring weeks may be required. 
Further, it may be possible, as is well known, to keep a supersaturated solution 
in a sealed tube for years without change; and it is also possible to start crystal- 
lisation in such a liquid by dropping into it a crystal of the dissolved substance, or 
of one isomorphous with it, or sometimes by shaking it. 
But it is, perhaps, not generally known that supersaturated solutions are of two 
sorts. 
In 1897 Ostwald published some experiments upon supercooled liquids and 
supersaturated solutions, which were carried out with the object of showing how 
extraordinarily minute are the quantities of solid material capable of starting crystal- 
lisation in such liquids, but at the same time that they have a limit of size. He 
called attention to the radical difference which probably exists between the state 
of a saturated solution which cannot crystallise spontaneously and that of the 
more strongly supersaturated solution which can do so, 
The former is one in which crystallisation can either take place spontaneously 
or can be induced by stirring or shaking, or a variety of causes: this Ostwald calls 
the /abile state. The latter is one in which crystallisation can only take place if a 
solid crystal of the dissolved substance, or a fragment of one, is brought into contact 
with the liquid: this he calls the metastable state. It is highly probable that no 
amount of stirring or shaking, or introduction of foreign substances, can make the 
metastable liquid crystallise. 
Until recently no attempt to ascertain the exact limit between the metastable 
and labile states, or even to establish the existence of such a limit, had been suc- 
cessful, and practically no attention has been paid to the difference between 
them. Tamman, who measures the spontaneous power of crystallisation by count- 
ing the number of the centres of growth or nuclei which appear in a super- 
saturated solution, does not recognise any distinction between the two states. 
During the present year a number of experiments carried on hy Miss F. Isaae 
and myself upon the strength of solutions from which crystals are growing have 
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