TRANSACTIONS OF SECTION C. 389 
shown that it is easy to determine the changing concentration of a cooling solution 
by an optical method, to show that it passes into the labile state, and to ascertain 
the temperature at which the transition occurs. We have found, for example, 
that a solution containing 48 per cent. of NaNO, is saturated at 26°, is metastable 
between 26° and 16°, and crystallises spontaneously below that temperature; one 
containing 52 per cent. of NaNO, is saturated at 44°, and becomes labile at 35°. 
In the metastable state inoculation by a solid germ of the dissolved sub- 
stance, or of one isomorphous with it, is necessary in order to cause the liquid to 
erystallise ; in the labile condition solid germs may be spontaneously generated 
from the liquid. Take, for example, a test tube filled with a solution of sodium 
nitrate containing 48 parts of the salt in 100 parts of solution, which is metastable 
at ordinary temperatures; if crystals make their appearance in this solution it 
will only be because the dust of the room contains minute particles of sodium 
nitrate which fall into the tube, or because crystals are deposited where drops 
have evaporated near the surface, and accordingly the first crystals appear at the 
surface of the liquid, and grow there until they are large enough to fall to the 
bottom. I find that such a solution, if enclosed in a sealed tube so as to prevent 
access of germs and evaporation, cannot be made to crystallise above the tempera- 
ture of 16°, although it is supersaturated at all temperatures below 26°. 
Again, let a hot solution of the same strength containing 48 per cent. of the 
salt be allowed to cool down while being stirred. If dust containing NaNO, can 
be excluded, the liquid will not crystallise until the temperature falls to 16°, when 
the solution passes from the metastable to the labile condition. A cloud of nuclei 
will then form throughout the liquid, and each will proceed to grow as a separate 
erystal; the immediate effect is to reduce the liquid to the metastable state so 
that no more crystals are produced, but each of these continues to grow from the 
liquid with which it is in contact. 
If dust be not excluded, crystals may make their appearance upon the surface 
of the liquid and will soon sink; but even though they be stirred about actively in 
the solution the liquid as a whole remains in the metastable state till a temperature 
somewhat below 16° is reached, when the labile region is entered and a cloud of 
new crystals makes its appearance. 
It follows, therefore, that in a cooling supersaturated solution, from which 
germs have not been excluded, there are normally two periods of growth: one 
in which a comparatively small number of isolated crystals are growing regu- 
larly, and a subsequent period in which a shower of small crystals is produced. 
Only if the rate of cooling be sufficiently slow, or the stirring be sufliciently 
violent, to keep the liquid in the metastable condition will there be no second 
period, no sudden precipitation of nuclei. 
These events take place in all the aqueous solutions which I have examined, 
and I am surprised that they have not been discovered before. They afford a 
possible explanation of two common features of igneous rocks, and of slags— 
namely, the growth of comparatively large and isolated porphyritic crystals, or pheno~ 
crysts, and the appearance of the same mineral at two or more different periods. 
The origin and the arrested growth of phenocrysts have generally been attributed 
to sudden change of temperature, of pressure, or of hydration, and no other 
plausible explanation has been given, although, as has been sometimes pointed out, 
they may occur in batholites where there is no independent evidence of such changes. 
Pirsson has recognised the utter impossibility of the ordinary theory and has 
recently suggested that each mineral has its crystallisation interval during which 
it continues to grow, and that this is terminated by the increasing viscosity of the 
magma, which checks the supply of further material to the growing phenocrysts 
and establishes new centres of crystallisation. A similar explanation was adopted 
by Crosby for the quartz-porphyry of the Blue Hills. He expresses it by saying 
that owing to the increased viscosity the rate of cooling overtaxes the molecular 
flow, which cannot keep pace with the crystallisation. It is so difficult to find any 
satisfactory theory for the growth of phenocrysts that they have even been attri- 
buted to the effect of earthquake shocks, 
Now in a silicate magma, in all probability, the temperature is sufficiently high 
