60 ME. CHABLES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 
crystals within the tube will become deformed, and rising to the surface, quickly melt ; 
hut if before plunging the tube into snow and water it he held for a short time in air, 
say, at about 50°, the crystals break up, and throw down a quantity of anhydrous salt, 
which, occupying the bottom of the tube, begins to combine with water and to heat, so 
that, on putting the tube into snow-water, the middle portion becomes quickly encrusted 
with ice, as before, while the bottom of the tube for about \ to f of an inch is protected 
from ice. 
By detaining the tube long enough in the freezing-mixture the solution becomes 
entirely solid, and on transferring it to ice-cold water it rapidly melts into a very bright, 
highly refractive liquid, without any separation of salt ; and the same liquid may be 
solidified and melted any number of times with the same results, provided the solution 
be protected from the action of nuclei. If the cotton-wool be taken out of the tube, 
even when the contents are solid, and be restored again to its place, there will be a sepa- 
ration of salt during the melting, in consequence of a nuclear particle being admitted 
from the air. 
Such is the behaviour of the solution made with one ounce of water. Although not 
supersaturated at ordinary temperatures, and not sensitive to the action of nuclei, yet it 
soon reaches a temperature at which it becomes supersaturated, and then it is very 
sensitive to such action. For example, in the case in hand, when the solution is at 50° 
or thereabouts, it may be touched, or the tube below the solution rubbed with a wire 
or a glass rod, and show no disposition to crystallize ; but at the temperature of 20° the 
same nuclei are powerfully active in inducing crystallization, crystals instantly attach 
themselves to the nucleus, or the part of the tube rubbed becomes at once chalky white, 
from which mark crystallization spreads 8 . 
Next, as to the behaviour of the solution made with half an ounce instead of an ounce 
of water. Such a solution, after being boiled and filtered, remains clear and bright, but 
retains its viscosity after twenty or thirty hours. Unlike the former solution, made 
with one ounce of water, this begins to crystallize at the temperature of the room, as 
soon as the cotton-wool is removed. The crystallization sets in from the surface in the 
form of minute needles spreading out like tufts of thistle-down and soon closing in the 
surface, so that the tube can be inverted, and the heat-currents generated in the viscous 
fluid consequent on change of state can be seen. 
But if, instead of subjecting such a solution to the action of the aerial nuclei, it be 
8 Supersaturated solutions of Glauber’s salt &c. are much less sensitive to the action of nuclei in warm than 
in cold weather. This has led to a good deal of misapprehension as to the real functions of nuclei, and has 
caused some observers to produce lists of bodies which will or will not induce crystallization ; whereas the 
same body, under the same conditions, only varying with the temperature of the solution, will induce crystal- 
lization at one time and not at another. This is strikingly shown in the case of calcic chloride (Ca Cl, 6 aq) ; the 
same nuclei which are inactive when the solution is at the temperature of the room act at once when the solu- 
tion is reduced to from 24° to 34°. A strong solution of this salt in an open evaporating-dish, which under 
ordinary circumstances becomes weaker by absorbing moisture, will, if the dish be put into a freezing-mixture, 
give a splendid crop of well-formed crystals. 
