ME. CHAKLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 
59 
bles and evaporating to dryness, the same quantity of salt was found to exist at four 
different parts of a column, 11 inches high, of a supersaturated solution of Glauber’s 
salt. 
The internal viscosity of such a solution will probably account for a case mentioned 
in the next section, where a highly supersaturated saline solution of Glauber’s salt was 
reduced to about 20° Fahr. without any deposit of the 7-atom salt. 
Section II. — On the Action of low Temperatures on Supersaturated Saline Solutions. 
I propose in this place to show that the solutions of certain salts which remain liquid 
and supersaturated at and about the freezing-point of water, by a further reduction in 
temperature to about the zero of Fahrenheit’s thermometer, become solid, but on being 
restored to the temperature of 32° recover their liquid state without any separation of 
salt. 
Ferrous sulphate (FeS0 4 , 7Aq) was dissolved in hot water, 1-25 of the salt to 1 of 
water, and was cooled rapidly, first, by placing the tube containing the filtered solution 
in water at 50°, then in snow-water, and lastly in a mixture of salt and snow at 0°. 
Beautiful tetrahedral crystals formed at the surface of the solution, and were propagated 
downwards, until the contents of the tube became solid. In snow-water at 32° the solid 
mass shrank from the sides of the tube, formed into a smooth rounded mass, and gradu- 
ally melted, leaving the solution clear and bright without any deposit. On removing 
the cotton-wool from the mouth of the tube, small but well-shaped rhomboidal crystals 
soon filled the solution. 
The double salt formed by mixing in atomic proportions solutions of the zincie and 
magnesic sulphates retains all the water of crystallization of the constituent salts, 
namely, 14 equivalents, and hence is well adapted to display the phenomena of super- 
saturation ; since, in general, the more highly hydrated a salt is, the more readily does 
it become supersaturated. One reason for this is that the water of crystallization adds 
to the volume of the menstruum and so assists solution. So much is this the case, that 
while the salt readily undergoes the watery fusion, the addition of a very small quantity 
of water is sufficient to cause the solution to remain entirely liquid in closed vessels, 
even under considerable reductions of temperature. On the other hand, if too much 
water be added to the salt in making the solution, saturation, and not supersaturation, 
is produced, so that on lowering the temperature the behaviour of the solution is quite 
different in the two cases. For example, 246 ’3 grains of the magnesic sulphate, 287 
grains of the zincic sulphate, and one ounce of water, boiled and filtered into clean tubes, 
forms only a saturated solution at ordinary atmospheric temperatures. If such a solu- 
tion be reduced to 0°, or from that to -—5° or —8°, a portion of the solution solidifies. 
It first forms an opaque growing mass of snowy whiteness, adapting itself to the smooth 
curved surface of the tube, but within, directed towards the axis, covered with well- 
shaped tetrahedral crystals. If at this low temperature the tube be transferred to a 
mixture of snow and water, it will of course be thickly encrusted with ice, the mass of 
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