SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 199 



little from what takes place when its non-saturated solution is brought to shed part of 

 its water as crystals of ice. In each case considerable depression of the temperature 

 of the solution below that of crystallisation is required before crystallisation sets in. 

 When it does set in, an immediate rise of temperature takes place in the solution, and 

 it stops only when the ordinary temperature of crystallisation has been reached. The 

 same sequence of events is observed when water, containing no salt, is brought to 

 crystallisation ; it also rises to its ordinary temperature of crystallisation. Unless these 

 experiments are made in conditions which are unusual in chemical laboratories, none of 

 these liquids begins to crystallise immediately when its temperature has been lowered 

 exactly to the crystallising point. 



The extent to which any particular solution can be cooled below its crystallising 

 temperature, and the amount of mechanical disturbance which it can withstand when in 

 this condition, vary with the nature of the solution. We have seen that the resistance 

 so offered by a supersaturated solution of calcium chloride is considerable. But even 

 that offered by pure water to the starting of crystallisation is greater than is generally 

 believed to be the case. 



When the mass of water used is small, and the capacity for heat of the vessel which 

 contains it is large, it is possible, with care, to reduce the temperature of the system so 

 far that, when crystallisation takes place, the whole of the water is transformed into 

 ice and its temperature does not rise so high as 0° C. An instance of this is given in 

 the following passage quoted from a lecture on " Ice and its Natural History " which I 

 delivered before the Royal Institution on 8th May 1908 : — * 



" Evidence of the uncertainty which exists regarding the temperature at which ice 

 begins to form in water, when it is cooled in contact only with a solid other than ice, 

 is furnished by the wet-bulb thermometer when it is being prepared for use at tem- 

 peratures below 0° C, by freezing on it the quantity of water which is supported, against 

 gravity, by the perfectly clean bulb. When this is rotated in air of — 10° to — 20° C, 

 ice never begins to form until the temperature of the bulb of the thermometer has fallen 

 to —2° or —3° C, and rarely before it has fallen to -4° C. In many cases I have 

 observed it fall to temperatures as low as —8° or —9° C. ; and in such cases, when 

 freezing begins, the whole of the water is frozen without its being able, by the liberation 

 of latent heat alone, to raise the temperatui-e of the bulb of the thermometer to 0°C." 



Whether, when in this unstable state, it would stand the mechanical disturbance 

 which is resisted by a supersaturated solution of calcium chloride, can only be determined 

 by experiment. This I have not as yet attempted. All that is required is to set about 

 determining the specific gravity of pure water hydrometrically in a laboratory having 

 the constant temperature — 4° or — 5° C, and using at least equal precautions with those 

 observed in the case of supersaturated solutions at ordinary room temperatures. 



If water, in these conditions, is sufficiently unsensitive to mechanical disturbance, it 

 will undoubtedly do its part in manifesting its unrest ; it will then be the part of the 



* Proceedings of the Royal Institution of Great Britain, 1909, xix., Part I. p. 248, 



