SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 13 



PAR. PAGE 



in the solution in its efforts to become a mother-liquor. In this it is finally successful, 

 but not before it has succeeded in forcing the door which confined its store of heat. 

 The birtli of the crystal was synchronous with and dependent on the liberation 

 of heat. 



114. It is shown that the change of displacement which occurred in the transition of the solution 196 



7*225 CaCl2 from a condition of supersaturation to that of a mixture of saturated solu- 

 tion and crystals at the common temperature 19"5° C. is a shrinkage amounting to 2-2 

 per cent, of the original volume of the supersaturated solution. 



115. Comparison of the behaviour of supersaturated solutions of MgClg and CaClj with respect 196 



to readiness in starting, and heat exchange accompanying, crystallisation. The variations 

 of the density of the liquid before the first element of crystal appears revealed only by 

 the skilled use of the hydrometer. The diagram illustrates the clianges of displacement 

 corresponding to the changes of density in the 7"225 CaClg and the 7"196 CaClj super- 

 saturated solutions, compared with the accidental changes observed in tlie stable solu- 

 tion 6'3 CaCl2. In the case of tlie 7'225 CaCl2 solution the state of unrest persisted 

 during the 140 minutes that the experiments lasted, and it seems to be not improbable 

 that a supersaturated solution is never at rest even in a closed vessel. 



116. Analogy between the crystallisation of a supersaturated saline solution and the formation 197 



of ice when a non-saturated solution or when pure water is cooled below its freezing 

 point. When the mass of water is small and the capucity for heat of the vessel which 

 contains it is large, the temperature of the system may be reduced so far that when 

 freezing begins the whole of the water may be frozen without the temperature of the 

 system rising to 0° C. Experimental illustration of this. Possibility of detecting 

 oscillations of density in water before freezing begins, by determining its specific 

 gravity hydromelrically with the necessary precautions in a room having a constant 

 temperature between - 4° and - 5° C. 



117. Calculation of the increment of pressure required to countoract the stretching of the 200 



7'225 CaClg solution before the beginning of crystallisation. It is found to be 38 

 atmospheres. 



118. Resemblance between the state of unrest preceding the crystallisation of a supersaturated 200 



solution and that preceding the liquefaction of a gas under a pressure not inferior 

 to its critical pressure, when its temperature is reduced slightly below its critical 

 temperature. 



119. It is only in the conditions of Andrews' experiment on COg that we can witness a substance 201 



persisting in the gaseous state under a pressure greater than its critical pressure, and 

 having a temperature lower than its critical temperature, because it is only when the 

 gas and the envelope which contains it have been maintained at a temperature higher 

 than the critical temperature of the gas, that the inner walls of the envelope have a 

 chance of being perfectly dry, that is, free from every trace of the liquid substance. 

 We do not know the temperature at which a dry gas can liquefy on the dry walls of its 

 envelope, but so soon as the first, even the minutest, trace of the liquid substance 

 appears, the temperature of liquefaction is defined, because the gas is then condensing 

 on itself as a liquid. 



