242 Ice and its Natural flistory 



the residual water, which becomes more and more concen- 

 trated as the freezing proceeds. The amount of such solution, 

 which remains liquid at any time, depends on the temperature 

 of the ice and liquid, and the concentration of the liquid or 

 solution. When these are given, and the nature of the dis- 

 solved impurity is known, the amount of liquid present and 

 the consequent contraction follow necessarily. 



In order to illustrate this, it is necessary to select some 

 substance as representative impurity. Chloride of sodium 

 has been chosen, because it is the most widely disseminated 

 and the best studied ingredient of natural waters. 



The following extract from my paper on 'Ice and Brines ' 

 explains this in detail. 



"All natural waters, including rain-water, contain some foreign and 

 usually saline ingredients. If we take chloride of sodium as the type of 

 such ingredients, and suppose a water to contain a quantity of this salt 

 equivalent to one part by weight of chlorine in a million parts of water, 

 then we should have a solution containing 0*0001 per cent, of chlorine, and 

 it would begin to freeze and to deposit pure ice at a temperature of 

 -o-oooi C. ; and it would continue to do so until, say, 999,000 parts 

 of water had been deposited as ice. There would then remain 1000 

 parts of residual water, which would retain the salt, and would contain, 

 therefore, o'i per cent, of chlorine, and would not freeze until the tem- 

 perature had fallen to o'i C. This water would then deposit ice at 

 temperatures becoming progressively lower, until, when 900 more parts 

 of ice had been deposited, we should have 100 parts residual water, or 

 brine as it might now be called, containing I per cent, of chlorine, and 

 remaining liquid at temperatures above -i'oC. When 90 more parts 

 of ice had been deposited, we should have 10 parts of concentrated brine 

 containing 10 per cent, chlorine and remaining liquid as low as 13 C. 

 In the case imagined, we assume the saline contents to consist of NaCl 

 only, and with further concentration the cryohydrate would no doubt 

 separate out and the mass become really solid. On reversing the 

 operation, that is, warming the ice just formed, we should, when the 

 temperature had risen to about - 13 C., have 999,990 parts ice and 

 10 brine containing 10 per cent, chlorine. Now, owing to the remark- 

 able fact that pure ice, in contact with a saline solution, melts at 

 a temperature which depends on the nature and the amount of the 

 salt in the solution, and is identical with the temperature at which ice 

 separates from a solution of the same composition on cooling, the brine 

 liquefies more and more ice at progressively rising temperatures, until, as 

 before, when the temperature of the mass has risen to -o'i C. it consists 



