250 31r. J. Y. Buchanan [May 8, 



water, contain some foreign and usnally saKne 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 hy 

 weight of chlorine in a million parts of water, then we shall 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'OOOl" C. ; 

 and it would continue to do so until, say, 1)00,000 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, there- 

 fore, • 1 per cent, of chlorine, and would not freeze until the tem- 

 perature had fallen to - 0* 1' C. This water would then deposit ice 

 at temperatures becoming progressively lower, until when 000 more 

 parts of ice had been deposited, we should have loo parts residual 

 water, or brine, as it may now l)e called, containing 1 per cent, of 

 chlorine and remaining liquid at temperatures above - 1 • 0'^ C. When 

 00 more parts of ice had been deposited we should have 10 parts of 

 concentrated brine containing 10 per cent, of chlorine and remaining 

 liquid as low as - i:^" C In the case imagined we assume the 

 saline contents to consist of NaCl only, and with further concentra- 

 tion the cryohydrate would no dou])t 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 - l:-r C, have 000,000 parts of ice and 10 of brine coiitaininiu' 

 10 per cent, of chlorine. Now, owing to the remarkable fact that 

 pure ice in contact with a saline solution melts at a temperature 

 which depends on the nature and the anion nt 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'l'^C. it 

 consists of 000,000 parts of ice and 1000 parts of liquid water con- 

 taining 1 part of clilorine. The remainder of the ice will melt at a 

 temperature gradually rising from - • 1 to * 0^ C. 



" The consideration of this example furnishes an easy explanation 

 of the anomalous behaviour of ice formed from anything but the very 

 purest distilled water, in the neighbourhood of its melting-point. 

 This subject has been studied with great care and thoroughness by 

 Pettersson. The apparent expansion of all but the very purest ice. 

 when cooled below O' C, is ascribed by him in part to solid saHne 

 contents of the ice, which exercise a disturbing and unexplained influ- 

 ence on its physical properties. Viewed in the light of the fact that 

 the presence of even the smallest quantity of sahne matter in solution 

 prevents the formation of ice at 0^ C, and promotes its liquefaction 

 at temperatures below 0° C, we see that this apparent expansion of 

 the ice on cooling is probably due to the fact that we are dealing, not 

 with homogeneous solid ice, but with a mixture of ice and saline 

 solutions. As the temperature falls this solution deposits more and 



