1908] on Ice and its Natural History. 24:9 



freezes exactly at 0" C. In fact the temperature at which ice 

 melts in nature depends on the medium in which it melts as well as 

 on the pressure to which it is subjected. If the pressure is 

 constant, it varies with the nature of the medium ; and, if 

 the nature of the medium is constant, it varies with the 

 pressure. 



The effect produced by both these agencies is the same in kind : 

 each, by its presence, induces the melting of ice and the freezing 

 of water at a lower temperature than would be the case in its absence. 

 In the case of dissolved salt this inducing power is active only at 

 temperatures lying l)etween 0° C. and the cryohydric temperature of 

 the salt. Between these temperatin-es the solid salt, when exposed 

 to an atmosphere which is not perfectly dry, is deliquescent 

 ('Antarctic Manual' p. 81). Below the cryohydric temperature 

 ice and salt are indifferent to each other. Of the two agencies the 

 one which has the more potent influence on the natural history of 

 ice is the nature of the medium in which it freezes or melts. 



Tlie Inflaetice of Salt in imlii,oin(j Vie Melting of Ice at Tem- 

 peratures between 0^ C. and its Cryohydric Point fur nis lies a quanti- 

 tative ex^planation of ol) served Anomalies in its Physiccd Constants. — 

 This influence furnishes a simple and natural explanation of the 

 anomalies so often noticed in the physical behaviour of ice. Thus 

 the belief that ice, at temperatures near 0" C, does not contract but 

 expands on Ijeing cooled, has been maintained by such experienced 

 observers as Hugi and Petzold as well as Pettersson. It is impossible 

 to arrive at this conclusion without close and accurate observation. 

 The observations in each case were exact, but the interpretation 

 of them was faulty. 



When due weight is given to the influence of the medium, the 

 anomaly disappears, and it is found that ice does not behave in 

 the capricious way supposed, but conforms to the usual custom 

 by expanding when warmed and contracting when cooled. The 

 disturbing agency is the impurity which is present in even the purest 

 water. This is excluded from the ice in the process of freezing, 

 and remains in solution in the residual water, which becomes more 

 and more concentrated as the freezing proceeds. The amount of 

 such solution, which remains hquid 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 dissolved 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, pp. 

 IJr^-liG, explains this in detail. '* All natural waters, including rain- 



