Ice and its Natural History 241 



ice never melts and water never freezes exactly at o 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 between 

 o C. and the cryohydric temperature of the salt. Between 

 these temperatures the solid salt, when exposed to an atmo- 

 sphere 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. 



The Influence of Salt in inducing the Melting of Ice at 

 Temperatures betiveen o C. and its Cryohydric Point furnishes 

 a quantitative explanation of observed Anomalies in its Physical 

 Constants. This influence furnishes a simple and natural ex- 

 planation of the anomalies so often noticed in the physical 

 behaviour of ice. Thus the belief that ice, at temperatures 

 near oC., does not contract but expands on being cooled, has 

 been maintained by such experienced observers as Hugi and 

 Petzold besides Pettersson. It is impossible to arrive at this 

 conclusion without close and accurate observation. The ob- 

 servations in each case were perfectly exact, but the interpre- 

 tation 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 



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