240 Ice and its Natural History 



This temperature was for long held to be invariable ; 

 indeed, it is little more than half a century since it was 

 established that it is lowered by increase of pressure and 

 raised by relief of the same, the quantitative proportion being 

 that of iC. to 135 atmospheres. 



In the above definition of the freezing and melting tempe- 

 rature of H 2 O, that substance is specified as being present 

 partly in the solid and partly in the liquid state. The 

 temperature at which ice begins to take form in water 

 which is cooled when in contact only with itself, or with 

 a solid other than ice, has not been determined, and is 

 in fact uncertain. The moment the smallest particle of ice 

 is present, the water has the opportunity of passing, as a 

 liquid, into itself as a solid : but not till then. 



Evidence of the uncertainty which exists regarding the 

 temperature at which ice begins to form in water, when it is 

 cooled in contact only with a solid other than ice, is furnished 

 by the wet-bulb thermometer when it is being prepared for 

 use at temperatures below oC, by freezing on it the quantity 

 of water which is supported, against gravity, by the perfectly 

 clean bulb. When this is rotated in air of 10 to 20 C. 

 ice never begins to form until the temperature of the bulb 

 of the thermometer has fallen to 2 or 3 C., and rarely 

 before it has fallen to 4C. In many cases I have observed 

 it fall to temperatures as low as 8 or 9 C. ; and in such 

 cases, when freezing begins, the whole drop of water is frozen 

 without its being able, by the liberation of latent heat alone, 

 to raise the temperature of the bulb of the thermometer 

 to oC. 



In our definition of the freezing and melting temperature 

 of H 2 O, no substance is specified except H 2 O. But H 2 O, 

 that is, absolutely pure water, rarely, if ever, occurs in nature. 

 Therefore our definition is not directly applicable to water as 

 it occurs in nature. It has been found by experiment that 

 when, in a mixture of ice and water, the water contains ever 

 so little foreign, especially saline, matter in solution, the 

 temperature at which it freezes, and that at which pure ice 

 melts in it, is lowered. It is therefore probable that in Nature, 



