xvi Contents 



PAGE 



Glacier-ice is protected from sun-weathering by ice and 



by water 231 



It is the dependence of the melting-point of ice on the 

 nature of the medium in which it melts that enables glaciers 



to move under the action of gravity 2 3 2 



No. 6. ICE AND ITS NATURAL HISTORY. (From the Proceedings of 



the Royal Institution of Great Britain, 1909, Vol. xix, p. 243) 233 



Recapitulation of principles and laws established in earlier 

 work and applicable to the matter handled in this lecture . 233 



The freezing-point of a substance is denned to be : the 

 temperature at which it, as a liquid, passes into itself as a 

 solid ; and its melting-point to be the temperature at which 

 it, as a solid, passes into itself as a liquid . . . .238 



When ice is melting in a mixture of ice and water, the 

 temperature of the water must be a little higher than that of 

 the ice, else there would be no inducement for heat to pass 

 from the water to the ice ; similarly, when ice is being formed 

 in a mixture of ice and water, the temperature of the water 

 must be a little lower than that of the ice produced . . 239 



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 240 



It is probable that in nature ice never melts and water 

 never freezes exactly at o C. The law which regulates the 

 melting of ice in nature may be stated as follows : 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 influence of salt in inducing the melting of ice at 

 temperatures between o C. and its Cryohydric point furnishes 

 a quantitative explanation of observed anomalies in its physical 

 constants. 



The belief that ice, at temperatures near o C., does not 

 contract but expands on being cooled, has been maintained 

 by experienced observers, such as Hugi, Petzold and Petters- 

 son. Their observations were exact but their 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 con- 

 forms to the usual custom, by expanding when warmed and 

 contracting when cooled 241 



