xviii Contents 



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chloride of sodium to that at which liquefaction is complete ; 

 and the actual facts of the apparent expansion have been 

 accounted for step by step by the cryoscopic reaction of NaCl 



on ice between 2 172 C. and o C. 249 



For blocks of ice which contain per 100 parts by weight 

 of ice less than 29-97 and more than 17164 parts of NaCl, 

 the coefficient of apparent expansion is negative at all tempe- 

 ratures above -2i72C. For solutions of higher dilution 

 Table 1 1 1 gives the critical temperature at which the coefficient 

 of apparent dilatation changes sign and becomes negative. 



This Table shows that in the case of ordinary potable 

 waters this critical temperature is sufficiently removed from 

 the temperature of congelation to be easily determined, and 

 it furnishes the best method of arriving at the concentration 

 of fresh water, or solutions of very high dilution. Thus, when 

 the dissolved matter is equivalent to no more than one gram 

 of chlorine, present as chloride of sodium, in one hundred 

 thousand grams of water, the critical temperature is o725 C. 

 The critical temperature of the apparent expansion of ice 

 affords a means of detecting impurity equivalent to quantities 

 of chlorine as small as i gram in ten tons, and even i gram 



in one hundred tons of water 251 



Cryoscopic equivalence between pressure and salinity. 

 This subject has been worked out in much the same way as 

 the similar phenomenon of the elevation of the boiling-point 

 of water by pressure and by dissolved salt, for which we used 

 the device of the elastic tank of uniform depth. In both 

 cases the same Law is found to exist. For a given elevation 

 of the boiling-point in the one case and depression of freezing- 

 point in the other, there is the same approximate proportion- 

 ality between the area of the surface of the water exposed to 

 the pressure and the volume of the water which holds the 



equivalent amount of salt 252 



Influence of impurity on the apparent latent heat of ice . 255 



Size of glacier grains 258 



Sun-weathering of granular ice produces white surface of 



glacier 262 



Snow, nevd and glacier 263 



The grain of lake-ice 265 



Characteristics of an advancing glacier .... 267 



Grooving of ice by rock 268 



External work of a glacier 270 



The real region of mechanical erosion and attrition is the 



sea-shore . 



272 



