260 Mr. J. Y. Buchanan [May 8) 



This exam-pie illustrates also the effect of impurity on the 

 apparent specific heat of ice. 



It will thus he seen how powerful is the influence of medium on 

 the behaviour of ice in the laboratory : it will now be shown that, in 

 nature, this influence is equally powerful and much more far-reach ins:. 



The nature of the medium is responsible, in the case of sea ice. 

 for depressions of freezinsr and meltins: temperature of 80, 40, and 

 even more decrees of Celsius' thermometer, while the srreatest pressure 

 to which fresh-water ice is exposed in nature, cannot well produce 

 an alteration of freezin^^ and melting-point amounting to as many 

 hundredths of a degree. 



The ice which surrounded the " Yega " dnrinsr her winter's im- 

 prisonment in the Arctic Ocean, had a pastv semi-liquid consistence, 

 although thp temperature of the air was at or below - 80° C. It 

 remained stationary onlv because it was on a level surface. Had it 

 been shovelled up on an inclined plane it would have quickly flowed 

 down it until it reached the lowest level asrain. If we pick up a piece 

 of ice floatinsr in the Polar Sea, we know that it will prove to be very 

 far from homogeneous. It mav have a foundation of genuine primarv 

 sea ice : but the ice forming: the superstructure is sure to consist of 

 snow, frozen spray, and very likely fra2:ments of land ice. all cemented 

 together into a species of con2:lomerate. "We have seen that when this 

 is exposed to warmth it begins to melt, at a temperature which may be 

 one or two degrees below the melting-point of pure ice : and the 

 liauid furnished by the meltinsr is salt water. The further melting 

 takes place in the ascendina: order of temperature : the salt ice of 

 low melting-point disappearinir first, and the purer ice melting later. 

 We thus see how ice can be cemented by ice, iust as metallic obieets 

 may be united by solder. In both cases the binding material differs 

 from the objects united, chiefly in being more easilv fusible. . 



If we have a number of cubes of pure ice, which fit each other 

 exactly, and, i^ after being moistened with saltwater thev are exposed 

 to frost, they will solidify to a single block. If this be exposed to 

 the sun, the cementing salt ice will melt first, and, when it ceases to 

 bind, the constituent cubes of pure ice will fall asunder, having them- 

 selves suffered practically no diminution due to melting. 



The GlaripT Grains. — Now this is precisely what happens whcii a 

 block of sound glacier ice is exposed to the ravs of the sun for a short 

 time : and it is one of the most strikinir and instructive experiments 

 that can be made. Under the influence of the sun's rays, the binding 

 material melts first, the continuity of the block is destroyed, the 

 individual grains become loose and rattle if the block be shaken, and 

 fin ally'they fall into a hpap. A block of glacier ice is a q-eometrical 

 curiosity. It consists of a number of solid" bodies of different sizes 

 and' of quite irregular shapes, yet they fit into each other as exactly 

 and fill space as completely as could the cubes above referred to. 



Tlie granular constitution of glacier-ice can nowhere be better 



