Ice and its \alnra! Ilistorv 265 



while the white spongy rind on the outside is more of the 

 nature of snow, like the highest neVe, and passes by layers 

 into the compact central grain. Also in the case of the in- 

 dividual grain, the nucleus or kernel is the first and oldest, 

 and only by continued development does the rind shape itself 

 and gradually pass into the mass of the nucleus and so be- 

 come a glacier-grain, which then continues its development 

 as the glacier itself continues its own development. In these 

 relations lies the foundation of the whole natural history of 

 the glacier. It will be observed that eighty years ago Hugi 

 held very modern ideas on the subject of development. 



The Grain of Lake-Ice. It is not glacier-ice alone which 

 suffers disintegration when exposed to a powerful sun. Lake- 

 ice behaves in a similar way. Beautiful examples of this can 

 be seen in Alpine seas every winter. During the harvesting 

 of the ice from the lake, the blocks often lie for a day or 

 more before they are carted away to the ice houses. Occa- 

 sionally some of them get overlooked and remain for many 

 days exposed to the powerful sun of February, while main- 

 taining the low temperature of the air usual in that month. 

 No melting takes place ; but, after even a few hours' exposure 

 to the sun, the block shows the figure of its grain in develop- 

 ment. It is being etched by the sun's radiation. 



The grain of lake-ice has a very different appearance from 

 that of glacier- ice, but both are individual crystals. The 

 difference in their appearance is to be traced to the difference 

 of treatment which they have received during their existence. 

 The glacier grains have been practically rolling over each 

 other during their descent, while those of the lake have 

 established themselves at right angles to the surface of the 

 water and have remained there. So long as the ice is in- 

 creasing in thickness, the temperature of its upper surface 

 is very low. It is perfectly transparent, and its surface is 

 smooth, dry and polished like glass, and it shows no trace 

 of crystalline figure. When the ice is undisturbed, this de- 

 velops itself only at the end of the season when the thaw 

 sets in. Then the whole ice-sheet rises to its melting tem- 

 perature and is at the same time exposed to the direct 



