IX THE ATMOSPHERE. 215 



its geological operations "being those, of course, to which 

 the attention of the reader will be more especially directed. 



In treating of Ice in a popular way, it may be conveni- 

 ently arranged under three great categories ice in the 

 atmosphere; ice on land; and ice on water. The ice or 

 frozen water in the atmosphere is the great nursing parent 

 of the ice on land ; and the ice accumulated on the land 

 becomes in the long-run one of the most remarkable fea- 

 tures of the ice that floats on the water. As the vapour 

 that ascends from the ocean is condensed and falls as 

 rain on the land, and this rain finds its way again by runnel 

 and river to the sea ; so the vapour frozen in the air descends 

 on the land, where, accumulating for ages, it slowly grinds 

 and pushes its way once more to the ocean. In this way 

 all the forms of ice are inseparably connected in one great 

 and incessant round of circulation, and we only separate 

 them provisionally for the purpose of intelligible descrip- 

 tion. First, then, as regards the ice in the atmosphere, the 

 most casual observer must have noted the frequent forma- 

 tion of hoar-frost, snow, and haiL The rapid radiation of 

 heat from the earth's surface, by which the invisible vapour 

 of the atmosphere is converted into dew, has only to be 

 carried beyond the limit of 32, when hoar-frost is produced, 

 crisping the herbage, or floating, in still conditions of the 

 air, in clouds of crystalline spicules. A more rapid con- 

 densation of rain or vapour produces hail, which may occur 

 at all seasons and under every latitude, and may fall in soft 

 snowy drops, or pellets of ice, from the size of a coriander- 

 seed to that of a pigeon's egg, and often with destructive 

 effect on the crops of the farmer. Neither hail nor hoar- 

 frost, however, exercises any perceptible influence on the 

 rocky surface ; and it is chiefly in the condition of snow that 

 ice in the atmosphere becomes of interest to the geologist. 



