30 GLACIERS 



for the complete change in the appearance, character, and 

 behaviour of snow when it is piled in vast thickness on 

 the slopes of mountains so high that it is ever renewed, 

 and never melts away on their peaks and shoulders. 



We are accustomed to see snow slowly melt and run 

 away as water, and the more observant will have noted 

 that in prolonged frost, snow, even when piled in heaps 

 by the roadside, disappears without thawing. It evapor- 

 ates, slowly but surely, straight away into the form of gas 

 invisible aqueous vapour. That is a rather unusual 

 property for a solid body to possess. In that way a 

 certain return of evaporated snow to the atmosphere from 

 which it was precipitated in crystalline flakes takes place. 

 But the amount is small. We are not accustomed to find 

 a solid body evaporating. Volatile liquids are common, 

 but volatile solids are unusual. The metals and rocks do 

 not behave in this way. The only familiar parallels to 

 ice and snow in this respect are the vegetable product 

 camphor and some allied bodies. They pass directly 

 from the solid to the gaseous state, and the invisible 

 gaseous camphor can be precipitated as " a snow " of 

 crystalline camphor on a glass shade placed over a lump 

 of that substance. 



There are some bodies the metal bismuth is one of 

 them, sulphur and hard paraffin also are of the number, 

 and water is another which in passing from the liquid to 

 the solid state expand actually increase in volume. It 

 is far more usual, and seems to us a more " natural " thing, 

 for a liquid to contract when, owing to cooling, it becomes 

 solid. The exceptional property possessed by water of 

 expanding when frozen is of enormous effect in the wear 

 and tear of the earth's surface. It is thus that the strongest 

 water-pipes, which the combined wickedness and ignorance 

 of plumbers and architects lead them to place on the out- 

 side of our houses instead of inside near the chimneys, 



