214 ICE ITS FOKMS AND FUNCTIONS. 



atures it becomes solid and crystalline. At 39 Fahr., or 

 thereabouts, it appears to be at its maximum density ; at 

 that temperature and above it, under favourable conditions 

 of the atmosphere, it is incessantly passing off as invisible 

 vapour, and diffusing itself through the air; and at 212, 

 under the usual sea-level pressure of the atmosphere, it 

 boils, and is rapidly converted into steam. From 39 down 

 to 32, it appears to suffer little change in density; but at 

 and under 32 for fresh water, and 28J for salt, it is sud- 

 denly converted into the crystalline or solid state, and is 

 then known as ice. In this state it has expanded, become 

 lighter, and necessarily floats on its own liquid surface. 

 Ice, then, in ordinary language, is solid or frozen water. 

 From its crystalline structure it occupies more space than 

 when in a liquid state ; hence its lightness and flotation, 

 which are further increased by the number of air-cells which 

 are always less or more entangled in its mass.* Compared 

 with water at 60, whose specific gravity is 1, ice is found 

 to be only .912 ; hence it floats with about one-ninth of its 

 mass above water, and the remaining portion below. Being 

 formed at 32, ice may be said to be then in its normal 

 condition ; but at lower temperatures it slightly contracts, 

 as was long ago proved by experiments on the ice of the 

 Neva at St Petersburg, t It is this ice, its various aspects 

 and functions, that forms the subject of the present Sketch; 



* It is only from water that has been subjected to boiling that ice free 

 from these air-cells can be obtained. 



j- In further proof of this contraction, we may cite Sir James Eoss, 

 who says " We have often in the arctic regions witnessed the astonish- 

 ing effects of a sudden change of temperature during the winter season 

 upon the ice of the fresh- water lakes. A fall of thirty or forty degrees of 

 the thermometer immediately occasions large cracks, traversing the whole 

 extent of the lake in every direction ; some of the cracks opening in 

 places several inches by the contraction of the upper surface in contact 

 with the extreme cold air of the atmosphere." It is also partly by this 

 contractile force that the ice-barriers and ice-walls of the polar seas are 

 broken into floes and fragments. 



