402 THE POPULAR SCIENCE MONTHLY. 



been solidified, the tendency to freeze being held in check by pressure, 

 but occurred the instant the pressure diminished. 



The expansion of water at freezing is vastly important in its rela- 

 tions to life. Without this property, water in high latitudes would 

 become permanently solid, and the aspect of Nature be one of lifeless 

 desolation. Water, in cooling from a high temperature, contracts in 

 volume, and the cooled particles sink until the mass is reduced through- 

 out to a temperature about seven degrees above the freezing-point, 

 when an important change takes place. Contraction of volume ceases, 

 and expansion begins. The chilled particles remain at the surface 

 from their lightness, and there solidify, while the water beneath, in 

 its deeper portions, may be 7° warmer than the point of freezing. By 

 this means a temperature of water in lakes is maintained adequate 

 to the wants of life. 



The structure of ice is crystalline, and the fundamental pattern of 

 the crystals is six-rayed stars. But it is only in entire freedom of 

 molecular motion that crystals attain perfection of symmetry. They 

 form upon the surface of water, when the cold is severe, with great 

 rapidity ; but are modified in their arrangement or aggregation the 

 instant the first crust is produced. The additions to the thickness of 

 the ice are always at its underside, and the result is a prismatic form. 



Fig. 4. 



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1'"lo\vek3 01' Ice rKo.iECTKD ox a Scheen. 



the prisms growing downward. These prisms are hexagonal in shape, 

 and are so joined .at their sides as to present an apparently homoge- 

 neous structure. That such is not the case, however, will be seen 

 when we speak of the decay of ice. That the internal structure of ice 

 is upon the stellate type is shown when a small cube of it is dissected 

 by a beam of light. By the heat rays of the beam, the ice is decrys- 

 tallized — its molecular architecture is taken down, and the result 

 appears in stellate figures of exquisite beauty upon the screen. These 

 figures are areas in the ice which have been liquefied by the beam, 

 which thus throws on the screen an " image of its own work." In 

 Fig. 5 we have magnified pictures of the crystalline structure of ice. 

 They seem shadows of living objects, rivaling fern-leaf and blossom in 



