THE PHYSICS OF ICE. 4.05 



aggregate in six-raved figures, but in endless diversity of patterns. 

 Captain Scoresby figured 96 of these, beautiful illustrations of wliich 

 are shown in the plate. And Nature is profuse of these "frozen flow- 

 ers." On mountains, and amid solitudes of the North unseen by man, 

 she scatters them as she does those which waste their perfume in the 

 desert. The delicate lace-like figures which follow the touch of frost 

 on the window-pane are of the stellate type, but, being modified by 

 disturbing influences, develop into gorgeous patterns. 



Ice, when decayed, is weakened and becomes soft throughout, 

 scarcely more compacted than snow. By rains and mild weather of 

 spring, ice on our Northern lakes is thus impaired. In this condition 

 it is said to be " honey-combed ; " and, while yet many inches in thick- 

 ness, and apparently solid, is unsafe to travel over. The foot of a 

 horse will pass through it, displacing merely the portions beneath, and 

 without fracture of the surrounding parts. This arises from the pris- 

 matic structure already noticed ; and it is along the lines of adhesion 

 of the prisms that the ice first yields to the invasion of heat. When 

 thus weakened, it will sometimes disappear from the surface of a lake 

 by a few hours of heavy storm, or, if any portion remain, it will be in 

 the form of crystals, thoroughly permeated by water. So rapidly has 

 it vanished in many instances from lakes, that its sinking was insisted 

 on, but it is now known that it disintegrates and disappears by in- 

 ternal liquefaction. 



A most interesting and important property of ice remains to be 

 noticed. We refer to that by which it may be moulded into almost 

 any form by pressure. Cubes of solid ice have been pressed into balls, 

 cups, rings, and other shapes, showing its extraordinary plasticity. 

 At a temperature of 32°, ice is by no means a rigid substance, but 

 readily yields to pressure. Placed in the cavdty of a mould (Fig. 6), 

 it is broken into innumerable fragments as pressure is applied. It has 



Fig 



m 



:\louLD AND Ball of Ice formed by Compression. 



been shown, however, that, unless the crushing be sudden, the ice 

 is not reduced to a granular or powdery mass, but maintains its co- 

 hesion while it undergoes change of form. During the process a por- 

 tion of the ice becomes liquefied, and the water escapes, carrying with 

 it the heat liberated in the liquefaction, but the portions remaining are 

 moist, and at each point of contact directly adhere together by freez- 

 ing of the moisture. This refreezing takes place throughout the mass, 



