266 Ice and its Natural Hist or v 



radiation of the sun. This produces disarticulation of the 

 ice into groups of vertical prisms, which are then floating 

 independently : they are kept together only by crowding. 

 Ice in this state is said to be rotten ; and it will be recognised 

 that, however thick the ice-sheet may be, when it gets into 

 this condition it is dangerous. In the neighbourhood of the 

 outflow the crowding is relieved. The disarticulated groups 

 become disengaged, the smaller groups and individual prisms 

 are able to assume their attitude of stability and to float on 

 their sides. All then drift towards the outlet. The ice breaks 

 up, and the lake is cleared in an astonishingly short time. 



If it were not for the law that even impure water in 

 freezing always forms pure ice, the impurity remaining in the 

 liquid and generally entangled in the interstices of the grains, 

 and that the pure ice which is in contact with this impure 

 liquid melts at a lower temperature than that which is in 

 contact with nothing but the water formed by its own melting, 

 the ice covering a lake would be a continuous sheet offering 

 no points of weakness, and it would have to melt as a whole. 

 It is doubtful if lakes such as those met with in the upper 

 Engadine would get rid of their ice-covering at all. On the 

 Silser See the ice is usually over 60 centimetres thick when 

 the thaw sets in. but when once the ice begins to break up, the 

 lake is cleared in a day. Sixty centimetres of ice would take 

 a long time to disappear on the basis of surface melting 

 alone. 



While the winter lasts the ice on the lake shows no 

 crystalline structure. This develops only after removal from 

 the water and exposure to the sun. The ice then splits up 

 into prisms in a vertical plane. These are at first of irregular 

 section, and as sun-weathering proceeds the thicker prisms 

 split up into thinner. When a block has lain exposed to the 

 February sun and cold, it may fall to pieces, each piece being 

 a long, thin triangular prism, with some resemblance to a 

 razor blade. When the ice is cold and dry, the outlines of 

 the grains are lines ; when the ice has a temperature of o C., 

 it melts perfectly round the grain, forming troughs in which 

 the water collects, and the aspect is that of a dark polygon, 



