of the sea surrounding it. Usually the bottom of such reservoirs proves to have melted through. 

 Blue reservoirs (blue walls, bluish water) , always prove to be completely fresh. * 



Ice structure also affects the color of ice. Green ice consists of weakly expressed and 

 irregularly arranged crystals (granular ice). In blue ice, the isoline structure is sharply ex- 

 pressed and the crystals are orientated alike. Such ice splits well along the axes of the crystals, 

 it is more durable in a perpendicular direction, and when broken it yields an angular surface. 

 Thus, basically, a plate of ice which had frozen under calm conditions, is blue. 



The color of the ice at the initial moment of ice formation is also characteristic. Slush, 

 brash and also thin, completely wet, ice is of dark-grey, steel color (dark-grey nilas). In ratio to 

 the increase in thickness, the color of the ice changes to light grey (light-grey nilas), and then in- 

 to white when a considerable part of the ice begins to rise above the water. Separate little ice 

 chunks wet with water, which form during melting as a result of the disintegration of large ice 

 floes, appear completely dark. 



The color of ice colored by bacteria and plankton, concerning which more will be said later, 

 should be investigated particularly. 



As Burke correctly points out, in practice, there always exists the necessity to distinguish 

 hard ice from the more friable ice. This can be done only with great experience, differentiating in 

 the color of the ice sometimes by very slightly differing hues. 



The change in the color of ice fields in connection with a change in weather is characteristic. 



In clear weather with strong solar radiation and strong emission, ice fields turn white. The 

 impression is created that they are covered with frost or freshly fallen snow. In overcast weather 

 conditions, during high air temperatures, the ice turns grey and takes on dirty hues. An explana- 

 tion of this phenomenon will be given in Section 115. 



As a rule, the whiter the ice, the more frangible it is, but this rule has many exceptions. 

 Sometimes very white ice is encountered and in spite of its considerable thickness a ship may pass 

 through it easily. Thus for instance, in August 1935, when we were on the Sadko in the Barents 

 Sea at 79°88' north, 33°27' east, we encountered very jumbled, frightening, but completely white 

 9 point ice, through which we sailed freely at a slow speed. On the other hand, dirty ice was found 

 which was almost impossible to pass through with a vessel. An additional sign of the latter is a 

 rounded, eroded form of ice. 



LITERATURE: 23, 62, 77. 



Section 72. Hardness 



If ice is considered as a mineral, its hardness can be determined by the so called "hardness 

 scale" of Mohs, i. e. , by the resistance shown to scratching by a determined testing mineral 

 (table 58). 



*It should be added that another indirect sign of the use of water for drinking is a higher water 

 level in the reservoir in comparison with the level of the sea. This sign, however, is not observed 

 in all cases. 



181 



