GROUND ICE. 403 



of Fahrenheit's thermometer, it becomes gra- 

 dually heavier and more heavy ; but when it 

 has reached this point (40 F.), further cooling 

 reverses the state of things, and the particles 

 actually become specifically lighter ! The 

 descent of water cooled below 40 is thus effec- 

 tually arrested in the simplest manner. It is 

 difficult to explain this curious phenomenon. 

 It is supposed that, since the particles of ice, in 

 consequence of their angular form, are lighter, 

 in consequence of their occupying a larger 

 space than the particles which form fluid water, 

 the expansion of water below 40 may be due 

 to its particles being in a process of arrange- 

 ment preparatory to their becoming visible 

 crystalline, as they do below 32. "Ground 

 ice, n as it is called, seldom is formed in still 

 water, but not unfrequently in shallow running 

 water. When ice is formed it is so light as to 

 float on the surface of the water, and, in so 

 doing, covers and protects water actually warmer 

 than itself below, the propagation of cold from 

 above downwards being extremely slow in fluids, 

 and being, in the case of water, rendered addi- 

 tionally difficult by the refusal of the cooled 

 particles to sink when their temperature is 

 reduced to the point in question. 



A curious fact is related by Krusensteirn, 

 with reference to the temperature of a part of 



