136 On the Ground Ice observed in 



We have no observations which prove that this kind of ice 

 is seen, until the temperature of the whole of the water is 

 at zero. It is not certain that the little icy particles floating 

 On the water, mentioned by Mr Knight, and which may have 

 acquired, by coming into contact with the air, at least on their 

 upper surface, a temperature considerably below zero, do not 

 play an important part in this phenomenon, which I have en- 

 tirely overlooked ; that, viz. of cooling the stones covering the 

 bed of the river, when dragged thither by currents. Is it not 

 possible that these floating filaments were the principal elements 

 of the spongy ice which was afterwards to be formed ? 



Our theory does not explain in what manner this ice, once 

 formed, only increases in a downward direction. If the remark 

 of Desmarest be correct, there is something wanting to com- 

 plete it. 



During the congelation of the bottom of the Aar, at the place 

 where the ice was formed, M. Hugi immersed pitchers filled 

 with hot and cold water. The^r^^, he says, on being brought 

 up, was covered with a layer of ice of one inch thick, the other 

 had no marks of congelation. Bullets covered with cloth, warm 

 as well as cold, afforded similar results. 



These remarkable experiments cannot be kept out of view. 

 They ought to be repeated in a variety of ways : we should be 

 sure whether these two bodies, on being immersed, do not differ 

 but in temperature; that their surfaces are equally polished ; 

 and if, after all the minute precautions with which an able phi- 

 losopher is sure to avail himself, it be found that the body, ori- 

 ginally hot at the moment of immersion^ is covered, as we 

 are assured by M. Hugi, with more ice than the cold one, it 

 will perhaps be necessary to attribute this singular phenomenon 

 to the internal movement of the liquid ; to currents which, 

 being caused at first by the presence of a hot body, still conti- 

 nued after it became cold ; to currents which incessantly conti- 

 nued to throw over this cold body filaments frozen on the sur- 

 face. 



Before coming to the conclusion, that the question which we 

 have been discussing is completely solved, it would be necessary 

 to subject the texture of the ices at the bottom to additional ex- 

 periments ; we must ascertain accurately whether the vesicular 

 cavities, which traverse it in every direction, contain any air, — 



