M'Keevor's Voyage to Hudson's Bay, 13 



in a glass tube into a freezing moisture; if kept at perfect 

 rest, no crystallization will take place, at least not until the 

 entire mass is reduced to a very low degree of temperature ; 

 but if slightly agitated, as by striking the side of the tube with 

 a piece of money, ^he whole instantly starts into a solid mass. 

 Secondly, by serving as so many nuclei, from which crystalliz- 

 ation will spread on all sides ; thus operating, I conceive, on 

 the same principle as the crystal of a salt does when dropped 

 into a saline solution. Mr. Kerwan was the first, I believe, 

 who remarked, that when a crystal of the same kind of salt 

 with that held in solution was dropped in, the process of 

 crystallization went on still more rapidly. Now, crystallized 

 snow and hail-stones, being merely small portions of congealed 

 water, may, perhaps, operate in a similar way.* 



To the sources already enumerated, I have lastly to add, 

 that of the formation of ice at the bottom of the ocean, and 

 which becoming detached by the force of the currents, will, 

 by its diminished and specific gravity, rise to the surface and 

 become, as it were, a centre for further accumulations. From 

 the difficulties attendant on an explanation of this curious 

 phenomenon, some have considered it as altogether improba- 

 ble, while others have gone the length of denying it altoge- 

 ther. The circumstance however, at least with regard to fresh 

 water, is now put beyond the possibility of all doubt, and we 

 can very readily conceive, that what a lesser degree of cold 

 will effect in the beds of lakes and rivers, a still greater wiH 

 be able to accomplish at the bottom of the ocean. Mr. Lesslie, 

 in a note prefixed to his very interesting work on the subject 

 of heat, tells us, that many of the rivers in Siberia and Swit- 

 zerland are found to have their beds lined, during the greater 

 part of the year, with a thick crust of ice. Saussure describes 

 a similar appearance in the lakes of Geneva. Mr. Garnet, in 

 a very interesting paper contained in the last number of the 

 Journal of Science and of Arts, gives a very minute account 

 of this singular appearance. He mentions one place, in parti- 

 cular, where this phenomenon is to be observed in a very 

 striking manner. As the very valuable publication, in which 

 this interesting paper is contained, is in the hands of few, ex- 

 cept scientific readers, I trust an account of it will not be 

 deemed superfluous. 



* When, observes Mr. Lesslie, we examine the stricture of a hail-slonc, 

 we shall perceive a snowy kernel incased by a harder crust. It has Tery 

 nearly the appearance of a drop of water suddenly frozen, the particles of 

 air being driven from the surface towards the centre, where they form a 

 spongy texture. See LESSLIE on Heat and Moislurc, 



