I C E. 



637 



!- 



Optical pro- 



uc. 



F..V u: 

 fee. 



I ) Antic, and Mr .Scoresby, have sometimes found them 

 in the state of oclohedrong, composed of two four-sided 

 pyramids. 



' n c** 01 ' 11 ' 11 ^ the optical properties of ice, Dr Erew- 

 ster found, that even large masses two or three inch- 

 es thick, formed upon the surface of standing water, 

 was as perfectly crystallised as rock crystal or calcare- 

 ous spar, all the axes of the elementary crystals, corre- 

 sponding with the axes of the hexahedral prisms, being 

 exactly parallel to each other, and perpendicular to the 

 horizontal surface. 



This unexpected result was obtained by transmitting 

 polarised light through the plate in a direction perpen- 

 dicular to its surface. A series of beautiful concentric 

 coloured ring* with a dark rectangular cross passing 

 through their centre, were thus exhibited. These 

 ring* were of the same nature as those seen along 

 the axis of Zircon ; for when they were combined 

 with the rings produced by this substance, a system 

 of rings was formed smaller than that which was pro- 

 duced by either singly; whereas, had they been of the 

 opposite character, the rings produced by the combi- 

 nation would have been greater than one of the sys- 

 tem* produced separately. Hence, ice belongs to the 

 attractive clas* of crystals. The polarising force of ice 

 i* TrrT> that of rock crystal being T J .. Its refractive 

 power is 1 .507 less than that of water, and its specific 

 gravity is also less than that of water. For an account 

 of the chemical laws on which the congelation of water 

 depend*, see CHCMISIW. 



Under the article COLD, and also under GREENLAND, 

 tome of the phenomena which accompany the forma- 

 tion of ice in polar region* are briefly mentioned Their 

 history, however, is sufficiently important to require a 

 minute description, on account of the great scale on 

 which congelation i* fleeted, the variety of the ap- 

 ptarancfs presented, the striking grandeur of some, and 

 the singularity of others, together with the suggestions 

 which some known circumstances concerning it afford 

 for the prosecution of inttTesting objects. 



The appearances which the ice presents in its out- 

 line, especially when existing in a detached state, are 

 diversified ; and, Iteing of great interest to mariners 

 who navigate the polar seas, they are designated by 

 dWnct terms with as much familiarity as the varieties 

 of form which occur on land. 



A continued sheet of ice, so extensive that its ulterior 

 boundary cannot be seen from the nuut-head of a ship, 

 is called *JUUL 



Field* are famed from ice which is in the first in- 

 fixed. They owe their origin to a process of 

 on the surface of the ocean, to which the 

 vicinity of land it not, as some have supposed, neces- 

 sary. This process takes place both in a rough and in 

 smooth state of the surface, when the temperature is 

 sufficiently low. It requires a temperature considera- 

 bly inferior to the freezing point of freh water, as the 

 face must be sufficient to surmount the attraction of 

 the salt for the water which holds it in solution, for 

 more or lets separation always take* place : the ice 

 formed ander these circumstances, never affords water 

 ef equally strong impregnation as the original water of 

 the ocean. 



On a rough surface, this process begins with the for- 

 of detached crystals, called by the sailors tludge, 

 resemble mow immersed in water without un- 

 liquefaction ; by the union of the crystals, 

 - : of the sludge, the surface is ren- 



dered smooth. The continued sheet now formed is I. 

 soon broken into fragments of about three inches in V ""*V^' 

 diameter; these again coalesce and form a continued 

 texture of a stronger kind, which, in its turn, is bro- 

 ken into masses of much larger size. These are round- 

 ed at the edges by mutual attrition, and receive the 

 name of pancake ice. 



In sheltered situations and in still weather the conge- 

 lation goes on more regularly, and to appearance more 

 rapidly. The thin sheet formed in the first instance, 

 receives accessions to its thickness from beneath. In 

 twenty-four hours of keen frost it often acquires a 

 thickness of two or three inches ; and in less than two 

 days is strong enough to bear the weight of a man. 

 This is termed bay ice, from being formed in sheltered 

 bays. That which is of older formation is distinguish- 

 ed into two kinds, according to its thickness, being call- 

 eil light ice when from a foot to a yard thick, and when 

 above a yard heart/ ice. 



Some fields of ice are so smooth in their surface, so 

 transparent in their texture, and so exactly similar to 

 the ice formed on fresh lakes, that it has been believed 

 scarcely possible that they should be produced by the 

 freezing of the ocean, and they have been considered 

 as owing their origin, at least in part, to the freezing of 

 rain or melted snow, which had settled on a flat surface 

 of young ice, encircled by a ledge of older ice, which 

 retained the water like a cup. 



Loosened pieces, which are smaller than fields, but 

 still of very large dimensions, are called Jloet. Pieces Floe. 

 much smaller, detached from the angles of larger ones, 

 and floating in a congregated state, are called brash Brash ice. 

 ice, 



Ice of any form or size, floating in a state sufficiently Drift i<*. 

 loose to allow a vessel to sail freely among it, is culled 

 loosf, open, or drift ice. 



A number of large pieces in close contact, forming Vida and 

 a congeries which cannot be seen over from a mast P tcl '*- 

 bead, are called a pack. A similar congeries, which 

 can be seen over, is called a patch. When a congeries 

 of either kind is of an oblong shape, it is called a stream 

 of ice. 



A protuberance, considerably elevated above the Hummocks, 

 common surface of flat ice, is called a hummock. Hum- 

 mock* often attain the height of thirty feet or upwards. 

 They are sometimes formed by field* of ice crushing each 

 other, so that large pieces, separated from the margin, 

 are raised on edge, or a numerous wreck is accumula- 

 ted on the top of a field. Hummocks are generally 

 near the margin, but sometimes they extend to the 

 middle of a field, showing that their origin is some- 

 times different from that now described. They com- 

 municate to the ice a variety of fanciful shapes, and 

 render the whole appearance of it highly picturesque. 



When the effect of concussions of pieces of moderate 

 size is to accumulate such masses above a comparative- 

 ly thinjloe, that the surface of the latter is depressed 

 beneath that of the water, this part is called a calf. clv. 

 Some of these cahes are sufficiently deep to allow n 

 vessel to sail over them. But it is dangerous to ap- 

 proach them, their depth being uncertain and unequal ; 

 and when they rub freely against the superincumbent 

 pieces, they often become so far detached that their 

 buoyancy raises them to the surface, and their momen- 

 tum during a change of situation is sufficient to stave 

 the timbers of a vessel, or reduce it at once to a wreck. 

 Sometimes even a gentle heaving motion of this sort, 

 by raising one end of a vessel, immerses the other end, 



