and the Crystallization of Ice. 407 



ing the crack was always much whiter than the surrounding ; it 

 was, in fact, traversed by tine cracks or flaws in a very regular 

 and remarkable manner. One of these cracks formed a conti- 

 nuous and somewhat waving line, which pig 

 occupied almost exactly the middle of the 

 vein, and from that proceeded, at an acute 

 angle, an immense number of smaller flaws 

 to each side (fig. 1), just like the lateral ribs 

 of a leaf issuing from the central rib. It 

 seemed rather difficult to give a thorough 

 explanation of this singular mode of frac- 

 ture ; but at least it was evident that the mass 

 in the vein had been subjected to a great 

 strain, owing to the pressure under wliich 

 the water was frozen, for immoveable ice on both sides had re- 

 sisted the dilatation which attends the process of congelation. 



Additional Remark {Ap7il ISod). — These observations, which 

 were remarkably favoured by the transparency of the water and 

 of the ice of Lake Baikal, show us a primitive sort of ice clearly 

 distinguishable from a later or injected kind, and, again, in the 

 former — 



1 . A division into horizontal strata. — Two of these strata, and 

 of course the most perfect ones, were observed ; but more of 

 them would perhaps have been found if the ice had been anato- 

 mized by Mr. Tyndall's ingenious proceeding of interior melting. 

 This horizontal fissility is evidently connected with the vertical 

 direction of an optical and crystallographic axis in the ice of un- 

 moved water, which was first pointed out by Sir David Brewster, 

 and is now acknowledged by all opticians. 



2. Divisions of each of the horizontal strata by perpendicular 

 planes into almost hexagonal columns. — The width of these per- 

 pendicular cracks or plane fissures is so small, that they only 

 become visible by the air or the vacuum in their interior and the 

 adjoining solid acting differently upon the light. It is by these 

 same circumstances of their small width and their emptiness, 

 that fissures of this kind differ from those which are formed by 

 the ice cooling beneath the freezing-point, and which are then 

 mostly filled up with the solid product of a new congelation. 

 We have therefore to consider — 



3. These cracks, onring to contraction, and 



4. Those that are produced in the ice of ])osterior formation, 

 viz. in that which is afterwards injected in the previously men- 

 tioned [sub 3) cracks by contraction. And so it seems that, 

 according to the producing forces, we must (hstinguish in lake- 

 ice, where form(;d without disturbance by waves or the like, 

 between — 



