R. M. Deeley and Q. Fletcher — Structure of Olacier-Ice. 155 



that there should be large and small ones. In the glacier proper 

 this alleged equality of size between the various crystal grains 

 certainly does not exist, as will be seen from the sections we give, 

 nor does it, as far as our memory serves, in the neve. Crystals 

 1^ inches long are frequently to be seen side by side with very 

 small ones. 



During this growth of the ice-grains dilatation does not, of course, 

 take place, neither is there a true absorption or liberation of heat, 

 for evaporation and solidification go on at the same rate. Although 

 it is not quite clear why some of the grains should increase in size 

 and others disappear, the transference of the molecules from crystal 

 to crystal offers no difficult^^ for when exposed to dry air ice 

 evaporates, surface molecules detaching themselves from the mass, 

 and even in air saturated with moisture particles doubtless get free, 

 and this is perhaps more nearly the condition obtaining between the 

 grains. When the water vapour has reached its maximum density, 

 as Maxwell says, "according to the molecular theory . . . evapora- 

 tion is still going on as fast as ever; only condensation is also 

 going on at an equal rate."^ In the interior of a glacier the surface 

 molecules of the grains are pi'obably more unstable than they are 

 on surfaces exposed to the atmosphere, but instead of the molecules 

 obtaining their freedom, they simply pass from granule to granule. 

 However, explain the fact as we may, such a growth of the granules 

 does take place, and it is clear that it must take place at the expense 

 of the smaller individuals. 



It is only under certain meteorological conditions that the granular 

 structure can be made out without the use of the polariscope. How- 

 ever, on several occasions, when it was looked for, it was seen 

 developed in perfection. The most favourable circumstances seem to 

 be a dry atmosphere a little below the freezing point. Under such 

 conditions, on looking obliquely at an ice surface, shaded from 

 the sun, the outline of each crystal grain can be clearly seen. 

 Occasionally, the surface of a grain will be quite smooth ; the majority 

 are, however, seamed with exceedingly small fuiTOws, which give 

 the surface much the appearance of watered silk, or the skin on the 

 palm of the hand. On each crystal the direction of the lines is 

 different. In the ice-caves at the foot of the glaciers the structure 

 is at times very beautifully shown. Figure 1 shows the nature 

 of this surface structure, and the arrangement of the grains. It 

 was sketched in the ice-cave of the Rhone Glacier. The conditions 

 did not allow us to determine the relation between these striae and 

 the directions of the optic axes on the glacier. The matter is now 

 receiving our attention. The striee are, doubtless, analogous to those 

 often seen on the surfaces of other crystals, and are due in all 

 probability to the alternation of two faces of crystallization. 



When neve reaches lower levels and becomes a portion of the 



glacier proper, quite a new structure begins to show itself. This 



is the well-known ribboned or veined structure. In some cases this 



structure develops roughly parallel with the stratification of the 



1 "Theory of Heat," seventh edition, p. 324. 



