August 20, 1908] 



NA TURE 



?8i 



finallv, they fall into a heap. A block of glacier ice is 

 a geometrical curiosity. It consists of a number of solid 

 bodies of different sizes and of quite irregular shapes, 

 yet they fit into each other as exactly and fill space as 

 completely as could the cubes above referred to. 



Particulars with regard to the size of the grain of the 

 Aletsch and the Bossons glaciers will be found in an 

 article in N.\ture (igoi), vol. l.xiv., p. 399. 



Disiiriiculation of the Grains of llu 

 Radiation. 



Glacier hv Solar 



In the Mergelin See, glacier ice can be studied in a 

 wav that is possible in no other place. The fragments of 

 the' Aletsch Glacier which float in it are veritable ice- 

 bergs. In the middle of summer they are exposed to a 

 very powerful sun, and the weathering and disintegration, 

 as well as the melting, proceed at a very rapid rate. 



The action of the sun's rays on glacier ice is 

 two-fold ; it disarticulates the ice into its con- 

 stituent grains, and it splits the individual grain 

 up into lamina; perpendicular to the principal axis 

 of the crystal, and bounded by the planes of fusion 

 described by Tyndall. These planes are the dis- 

 tinguishing ch'aracteristic of the individual ice- 

 grain. 



Under the influence of radiant heat an ice-grain 

 begins to melt at the surfaces which separate 

 these lamina;, and the process of disintegration 

 and decay is directed by their plane. On the 

 other hand, an ice-grain, floating in water and 

 losing heat, generates ice laminje which are 

 directed by the same planes, which form the con- 

 tinuation of the corresponding lamina; of the 

 parent crystal. .\s the grains in a block of 

 glacier ice are distributed quite irregularly, the 

 water-line of a floating block necessarily cuts a 

 great number of grains, all of which are oriented 

 differently. The ice which is formed during the 

 night along this line is oriented crystallographic- 

 ally by the grain with which it is in contact, and 

 from which it appears to spring in continuation 

 of its crystalline lamina;. This produces a re- 

 markable pattern of lines on the surface of the 

 lake ice contiguous to a block of glacier ice. 



Tyndall has described and figured the minute 

 features of the disintegration of the crystal under 

 the absorption of radiant heat. Similar and com- 

 plementary features are observed when ice is 

 generated from an existing crystal under the dis- 

 sipation of heat. To do justice to them, how- 

 ever, would require the services of a skilful, 

 patient, and resourceful artist. 



The disarticulating and analysing action of the 

 sun's rays is not accomplished without selection 

 and the expenditure of energy. .Accordingly wc 

 observe that one grain protects another. The 

 disarticulation into separate grains, although very 

 thorough near the surface of a glacier, does not 



penetrate far. A stroke or two with an ice-axe 



reveals the fresh blue ice. In the case of an ice- . 



faerg, whether floating in a lake or in the ocean, 

 only the grains that are exposed to the sky and 

 above water are disarticulated, and prolonged ex- 

 posure of this kind reduces a grain to the last stage of 

 dilapidation. The grains beneath the surface, whether of 

 ice or ivater, are almost completely unattacked. 



The importance of direct skylight for the disarticula- 

 tion of glacier ice into its constituent grains is very well 

 seen in the artificial grottos which are maintained at easily 

 accessible parts of most popular glaciers. The thickness 

 of the layer of completely disarticulated ice is so small 

 that it is hardly noticed, and the whole grotto appears to 

 be cut out of pure blue ice. If the observer, on penetrating 

 for a few paces, turns round and looks outwards, he sees 

 the surface of the ice-walls of the grotto etched with 

 strange line figures. These are most strongly marked 

 near the opening, and they extend as far as direct skylight 

 strikes the ice. The lines so developed are formed by the 

 intersection of the surface of the ice-wall of the cave with 

 the separating surfaces of contiguous ice-grains. The 

 picture thus presented is one of very great interest. 



NO. 2025, VOL. 78] 



Delineation of the Grain by Hoar-frost. 

 After the autumnal equinox very little melting of ice 

 takes place, and by the end of October it has, as a rule, 

 ceased entirely. The etched figures on the walls of the 

 entrance of the grotto, which were developed by solar 

 radiation during summer, disappear quickly with the 

 arrival of winter ; but the winter brings with it another 

 means of delineation of the grain which does not depend 

 on solar radiation. Even at the lowest of winter tempera- 

 tures the atmosphere contains vapour of water, which it 

 is prepared to relinquish under the same conditions as 

 those under which dew is formed in summer. In the 

 Alpine winter, however, it is deposited, not as dew, but 

 as rime, that is, not as water, but as ice. It is well 

 known that very fine etching on a polished surface, which 

 can with difficulty be seen without assistance, at once 

 becomes visible if the surface be breathed on. In winter 



f the glacier on the roof of the grotto in the Morlerat^ch Glacier elineated 

 by hoar-frost, January, 1907. 



the walls and roof of the grotto are cold, dry, smooth, 

 and polished like glass. The winter air entering from 

 without and circulating in the grotto breathes on the 

 polished surface of ice and develops the figure of the ice 

 by the rime which is deposited on it. As rime always 

 settles by preference on sharp edges, it seeks out the lines 

 of separation between the grains and settles on them, 

 showing the whole granular structure. In January, 1907, 

 there was a wonderful exhibition of this natural 

 damascening on the roof of the cave of the Morteratsch 

 glacier ; in January, 190S, however, it was quite inferior, 

 and would not have struck the eye. The illustration re- 

 presents a portion of the roof of the cave which I photo- 

 graphed in January, 1907. As the roof is not flat, but 

 made up of shell-like cavities worn by the hot air in 

 summer, the delineation of the grain is sharp in some 

 parts of the photograph and faint in others. 



;\ precisely similar phenomenon was observed in 18S6 



