1.888.] 



On the Plasticity of Glacier and other Ice. 



333 



a decided separation of the collars due to the plasticity of the conical 

 pieces of ice therein. 



We next took a bar of ice formed in the mould, applied tension and 

 took measurements in the same way. The extension was at the rate 

 of 0*048 mm. per hour per length of 10 cm. The crystalline structure 

 of this ice was highly irregular. As one principal object of our ex- 

 periments lay in their application to the theory of glaciers, it had 

 now become obviously most important to test actual glacier ice. We 

 therefore drove over to the Morteratsch glacier, which is now readily 

 accessible from St. Moritz even in the winter, and obtained some 

 specimens from the natural ice caves at the foot of the glacier. 



We tested three pieces, which were quite sufficient to disprove the 

 common notions, that glacier ice is only plastic under pressure, not 

 under tension, and that regelation is an essential part of the process. 

 They showed at the same time the extraordinary variability of the 

 phenomenon. The first extended at a rate of from 0*013 mm. to 

 0*022 mm. per hour per length of 10 cm., the variations in speed being 

 attributable to temperature. The second piece began at a rate of 

 0*016 mm. and gradually slowed down till it reached at the same 

 temperature a rate of 0*0029 mm., at which point it remained 

 tolerably constant, except for temperature variations, till a greater 

 tension was applied. The third piece on the contrary began at the 

 rate of 0*012 mm., increased its speed with greater tension to 0*026 mm., 

 and stretched faster and faster with unaltered tension, till it reached 

 the extraordinary speed of 1*88 mm. per hour per length of 10 cm. 

 We put on a check by reducing the tension slightly, whereupon the 

 speed fell at once to 0*35 mm. and gradually declined to 043 mm. 

 The lowest temperature reached during our experiments, except with 

 the intractable bath ice, was with this specimen. During 12 hours 

 with a maximum temperature —9° and a mean temperature probably 

 — 10*5°, the rate under the light tension of 1*45 kilo, per sq. cm. was 

 0*0065 mm. 



These three pieces were composed of a number of crystals varying 

 in thickness from two or three millimetres up to thirty or even a 

 hundred. These crystals are the "glacier grains" (gletscherkdrner) , 

 which play such a large part in glacier literature. Glacier ice is 

 a sort of conglomerate of these grains, differing, however, from a 

 conglomerate proper in that there is no matrix, the grains fitting 

 each other perfectly. In the winter, at any rate, the ice on the sides 

 of the glacier caves looks quite homogeneous. But, when a piece is 

 broken off and exposed to the sun's rays, the different grains become 

 visible to the naked eye, being separated probably by thin films of 

 water. Though the optical structure of each grain is found under 

 the polariscope to be perfectly uniform, the bounding surfaces are 

 utterly irregular, and are generally curved. The optic axes too of 



2 c 2 



