1888.] On the Plasticity of Glacier and other Ice. 335 



was high, averaging —1° C, yet it is difficult to suggest any theoreti- 

 cal reason for an increase in the number of interfaces producing a 

 decrease in the plasticity. 



We tried further two experiments on compression of ice, the 

 pressure being applied to three nearly cubical pieces at once. Of 

 three pieces of glacier ice, under a pressure of 3*2 kilos, per sq. cm., 

 the mean rates of contraction during five days were respectively 

 0*035 mm., 0*056 mm., and 0*007 mm. per hour per length of 10 cm. 

 These figures show that while the plasticity varies enormously in 

 different specimens, the rate of distortion is of the same order of mag- 

 nitude, whether the force applied be a pull or a thrust. 



The other experiment was on three pieces of lake ice, applying the 

 pressure in a direction parallel to the columns. The contraction was 

 scarcely perceptible. Under a pressure of 3*7 kilos, per sq. cm., the 

 mean rate of the three pieces during four days was 0*001 mm. per hour 

 per length of 10 cm. To fix the blocks of ice in position, we found it 

 necessary to cover their ends with paper frozen on, and the small con- 

 traction observed may well be attributed to the yielding of the films 

 of irregular ice with which the paper was attached. This view is 

 supported by the fact that nearly the whole of the contraction took 

 place in the first 36 hours. 



We have now shown by direct experiment that ordinary ice, con- 

 sisting of an irregular aggregation of crystals, exhibits plasticity, 

 both under pressure and under tension, at temperatures far below the 

 freezing point — in the case of tension at any rate down to — 9° at 

 least, and probably much lower — and also that a single uniform 

 crystal will not yield continuously either to pressure or tension 

 when applied in a direction at right angles to the optic axis. 

 We fully intended to test a crystal under tension applied along the 

 optic axis ; but we were unsuccessful in obtaining a crystal longer 

 in the axis than perhaps 8 cm., and when we had decided to be 

 content with that length, a thaw put a stop to all further operations. 

 We have, however, very little doubt that a crystal would refuse to 

 yield either to pressure or to tension in whatever direction they were 

 applied. 



The following reasoning seems tolerably conclusive as far as it goes. 

 We first assume the axiom that, if two systems of stresses produce 

 each by itself no continuous yielding, superposition of the two will 

 likewise produce no continuous yielding. This will probably be 

 admitted when we add the proviso that, when the nature of the 

 resultant stresses is found, their magnitude is to be reduced to the 

 same value as that of the simple stresses which are known to be 

 inactive. Take then a cube of ice, two of whose faces are perpen- 

 dicular to the optic axis. Apply tension to one of the other pairs of 

 faces. This according to our experiments produces no extension. 



