336 



Messrs. J. C. McConnel and D. A. Kidd. [June 21, 



Of course we do not take into account the slight elastic yielding. 

 Apply an equal tension to the other pair of faces which are parallel 

 to the axis. There is still no extension by the axiom. Now it can 

 hardly be supposed that an uniform hydrostatic pressure could pro- 

 duce continuous change of form. Apply then a pressure of such 

 magnitude as to neutralise the two tensions. We have then remain- 

 ing only a pressure along the optic axis, producing no continuous 

 yielding. 



In a similar way it ma'y be shown that tension along the optic axis 

 would produce no continuous yielding. It is true that the reasoning 

 cannot be extended to pressures and tensions oblique to the optic axis. 

 But if the plasticity observed had been due to the majority of crystals 

 extending, while a certain number remained unchanged, there would 

 surely have been numerous cracks found in every case ; while as a 

 matter of fact such cracks were only found in two cases, and then 

 they were very slight. Hence, while we think it desirable to experi- 

 ment further in the matter, we feel tolerably confident that single 

 crystals of ice are not plastic, and we attribute the apparent plasticity 

 of glacier ice to some action at the interfaces of the crystals. But 

 we are not at present inclined to venture any opinion as to the nature 

 of this action. 



The variation of plasticity with the temperature is of great intere st 

 both for the theory of glaciers and for the explanation of the plasticity 

 itself, but it is so difficult to disentangle the temperature variations 

 proper from the much larger alterations due to structural changes, 

 that our experiments throw very little light on this point. In the case 

 of the glacier ice in Experiment 7 the rate seems to have become 

 tolerably constant except for temperature changes. While at — 3*5° the 

 rate was 0*0029, two days before and two days afterwards it was 

 about 0*0020 at -5°, and a few days earlier 0*0013 at -8°. In the 

 icicle, when the temperature variations seemed paramount, the rate at 

 —2° was 0*0028, and at -0*2° 0*0034. This is a much smaller change 

 than we should have expected. In the case of compression the influ- 

 ence of temperature seems more strongly marked. In all three 

 pieces the rate rose at — 3° to about ten times its value at — 5°. An 

 increase which takes place in three pieces simultaneously can hardly 

 be attribuf-ed to structural changes independent of the temperature. 



The change in the rate of extension, produced by an alteration of 

 the tension, was in every case altogether out of proportion to the 

 magnitude of the latter. In the following table are collected all the 

 instances which occurred i — 



