Dec. 



27, I 



NA TURE 



203 



ON THE PLASTICITY OF GLACIER AND 



OTHER ICE} 

 T^HE nature of the motion of glaciers has been the 

 subject of an immense number of observations by 

 Forbes, Agassiz, Schlagintweit, Tyndall, &c.. and the 

 following facts amongst others have been established. - 

 \) Ihe velocity decreases gradually and continuously 

 from the centre to ihe sides, where it is sometimes almost 

 imperceptible, though in other cases it reaches one-third 

 of Its value at the centre. -(2) The motion is in general 

 continuous from day to day, and even from hour to hour. 

 '3) The motion is generally most rapid at the hottest 

 lime of the year, and slowest at the coldest, the ratio 

 being often 4 to i. But the effect of temperature is at 

 present by no means properly worked out. 



One main result of these observations may be summed 

 i]) in the statement that a glacier moves like a plastic 

 body. The most natural conclusion would be that ice is 

 plastic. But this conclusion was for a long time almost 

 universally rejected. Hand specimens of ice show no 

 sign of plasticity to casual observation, and no doubt 

 'i&w people realized what very slow yielding under stress 

 would account for the observed motion. So the rigidity 

 if ice was treated as an obvious fact. At any rate I have 

 not come across any mention of careful experiments 

 which failed to show plasticity within a few degrees of 

 the melting-point. As will be seen below, however, such 

 results might readily have been obtained on suitable ice. 

 True plasticity, then, being rejected, some other ex- 

 planation had to be found. The one generally adopted 

 is due to James Thomson. He proved theoretically that 

 the freezing-point of water is lowered by pressure at the 

 rate of 0-0075 C. per atmosphere. This was afterwards 

 verified experimentally by Sir Wm.Thomson. The former 

 held further that any kind of stress lowers the freezing- 

 point. Now glaciers are believed to be throughout at or 

 very near the temperature o' C. Thus the ice should melt 

 at places \\here the stress is most severe, and an equal 

 quantity should be formed elsewhere. There are at 

 least two difificulties in this explanation. In the first 

 place, the melting must absorb heat, and the work done 

 by pressure in the contraction of volume is quite an in- 

 significant source of heat. So the temperature would be 

 immediately lowered, and the process be brought to a 

 standstill, before it had well commenced, unless heat 

 were supplied by conduction. When we remember that, 

 even when the stress is most severe, the melting-point is 

 only lowered a few hundredths of a degree, and "that there 

 must be considerable distances between points of great 

 stress when the ice melts, and points of little stress when 

 It forms. It is difficult to believe that sufficient heat can 

 be conveyed from one to the other to produce much 

 effect. Some rough experiments I have made show ice 

 to be a far worse conductor than any rock, and nearly as 

 bad as wood. In the second place, it has vet to be 

 proved that the mass of the glacier is permeated by 

 water. Recent experiments by Prof. Forel {Arch, dcs 

 Sciences ^/yx, Geneva, July 1887) go far to show that 

 the capillary fissures containing water are confined to the 

 surface layer. 



But the point which I especially desire to bring out is 

 that this explanation is confessedly only a way out of a 

 dilemma. If glacier ice can be shown in the laboratory 

 to be plastic, the dilemma no longer exists, and there is 

 no necessity to have recourse to any other explanation 

 until It can be proved that the plasticity is insufficient, or 

 otherwise fails to account for the observed facts. The 

 existence of this plasticity in glacier ice we claim to have 

 established in our experiments last winter. 



The false plasticity due to melting and regelation is 



' ^°^'^'i'J '1*'=^''* o*" '''e experiments herein described see a Paper by 

 James C. McConnel and Dudley A. Kidd, published in the Royal Society's 

 i'roceediiTg.s, June i883. ^ ouc.ciy s 



^ See Heim's "Gletscherkunde," published by Engelhorn, Stuttgart, i£85. 



put out of the question by operating at a temperature 

 below even - o'-j C, for to lower the melting-point bv a 

 tenth of a degree requires a pressure of thirteen atmo- 

 spheres. If true plasticity is found at lower temperatures, 

 it is impossible to deny its existenre at the melting-point 

 itself. And plasticity has been found several degrees 

 below o' C. by many experimenters, such as .Matthews, 

 Bianconi, Aitken, Pfafif, &c.i Most of their experiments 

 were made on the bending of bars, in which case the 

 stress is too complicated to furnish any but the vaguest 

 idea of the relation between strain and stress. Further, 

 none of them dealt with glacier ice, for I do not include 

 i the experiments of Coutts Trotter, made at o" C. 



Matters were in this state when Dr. Main began his 

 experiments at St. Moritz the winter before last (Roy. 

 I Soc. Proc, vol. xlii. p. 329). A winter sojourn in the 

 j Engadine affords peculiar facilities for experiments of 

 this nature. During December, January, and February, 

 ! one can count on almost continuous frost. In a room on 

 j the north side of the house, with the window kept per- 

 j manently open, the temperature seldom rises above the 

 freezing-point. Dr. Main wished not merely to settle the 

 I question of the existence of plasticity, but also to deter- 

 I mine accurately its amount under various conditions of 

 stress and temperature. He decided to apply tension. 

 This has great advantages over other kinds of stress for 

 purposes of accurate measurement, as it is comparatively 

 easy to isolate from other stresses. Pressure, for instance, 

 applied to the ends of a bar of ice makes it bend, and we 

 have then a complicated set of stresses to deal with. And 

 if the bar be so short and thick that bending is im- 

 probable, the contraction to be measured becomes very 

 small. There are, however, certain obvious inconve- 

 niences in applying tension, viz, the difficulty of getting a 

 good grip of the ends of the bar of ice, and the constant 

 risk of fracture. 



Main used a mould for his ice, which turned out a 

 round bar with a conical enlargement at one end, which 

 would fit into a conical iron collar. A conical piece of 

 ice fitting another collar was frozen to the other end of 

 the bar of ice, and the tension was applied through the 

 two collars. Accurate measurements of the distance 

 between the collars were taken from time to time. In 

 this way he established the existence of plasticity in this 

 kind of ice at all temperatures down to - 6' C. It is 

 to be noticed that the ice cones are subjected to both 

 pressure and shearing stress, and some of the observed 

 extension must have been due to the distortion of these 

 cones ; but that nearly all of it was due to pure tension 

 in the bar he found by measuring the distance between 

 marks on pieces of paper gummed on to the bar itself. 

 In this last way he found the bar extended during three 

 days at the rate of 0-02 mm. per hour per length of 

 10 cm., while the temperature remained below - 2°. 



As his health prevented him from spending last winter 

 at St. Moritz, he suggested that I should continue the 

 experiments, kindly putting all his apparatus at my dis- 

 posal. I should not have been able to carry out such an 

 undertaking had I not been fortunate enough to secure 

 the assistance of an able coadjutor in Mr. Kidd, on whom 

 fell by far the greater part of the labour of experiment. 

 We started, like, I believe, all investigators before us, 

 under the impression that one piece of clear ice would do 

 as well as another, no matter how it had been formed. 

 Thus it was merely owing to the difficulty of obtaining 

 clear ice in the mould that we took our first experimental 

 bar from a different source. We imagined that since 

 Main had established the fact of extension under tension, 

 all that was left was to determine its amount at various 

 temperatures and under various tensions. So we were a 

 good deal surprised by the behaviour of our first bar. It 

 practically refused to stretch. We had taken the pre- 



' See Nature, vol. xxxii. p. 16. 

 paper by Matthews, Phil. Mag., i86 



id Heim, loc. cit p. 315, wlu cites a 



