102 Rev. C. Trotter. Physical Properties of lee [Jan. 29, 



sensible to the eye), its surface must be extended by more than a 

 metre for every 60 metres of vertical thickness of the glacier. And 

 it is much the same with the system of diagonal crevasses due to 

 differential motion. It is true that the daily linear extension required 

 to prevent fracture is in such cases usually very small, but a small 

 extension of an inelastic mass like that of a glacier would seem to 

 require molecular displacements which are much more complicated 

 than those involved in a slight yielding of an elastic solid. If no 

 yielding were possible, systems of diagonal crevasses would be as 

 universal as differential motion. What is really proved by the often 

 quoted observation of Tyndall (" Glaciers of the Alps," 1860, pp. 317 

 — 18), as to the extreme narrowness of crevasses when they are first 

 formed, is that ice is not capable of any appreciable elastic extension 

 before it gives way. Such want of elasticity is often very charac- 

 teristic of viscous as distinguished from strictly solid bodies. 



I believe, therefore, that the weight of evidence tends to show that 

 ice at or about 0° C, is just as truly viscous as pitch or sealing-wax 

 at temperatures at which they are brittle but yet capable of yielding 

 to the continuous application of a very moderate force. The viscosity 

 of ice, however, probably diminishes very rapidly with the tempera- 

 ture. M. Person ("Comptes Rendus," xxx, pp. 526 — 528), quoted 

 by Forbes, infers from experiments on the latent heat of fusion of 

 ice that ice begins to soften at about —2° C. Ice just before it melts 

 appears to be quite soft. When daring the preparation of the bars 

 for the experiments described in this paper, I had occasion to inter- 

 rupt for a few minutes my work in the comparatively warm atmo- 

 sphere of the moraine, I was repeatedly struck by the difference 

 between the sensation caused by the first one or two cuts with the 

 plane, and that experienced after half a millimetre or so of the 

 melting surface had been removed. Many persons must have noticed 

 the way in which a piece of ordinary clear ice which has been 

 wrapped in a coarse linen cloth on a summer's day adheres to the 

 cloth, and has the marks of the threads impressed upon it. It may 

 be said that this is an instance of the lowering of the melting point 

 by pressure and subsequent freezing ; but when it is considered that 

 the phenomenon may be observed when 1 lb. per square inch is a 

 liberal estimate of the pressure exerted, so that the lowering of the 

 freezing point would be about '0005° C, and the melting of 1 mgrm. 

 of ice would produce sufficient absorption of heat to cool down about 

 320,000 mgrm. of ice to the new melting point, it seems more natural 

 to suppose that the ice just before it melts passes into a soft some- 

 what sticky condition like that of sealing-wax at about 100° C. It is 

 scarcely surprising that this condition is not more often obvious, as it 

 is difficult to touch the melting ice with anything which will not 

 convey to it heat enough to produce complete liquefaction. 



