6i AQUEOUS AGENCIES. 



Regelation. — If two slabs of ice be laid one atop of the other, they 

 soon freeze into a solid mass. This will take place not only in cold 

 weather, but in midsummer, or even if boiling water be thrown over 

 the slabs. If a mass of ice be broken to pieces, and the fragments be 

 pressed or even brought in contact with one another, they will quickly 

 unite into a solid mass. Snow pressed in the warm hand, though con- 

 stantly melting, gradually becomes compacted into solid ice. This very 

 remarkable but imperfectly understood property of ice completely ex- 

 plains the phenomena of molding ice by experiment. By this property 

 the broken fragments reunite in a new form as solid as before. We 

 may possibly call this property of molding under pressure plasticity 

 (although it is not true plasticity, since it does not mold without rupt- 

 ure, but by rupture and regelation) ; but it can not in any sense be called 

 viscosity, for the true definition of viscosity is the property of yielding 

 under tension — the property of stretching like molasses-candy, or 

 melted glass ; but ice in the experiments, according to Tyndall, did not 

 yield in the slightest degree to tension. In the experiment, if, instead 

 of placing the straight bar at once into the curved mold, it had 

 been placed successively in a thousand molds, with gradually-increased 

 curvature, or, still better, if placed in a straight mold, and this mold, 

 while under pressure, curved slowly, then there would have been no 

 sudden visible ruptures, but an infinite number of small ruptures and 

 regelations going on all the time. The ice ivould have behaved precise- 

 ly like a viscous body. Xow, this is precisely what takes place in a glacier. 



Application to Glaciers. — A glacier, on account of its immense mass, 

 is, in its loicer parts, under the heavy pressure of its own weight 

 tending to mold it to the inequalities of its own bed, and in every part 

 under a still more powerful pressure — a pressure proportioned to the 

 height of the head of the glacier — urging it down the slope of its bed. 

 Under the influence of this pressure the mass is continually yielding by 



fracture of all sizes, but, after changing the position of its parts, again 

 uniting by regelation. By this constant process of crushing, change of 

 form, and reunion, the glacier behaves like a plastic or viscous body ; 

 though of true plasticity or true viscosity there is, according to Tyn- 

 dall, none. In fact, we have in the phenomena of glaciers the most 

 delicate test of viscosity conceivable ; but we find the glaciers will not 

 stand the test. For instance, the slope of the Mer de Glace at one 

 point changes from 4° to 9° 25' * (Fig. 55), and yet the glacier, although 



* Tyndall, Glaciers of the Alps. 



