﻿498 M. A. Heim on Glaciers. 



perceptibly displaced ; either their position is quite unaltered, 

 or they are completely separated — the piece breaks. This pro- 

 perty is also possessed by glacier-ice. Pressure does not alter 

 the arrangement of its particles, but separates them through 

 fracture at the capillary fissures ; and all the ice fragments which 

 are traced out thereby (the glacier-grains) behave now like the 

 molecules of semiliquid masses, which are plastic to pressure. 

 The glacier cannot possibly behave to traction like a "viscous 

 body" because the grains partly are separated from each other 

 and have not, like molecules, a sphere of attraction surrounding 

 them, partly unite by regelation and then constitute an aggre- 

 gate like a piece of water-ice, brittle. 



If with homogeneous semiliquid masses we would make ex- 

 periments on motion which can be brought into direct relation 

 with what is observed in glaciers, their molecules must be 

 merely displaced by pressure, and not merely displaced but se- 

 parated by traction. But in bodies of this class known to me 

 the pulling force necessary for separation is very small, less than 

 the weight of each single molecule; and this gives them the 

 property of thin liquidity. We cannot use them for experi- 

 ments on glacier-motion, because, if a fissure were to ensue, 

 the weight of the particles at its margin would immediately 

 drag them down into it, so that no fissure could remain. 



I see no simpler expedient than the following. We must 

 procure a thickly liquid mass, not homogeneous, but consisting 

 of solid corpuscles adherent so that, drawn asunder, they are 

 easily separated, while yet the adhesion is sufficient to neutralize 

 the weight of a small number of corpuscles, but not of a whole 

 mass. 



Wet sand, moistened powder, &c. would be such. Through 

 other labours I was first led to "killed" gypsum. "Burnt" 

 gypsum powder is seen under the microscope to be distinctly 

 crystalline. Moistened with water, after a little while it sud- 

 denly shows lively motion ; the form changes almost momenta- 

 rily, and other gypsum crystals appear. At the same time the 

 mass stiffens. After a few seconds, a multitude of new, quite 

 fine, needle-shaped crystals make their appearance, which, when 

 not disturbed, unite in beautiful radiate groups; they add to 

 the density of the texture formed by the other, larger crystals, 

 and thereby enhance the solidity of the stiffening mass. The 

 moment of violent motion is probably that at which the crystals 

 of " burnt " gypsum take up water. If now, at the moment 

 the circumcrystallization ensues, fresh water in proper quantity 

 be poured in, and the mass be briskly stirred or moved, the 

 crystals cannot interweave themselves, but will remain single 

 and free ; the mass will not stiffen again, and may be kept for 



