268 ANNUAL OF SCIENTIFIC DISCOVERY. 



freezes the water again in new positions, and thus a change of form, or plas- 

 tic yielding of the mass of ice to the applied pressures, has occurred. The 

 newly-formed ice is at first free from the stress of the applied forces, but the 

 yielding of one part always leaves some other part exposed to the pressure, 

 and that part, in its turn, melts and falls in temperature ; and, on the whole, 

 a continual succession goes on, of pressures being applied to particular parts 

 liquefaction occurring in those parts accompanied by evolution of cold, 

 dispersion of the water so produced in such directions as will relieve its pres- 

 sure, and re-congelation, by the cold previously evolved, of the water on its 

 being relieved from this pressure. The cycle of operations then begins again, 

 for the parts re-congealed, after having been melted, must, in their turn, 

 through the yielding of other parts, receive pressures from the applied forces, 

 thereby to be again liquefied, and to proceed through successive operations 

 as before. The succession of these processes must continue as long as the 

 external forces tending to change of form remain applied to the mass of 

 porous ice permeated by minute quantities of liquid water. The ice is thus 

 shown to be incapable of opposing a permanent resistance to the pressures, 

 and to be subjected to gradual changes of form while they act on it ; or, in 

 other words, it has been shown to be possessed of the quality of plasticity. 

 In the foregoing, I have supposed the ice under consideration to be porous, 

 and to contain small quantities of liquid water diffused through its substance. 

 Porosity and permeation by liquid water are generally understood, from the 

 results of observations, and from numerous other reasons, to be normal con- 

 ditions of glacier ice. It is not, however, necessary for the purposes of my 

 explanation of the plasticity of ice at the freezing point, that the ice 

 should be, at the outset, in this condition ; for, even if we commence with 

 the consideration of a mass of ice perfectly free from porosity, and free from 

 particles of liquid water diffused through its substance, and if we suppose 

 it to be kept in an atmosphere at or above centigrade, then, as soon as 

 pressure is applied to it, pores occupied by liquid water must instantly be 

 formed in the compressed parts, in accordance with the fundamental prin- 

 ciple of the explanation which I have proposed the lowering, namely, of 

 the freezing or melting point by pressure, and the cognate fact, that ice can- 

 not exist at centigrade under a pressure exceeding that of the atmos- 

 phere. I would further wish to make it distinctly understood, that no part 

 of the ice, even if supposed at the outset to be solid, or free from porosity, 

 can resist being permeated by the water squeezed against it from such parts 

 as may be directly subjected to the pressure ; because, the very fact of that 

 water being forced against any portions of the ice supposed to be solid, will 

 instantly subject them to pressure, and so will cause melting to set in 

 throughout their substance, thereby reducing them immediately to the porous 

 condition. Thus it is a matter of indifference, as to whether we commence 

 with the supposition of a mass of porous or of solid ice. Mr. Thomson then 

 referred to an experiment made by Prof. Christie, late Secretary to the 

 Royal Society, showing the plasticity of ice in small hand specimens, and 

 also to more recent experiments by Prof. Tyndall to the same effect, and 

 very interesting on account of the striking way in which they exhibit the 



