Mr. J. Thomson on the Plasticity of Ice. 549 
denoted in degrees Centigrade by ¢, will be expressed by the formula 
. t="0075 n. 
The phenomena which I there predicted, in anticipation of direct 
observations, were afterwards fully established by experiments made 
by my brother, Professor William Thomson, and described in a paper 
by him, published in the Proceedings of the Royal Society of Edin- 
burgh (Feb. 1850) under the title, “The Effect of Pressure in 
lowering the Freezing-Point of Water experimentally demon- 
strated *.”” 
The principle of the lowering of the freezing-point by pressure 
being laid down as a basis, I now proceed to offer my explanation, 
derived from it, of the plasticity of ice at the freezing-point as 
follows :— 
If to amass of ice at 0° Centigrade, which may be supposed at the 
outset to be slightly porous, and to contain small quantities of liquid 
water diffused through its substance, forces tending to change its 
form be applied, whatever portions of it may thereby be subjected to 
compression will instantly have their melting-point lowered so as to 
be below their existing temperature of 0° Cent. Melting of those 
portions will therefore set in throughout their substance, and this 
will be accompanied by a fall of temperature in them on account of 
the cold evolved in the liquefaction. The liquefied portions being 
subjected to squeezing of the compressed mass in which they origi- 
nate, will spread themselves out through the pores of the general 
mass, by dispersion from the regions of greatest to those of least 
fluid pressure. Thus the fluid pressure is relieved in those portions 
in which the compression and liquefaction of the ice had set in, 
accompanied by the lowering of temperature. On the removal of 
this cause of liquidity—the fluid pressure, namely,—the cold which 
had been evolved in the compressed parts of the ice and water, freezes 
the water again in new positions, and thus a change of form, or 
plastic 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, in its turn, acts in like 
manner; and, on the whole, a continual succession goes on of pres- 
sures being applied to particular parts—liquefaction in those parts— 
dispersion of the water so produced, in such directions as will relieve 
its pressure,—and recongelation, by the cold previously evolved, of 
the water on its being relieved from this pressure. Thus the parts 
recongealed 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 enter again on a similar cycle of 
operations. 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 water. 
Postscript received 22nd April, 1857. 
It will be observed that in the course of the foregoing communica- 
* The paper by Prof. William Thomson, here referred to, is also to be found 
republished in the Philosophical Mag@zine for August 1850, 
