120 Mr. J. Gill on Reg elation. 



Still the difficulty remains of the disposal of the heat of 

 liquidity of the frozen water. Professor Tyndall is no doubt 

 right in assuming that the interior portions of a mass of ice 

 require a higher temperature to liquefy than the surfaces, and 

 consequently that they may conduct and transmit heat without 

 melting. But as this very fact supposes them to be less suscep- 

 tible of individual molecular motion, it is difficult to imagine that 

 they can absorb or retain heat, unless from a source of appreci- 

 ably higher temperature — that is to say, a temperature above the 

 margin which distinguishes the capability of liquefaction between 

 the free and the confined particles, no cause of which is given ; 

 and it is not supposed that the film of water between the ap- 

 pressed surfaces is subject to any other cause of freezing than the 

 mere sympathetic action tending to induce synchronism of mo- 

 lecular motion, or identity of condition in the neighbouring- 

 particles. 



The explanation given by Professor James Thomson is based 

 on his interesting discovery of the lowering of the freezing-point 

 of water by compression, and is briefly as follows : — " The two 

 pieces of ice, on being pressed together at their points of contact, 

 will at that place, in virtue of the pressure, be in part liquefied 

 and reduced in temperature, and the cold evolved in their lique- 

 faction will cause some of the liquid film intervening between 

 the two masses to freeze." Of course a sufficient cause of the 

 pressure should be shown ; and moreover it must be supposed 

 that the liquid film between the appressed solid surfaces is free 

 (or comparatively free) from pressure. With this condition the 

 explanation seems satisfactory; and Helmholtz supposes this 

 condition to obtain in the case of glacier-motion, as the water of 

 the compressed ice can escape through fissures. This seems 

 questionable as a general fact ; but, supposing that in general 

 the water is confined and retained in the internal fissures, the 

 phenomena might still be explained by supposing the degree of 

 pressure to be continually varying in any given spot in the mass 

 from the extensive process of internal fracture and change of 

 shape always going on in the glacier from its descending motion 

 through a mould of very irregular conformation. When a block 

 of ice is compressed, layers of liquid water are formed in the 

 substance of the solid at right angles to the direction of pressure. 

 The temperature of the ice must be lowered by a quantity corre- 

 sponding to the heat of liquefaction of the water produced; and 

 the water, being under the same pressure, remains liquid in con- 

 tact with the cooled ice, because its freezing-point is lowered by 

 the pressure. On the removal of the pressure (supposing the ice 

 to return to its former dimensions, not by elasticity, but by a 

 new mechanical arrangement of its parts), the water freezes 



