122 Mr. J. Gill on Reg elation, 



distance of the particles from each other being too great to admit 

 of any appreciable mutual interaction between them. The eva- 

 porating surface of the ice is therefore a field of active mole- 

 cular operation ; in whatever way heat may be getting in, it is 

 certainly going out with each disgregrated or metamorphosed 

 particle which flies off from the surface of the solid. 



Water, within a small range above its freezing-point, expands 

 with cold ; and as it gives this marked evidence of being in an 

 exceptional state of thermic susceptibility under these circum- 

 stances, we might naturally suppose that in this condition its 

 thermic properties generally were in an inverted state. It is 

 certain, at least, that between 39^° and 32° water contracts with 

 heat. Very thin liquid films on the surfaces of ice maybe sup- 

 posed to be at the maximum limit of this inverted state of ther- 

 mometric action, and would certainly contract on being slightly 

 heated. 



"When two pieces of ice are brought together, we should ima- 

 gine that the motion of translation of the escaping disgregated 

 particles being mutually stopped, local heat must be the result. 

 In the case of two fragments of wet ice touching each other 

 without any appreciable pressure artificially applied, the actual 

 points of solid contact may be supposed to be very minute, with 

 comparatively large surfaces of water-films between ; and the 

 smallest extent of " spherical surfaces " which we can imagine 

 practically to come to touch each other should, when conceived 

 of molecularly, give the idea of considerable space, including 

 many of the "virtual points that touch other," as described by 

 Tyndall, with very shallow water-spaces between. Now the 

 sudden generation of heat, developed, between the pieces of ice 

 in the act of contact, from the arrested motion of translation of 

 the disgregated particles flying off from the surfaces, must raise 

 the temperature of the surfaces where it acts. The sudden 

 melting of the projecting prominences of solid ice would proba- 

 bly be an action of very limited extent compared with the heat- 

 ing of the intervening liquid films, which, shut in from free com- 

 munication with the exterior, and instantly contracting with the 

 increase of temperature, would cause locally a partial vacuum 

 between the contiguous solid points of support, while the atmo- 

 spheric pressure acting on the back of each piece of ice might 

 cause a very considerable amount of pressure on the solid points, 

 in addition, perhaps, to a kind of capillary attraction existing 

 between surfaces in contact. 



The effects of the pressure thus brought into action may be 

 conceived of as results of the interesting fact of the lowering of 

 the freezing-point of water by compression. Helmholtz (quoted 

 by Tyndall) says, "The pressed ice will become colder by a 



