IN CLOUDS AND RIVERS, ICE AND GLACIERS. 



139 



pansion, the water in consequence remaining 

 liquid at a temperature of more than 30 

 Fahr. below the ordinary freezing-point. A 

 bullet within the cylinder rattle"! about at 

 this temperature si owing that the water was 

 sti!. 1 liquid. On opening the tap the liquid, 

 relieved of the pressure, was instantly con- 

 verted into ice. 



433. It is only substances which expand on 

 solidifying that behave in this manner. The 

 metal bismuth, as we know, is an example 

 similar to water ; while lead, wax, or sulphur, 

 all of which contract on solidifying, have 

 their point of fusion heightened by pressure. 



434. And now you are piepared to under- 

 stand Professor James Thomson's theory of 

 regelation. When two pieces of ice are 

 pressed together, liquefaction, he contends, 

 results. The water spreads out around the 

 points of pressure, and when released re- 

 freezes, thus forming a kind of cement be- 

 tween the pieces of ice. 



63. FARADAY'S VIEW OP REGELATION. 



435. Faraday's view of regelation is not 

 so easily expressed, still I will try to give, 

 you some notion of it, dealing in the iirst 

 place with admitted facts. Water, even in 

 open vessels, may be lowered many degrees 

 below its freezing temperature, and still re- 

 main liquid ; it. may also be raised to a tem- 

 perature far higher than its boiling-point, 

 and still resist boiling. This is due to the 

 mutual cohesion of the water particles, which 

 resists the change of the liquid either into 

 the solid or the vaporous condition. 



436. But if into the over-chilled water you 

 throw a particle of ice. the cohesion is rup- 

 tured, and congelation immediately sets in- 

 And if into the superheated water you intro- 

 duce a bubble of air or of st^am, cohesion is 

 iikwise ruptured, and ebullition immediate- 

 ly commences. 



437. Faraday concluded that in the interior 

 of any body, whether solid or liquid, where 

 every particle is grasped, so to speak, by the 

 surrounding particles, and grasps them in 

 turn, the bond of cohesion is so strong as to 

 require a higher temperature to change the 

 stat? of aggregation than is necessary at the 

 surface. At the surface of a piece of ice, for 

 example, the molecules arc free on one side 

 from the control of other molecules ; and 

 they therefore yield to heat moie reaclily than 

 in the interior. The bubble of air or steam 

 in overheated water also frees the molecules 

 on one side ; hence the ebullition consequent 

 upon its introduction. Practically speaking, 

 then, the point of liquefaction of the interior 

 ice is higher than that of the superficial ice. 

 Faraday also refers to the special solidifying 

 power which bodies exert upon their own 

 molecules. Camphor in a glass bottle fills 

 the bottle with an atmosphere of camphor. 

 In such an atmosphere large crystals of the 

 substance may grow by the incessant deposi- 

 tion of camphor molecules upon camphor, at 

 a temperature too high to permit of the 

 slightest deposit upon the adjacent glass. A 

 similar remark applies to sulphur, phospho- 



rus, and the metals in a state of fusion. 

 They are deposited upon solid portions of 

 their own substance at temperatures not low 

 enough to cause them to solidify against 

 Other subst3nces. 



488. Water furnishes an eminent example 

 of this special solidifying power. It may be 

 cooled ten degrees and more below its freez- 

 ing-point without freezing. But this is not 

 possible if the smallest fragment of ice be 

 floating in the water. It then freezes accu- 

 rately at 32 Fahr., depositing itself, how- 

 ever, not upon th sides of the containing 

 vessel, but upon the ice. Faraday observed 

 in a freezing apparatus thin crystals of ice 

 growing in ice-cold water to a length of six, 

 eight, or ten inches, at a temperature incom- 

 petent to produce their deposition upon the 

 sides of the containing vessel. 



430. And now we are prepared for Fara- 

 day's view of regelation. When the surfaces 

 of two pieces of ice, covered with a film of 

 the water of liquefaction, are brought to- 

 gether, the covering film is transferred from 

 the surface to the centre of the ice, where 

 the point of liquefaction, as before shown, 

 is higher than at the surface. The special 

 solidifying power of ice upon water is now 

 brought into play on both sides of the film. 

 Under these circumstances, Faraday held 

 that the film would congeal, and freeze the 

 two surfaces together. 



440. The lowering of the freezing-point 

 by pressure amounts to no more than one 

 seventieth of a degree Fahrenheit for a whole 

 atmosphere. Considering the infinitesimal 

 fraction of this pressure which is brought 

 into play in some cases of regelation, Fara- 

 day thought its effect insensible. He sus- 

 pended pieces of ice, and brought them into 

 contact without sensible pressure, still they 

 froze together. Professor James Thomson, 

 however, considered that even the capillary 

 attraction exerted between two such masses 

 would be sufficient to produce regelatiou. 

 You may make the following experiments, 

 in further illustration of this subject : 



441. Place a small piece of ice on water, 

 and press it underneath the surface by a 

 second piece. The submerged piece may 

 be so small as to render the pressure infini- 

 tesimal ; still it will freeze to the under sur- 

 face of the superior piece. 



443. Place two pieces of ice in a basin of 

 warm water, and allow them to come to- 

 gether ; they freeze together when they 

 touch. The parts surrounding the place of 

 contact melt away, but the pieces continue 

 for a time united by a narrow bridge of ice. 

 The bridge finally melts, and the pieces for a 

 moment are separated. But capillary attrac- 

 tion immediately draws them together, and 

 regelation sets in once more. A new bridgo 

 is formed, which in its turn is dissolved, the 

 separated pieces again closing up. A kind 

 of pulsation is thus established between the 

 two pieces of ice. They touch, they freeze, 

 a bridge is formed and melted ; and thus the 

 rhythmic action continues until the ice di& 

 appears. 



