202 HEAT. 



strains to which the ice is subjected continually fracture it, and the 

 fracture allows slight yielding to the strain. Owing to the pressure, 

 regelation immediately occurs and the pieces join together again. This, 

 no doubt, is a partial explanation, but from experiments made by 

 M'Connel and Kidd (Proc. R.S., xliv., 1888, p. 331) it appears that ice 

 when consisting of an irregular aggregation of crystals behaves like a 

 viscous solid. They found that bars of ice of regular crystalline struc- 

 ture showed only exceedingly minute changes in extension and com- 

 pression under continued pulls and pressures, while bars of irregular 

 structure, formed of aggregations of crystals with their axes in all 

 directions, went on extending or lessening in length under continued 

 forces. The changes may be due to slipping along the cleavage planes. 

 The possession of a sensible vapour-pressure by ice shows a certain 

 mobility or power of escape of the molecules near the surface, and if we 

 suppose this mobility to exist within the mass we may suppose that there 

 will be considerable variations of strain from point to point in the irre- 

 gular aggregation. The mobile molecules will tend to move from points 

 of greater to points of less strain, and will perhaps fill up the gaps which 

 the slipping tends to form. Thus the mass keeps continuous. The 

 crystalline structure of glacier ice is of this irregular type. 



The viscosity of metals is probably due to a similar irregular 

 crystalline structure, and though we have not such clear indication of 

 internal molecular mobility it doubtless exists.* 



Regelation has been explained by supposing that the interior of the 

 blocks in contact is slightly below 0. The points in contact at are by 

 the contact surrounded by portions of ice below 0, and are thus cooled, 

 and the water on the surface is frozen so as to form a solid bridge. 

 Doubtless there is sometimes an effect of this kind. But it would take 

 place with very great slowness, as the latent heat of the newly formed 

 ice would have to be transmitted through the badly conducting surround- 

 ing ice to the colder ice inside the blocks. 



In order to remove this difficulty, Forbes and others have supposed 

 that ice passes gradually into water, so that on the surface of a melting 

 block there is a layer which may be regarded as either " plastic ice " 

 or "viscid water." When two such blocks are brought together, 

 any water between them need not give up its whole latent heat, but just 

 enough to become either "plastic" or "viscid," and the connecting- 

 bridge is formed. But there is no independent evidence of this plastic 

 condition. In freezing water, the ice always forms at definite points, 

 crystallising out from the water the water showing no tendency to 

 become viscid. Indeed, as we have seen, water may be cooled easily 

 below 0. A mixture of ice and water kept at 0, and neither gaining 

 nor losing heat from its surroundings, remains ice and water, whereas 

 if the plastic condition existed the whole of it should, as pointed out by 

 Helmholtz, gradually assume the viscid condition at a uniform tempera- 

 ture. The use of ice calorimeters, too, is founded on the supposition 

 that there is no such gradual transition but that the heat given up is 

 used to convert true ice to true water. If there is any intermediate 

 condition, we must make the improbable supposition that the contraction 

 in passing through any part of that condition is proportional to the heat 

 * See Properties of Matter, p. 59. 



