202 



thus appears to put it out of the condition in which my theory has 

 clearly indicated a cause of plasticity, and I presume makes it cease, 

 or almost entirely cease, to be plastic. I believe no plastic yielding 

 of ice to tension has been discovered by observation in any case, and 

 I think there are theoretical reasons why ice should be expected to 

 be very brittle in respect to tensile forces. The isthmus then being 

 supposed devoid of plasticity at its extended side, ultimately breaks 

 at that side, when the opening motion caused by the feathers has 

 arrived at a sufficient amount to cause fracture, and the ice newly 

 formed on the compressed side comes now to act as a tie instead of 

 the part which has undergone disruption, and holds together the two 

 pieces of ice, or serves as a fulcrum under tension to communicate a 

 compressive force to the points of the two pieces of ice immediately 

 beyond it ; and so the rolling action with a constant union at the 

 point of contact goes forward. It is to be observed that the leverage 



probably in many of them must, be so ; but in the second case, the diminution 

 of cubical pressure in the external water effects the same diminution of press- 

 ure in the ice, and also in the water occupying pores in the ice. The theory 

 and quantitative calculation which I originally gave (Transactions Koy. Soc. 

 Edin. vol. xiv. part 5. 1849, and Cambridge and Dublin Math. Journ. Nov. 

 1850) of the effect of increase of pressure in lowering the freezing-point of 

 water, and of course also of the effect of diminution of pressure in raising it, 

 applied solely to effects of pressure communicated to the ice through the water, 

 and therefore equal in all directions, and equally occurring in the ice and the 

 water j but when changes of pressure in one or more directions are applied to 

 the ice as distinguished from the water, the theory does not apply in any precise 

 way to determine the conditions of the melting of the ice, or of its growth by 

 the freezing of the adjacent water to its surface. There seems to me to be yet 

 a field open for much additional theoretical and experimental investigation in 

 this respect ; but so far as I have applied the principle of the lowering of the 

 freezing-point of water by pressure in developing or sketching out a theory of 

 the plasticity of ice, I think I have done so correctly. I perceived that the 

 application of pressures tending to change the form of the ice must necessarily 

 produce volume- compression in some parts of the mass, accompanied by the 

 occurrence of increased fluid pressure in the pores which might already exist in 

 those parts, or which would arise in them as a consequence of the pressure ; 

 and this I thought was a sufficient basis on which to rest the theory, even with- 

 out precise knowledge of all the varying influences on the melting or freezing 

 of the ice or water, of all the possible varieties of pressures or stresses that 

 could be applied to the ice, and of flttid pressure that could occur in the water 

 contemporaneously with those stresses in the ice. Some additional develop- 

 ments of this part of the subject, which have occurred to me, may, I hope, form 

 the subject of a future paper. 



