116 [May 22, 



4. But if the determinate property of viscosity, as here defined, be 

 not recognized in ice, what, it will be asked, is really the idea which 

 has been attached to the term plastic or viscous ? The question, as 

 already observed, is difficult to answer. Perhaps the best way of doing 

 so is to refer to the Prefatory Note to Principal Forbes's ' Occasional 

 Papers J (p. xvi). He there intimates that the expressions " bruising 

 and re-attachment," and " incipient fissures re-united by time and 

 cohesion," used by him in 1846, are to be regarded as having the 

 same meaning as the expression " fracture and regelation," first in- 

 troduced into the subject in 1857. Now there is no ambiguity what- 

 ever in this latter expression. "Fracture" means the breaking and 

 splitting of the ice regarded as a brittle and crystalline solid, and 

 could never be intended to have the slightest reference to viscosity. 

 In fact the expression is altogether inapplicable to any body which can 

 be called viscous without a violation of scientific language. Still this, it 

 may be said, may be only a want of strict accuracy of expression, rather 

 than of accuracy of conception. But if a notion of cracking and break- 

 ing, so foreign to any idea of plasticity, should be admitted, it could not 

 be said that a glacier moved as it is observed to move, because it was 

 plastic, but merely that it moved as if it were plastic. The true 

 inference from the motion would have been that glacier ice possessed 

 not necessarily real plasticity, a definite property of bodies, but a 

 quasi-plasticity, which expresses no determinate property at all, but 

 may consist with many different properties. It merely expresses, in 

 fact, the power of the component elements of the mass of changing to 

 a certain extent their relative positions. But this is not the peculiar 

 property of ice ; it is common, indeed, to all bodies exposed to dis- 

 ruptive forces which, as in the case of ice, the cohesive power is un- 

 able to withstand. The mass of any other substance, as well as that 

 of a glacier, will then be broken into fragments sufficiently small to 

 allow it to follow the impulses of the external forces acting on it. To 

 say, therefore, that a glacier moves as if it were plastic is not to as- 

 sign to ice any property peculiar to itself, and therefore does not pro- 

 perly constitute & physical theory of glacier motion at all. 



5. But if we pass over the difference between true plasticity and 

 that which, as we have pointed out, is merely apparent, there still 

 remained the great difficulty, which was only removed by the experi- 

 ments of Mr. Faraday and Dr. Tyndall. Every one who believed ice 



