Remarks on Glacier Theories. 335 



and he has spoken of the sliding or gravitation theory as in*- 

 volving the hypothesis of the rigidity of glacial ice*. 1 have 

 protested against his charging that theory with any such phy- 

 sical absurdity. Whether solidity as a relative term may or 

 may not with propriety be opposed to rigidity as an absolute 

 one, is a question not involved in anything I have advanced. 

 De Saussure undoubtedly regarded glacial ice as having soli- 

 dity ; but to assert that he propounded a theory which tacitly 

 involves his attributing to glacial ice either the property of 

 rigidity, or that of solidity in a degree which would render 

 the motion of a glacier a manifest impossibility under existing 

 conditions, would be a most unjustifiable reflection on the sci- 

 entific character of that distinguished traveller. 



The terms semijiuid^ -plastic and solid are doubtless compa- 

 rative terms; but we are not justified in using them without 

 discrimination because the properties denoted by them gra- 

 duate into each other by insensible degrees. Red and green 

 are equally comparative terms ; but no one thinks of calling 

 a green field red, or a common brick wall green, because the 

 colours of the spectrum graduate into each other. To call a 

 hard crystalline substance, like glacial ice, semifluid or viscous 

 is scarcely a less departure in my opinion from all propriety of 

 language. It is not, however, on any popular misconception of 

 the nature of glacial ice which may thus arise that I ground the 

 most serious objection to too indiscriminate a use of such terms 

 as those above mentioned. It is that an entirely erroneous con- 

 ception may thus be formed of the real nature of the mecha- 

 nical problem which the motion of glaciers presents. All 

 problems which have for their object the determination of the 

 effects of assigned forces on a given mass with reference to its 

 motion, form, internal pressures and tensions, &c. at any pro- 

 posed time, must necessarily separate themselves, indepen- 

 dently of minor subdivision, into two classes; first, that com- 

 prising the cases in which the continuity of the mass is pre- 

 served duringtheaction of the forces up to the proposed instant; 

 and secondly, that comprehending the cases in which the con- 

 tinuity is destroyed by the action of the forces, as in the in- 

 stances of the formation of open fissures or internal cavities. 

 These classes are not only mechanically distinct, they are ma- 

 thematically so; for the complete solution of the first class would 

 not involve that of the second. Now if we are to use the 

 terms solid and semijiuid with any reference to their esta- 

 blished popular or mechanical significations, it would appear 

 to me an absurdity to designate as semifluid any mass in which 



* "As I understand the gravitation theoi-y, it supposes the mass of the 

 glftcter to be a rigid one, &c." — Travels, p. 362. 



