﻿84 Mr. J. Ball on the Cause of the Motion of Glaciers, 



every such instance the motion is to be accounted for only by 

 supposing conversion into the fluid form of the particles at the 

 moment when they are changing their relative positions ? Or if 

 not, why should he imagine that a special hypothesis is neces- 

 sary in regard to ice ? So far as I know, the fact that ice will 

 change its form under moderate pressure without fracture at a 

 temperature notably below its melting-point was, for the first 

 time, established by the experiment published by Mr. W. 

 Mathews in the Number of ' Nature ' for the 24th of March 

 last. In ascertaining that a thick plank of ice supported at both 

 ends subsided about 1J inch in twenty-four hours, during a 

 sharp frost, under the pressure of its own weight alone, Mr. 

 Mathews did not disclose a fact which ought to excite surprise. 

 So far from being anomalous, this property of ice is in full ac- 

 cordance with what we know of the behaviour of other similar 

 bodies. I am, however, at a loss to understand whether Mr. 

 Croll maintains that the fact can be accounted for only by assu- 

 ming that each molecule of the ice momentarily became fluid as 

 it shifted its relative position so as to allow of the bending of the 

 ice. If this occurs when the temperature is considerably below 

 the freezing-point, the so-called molecular hypothesis cannot 

 stop here. It must be extended to all bodies which are capable 

 of changing their form under pressure without fracture at tem- 

 peratures below that of liquefaction. If, on the other hand, 

 Mr. Croll does not consider his hypothesis to be applicable to 

 the case in question, I must ask why he holds it to be requisite 

 in the case of a glacier, where the amount of relative disturbance 

 of the particles is very much less than in the experiment of Mr. 

 Mathews. 



I would especially beg Mr. Croll to consider the many 

 grounds, both theoretical and experimental, on which we are 

 led to hold that the temperature of the interior of great glaciers 

 is very nearly, if not absolutely, constant, and how impossible it is 

 to hold that their advance can be dependent on the passage 

 through their mass of any sensible quantity of heat such as 

 could-effect the molecular changes which he contemplates. 



If the molecular theory of Mr. Croll is no longer to be ap- 

 plied specially to the conduct of ice under pressure, but is ex- 

 tended generally to all, or most, plastic bodies that yield slowly 

 to pressure, I shall merely say that it appears to me to lack an- 

 tecedent probability, and to be at least premature. In spite of 

 recent progress, our knowledge of the molecular constitution of 

 matter is yet far too imperfect to enable us to reason with any 

 confidence on the general question raised by Mr. Croll. Should 

 he be induced to follow up the inquiry, it may in the hands of 

 so acute a thinker lead to important results ; but in the mean 



