Revieiv of the Controversy Regarding the Motion of Glaciers. 1 39 



which are constantly passing from the solid to the liquid state, and vice 

 versa, nevertheless retain an unvarying hard and brittle condition. 

 This is due to the property of regelation. When two fragments of 

 ice at :->2° are brought into contact, they instantly unite to form one 

 solid piece. What is, then, true of ice in the mass, is equally true 

 of its most minute atoms. Whenever these impinge upon one another, 

 they immediately freeze firmly together. No sooner does a molecule, 

 which has momentarily melted, resume the solid state, than it imme- 

 diately unites to the other solid particles by which it is surrounded ; 

 hence the continuity of the whole is preserved, and throughout its en- 

 tire bulk the ice remains a solid body. Such is the ingenious theory 

 advanced by Mr. Croll."* 



Mr. CroU's hypothesis, then, briefly is this, that the glacier moves 

 not as ice, but as water, during the constant though momentary lique- 

 faction of its particles, one after another, and that this liquefaction is 

 caused by the transmission of heat into the ice from without, in the 

 form of a liquid wave, the substance being incapable of conveying this 

 motion and at the same time of retainhig its solidity. 



In considering this theory, it will be well to define it at the outset a 

 little more clearly. 



In the first place it is evident that the only kind of heat really in- 

 volved in the hypothesis is that which is carried into the interior of 

 the glacier by the process of conduction. Radiant heat, as it enters 

 and passes through the ice, is not concerned directly in the production 

 of the motion. We will, however, consider this subject further on, 

 taking into account at present only the heat of conduction. 



Now, in order that conduction of heat may occur between two 

 bodies, the first and essential condition is, that they must be of different 

 temperatures. The greater this difference, the more rapid the conduc- 

 tion, and vice versa. A heated cannon ball cools rapidly at first, even 

 in its interior, and then more and more slowly, as its temperature foils, 

 until equilibrium is established, when all conduction ceases. Applying 

 this fact to the theory in question, we have three bodies : overlying air, 

 the superficial layer of ice, and the glacial mass below. Of these 

 three, the last is almost invariable in temperature, while the first 

 varies widely between day and night, summer and winter. The second 

 holds an intermediate position, and forms a transition link. It re- 

 ceives heat by conduction from the air at one time, and gives it up at 

 another. But it is subject to the limitation that it can not be raised 



* The Great Ice Age, p. 41. 



