﻿204 Mr. J. Croll on the Physical Cause 



on energy from a heated body without melting ; and we know 

 also equally well that a slab of ice at 32°, notwithstanding 

 this, still, as a mass, retains its solid state while the heat is being- 

 transmitted through it. This proves that every molecule resumes 

 its crystalline form the moment after the energy is transferred 

 over to the adjoining molecule. 



This point being established, every difficulty regarding the 

 descent of the glacier entirely disappears; for a molecule the 

 moment that it assumes the fluid state is completely freed from 

 shearing- force, and can descend by virtue of its own weight with- 

 out any impediment. All that the molecule requires is simply 

 room or space to advance in. If the molecule were in absolute 

 contact with the adjoining molecule below, it would not descend 

 unless it could push that molecule before it, which it probably 

 would not be able to do. But the molecule actually has room 

 in which to advance ; for in passing from the solid to the liquid 

 state its volume is diminished by about j 3 ^, and it consequently 

 can descend. True, when it again assumes the solid form it will 

 regain its former volume ; but the question is, will it go back to 

 its old position ? If we examine the matter thoroughly we shall 

 find that it cannot. If there were only this one molecule affected 

 by the heat, this molecule would certainly not descend; but all 

 the molecules are similarly affected, although not all at the same 

 moment of time. 



Let us observe what takes place, say at the lower end of the 

 glacier. The molecule A at the lower end, say, of the surface, 

 receives heat from the sun's rays ; it melts, and in melting not 

 only loses its shearing- force and descends by its own weight, but 

 it contracts also. B immediately above it is now, so far as A 

 is concerned, at liberty to descend, and will do so the moment 

 that it assumes the liquid state. A by this time has become 

 solid and again fixed by shearing-force; but it is not fixed in its 

 old position, but a little below where it was before. If B has 

 not already passed into the fluid state in consequence of heat de- 

 rived from the sun, the additional supply w r hich it will receive from 

 the solidifying of A will melt it. The moment that B becomes 

 fluid it will descend till it reaches A. B then is solidified a little 

 below its former position. The same process of reasoning is in 

 a similar manner applicable to every molecule of the glacier. 

 Each molecule of the glacier consequently descends step by step 

 as it melts and solidifies, and hence the glacier, considered as a 

 mass, is in a state of constant motion downwards. The fact ob- 

 served by Professor Tyndall that there are certain planes in the 

 ice along which melting takes place more readily than others 

 will perhaps favour the descent of the glacier. 



We have in this theory a satisfactory explanation of the origin 



