148 Proceedings of the Royal Physical Society. 



and his brother, Professor J. Thomson, have shown that the 

 melting point of ice is lowered 0°-0135 F. for each additional 

 atmosphere of pressure, which is equivalent to the weight of 

 a column of ice 3 7 '7 feet in height. 



Another factor in this connection is the important one that 

 water in freezing expands to such an extent that 174 volumes 

 of water become 184 volumes of ice, and that the force 

 necessary to counteract that expansion is 1110 lbs. (about 

 half a ton) per square inch of surface affected, for a fall of 

 temperature of one degree Fahrenheit. So when water flows 

 into a crevasse and freezes there, it is equivalent to driving 

 in a wedge of the same dimensions as those of the crevasse, 

 with a force equal to half a ton per square inch. 



Lastly, but by no means the least important, is the fact, 

 well-known to even the earlier students of glacial physics, 

 that pure ice is nearly diathermanous to the solar rays 

 of higher refrangibility.^ These pass through sound and 

 clean ice without sensibly raising its temperature, and the 

 ice, like the air, is warmed, not so much by the direct rays 

 of the Sun, as by radiation from other bodies which have 

 been themselves previously warmed by that luminary. The 

 meaning of this may be made clearer by reference to the 

 fact that lenses of clean ice are sometimes used as burning- 

 glasses, and that, in using them, the surface of the ice-lens 

 farthest from the Sun and nearest the object heated, melts 

 much faster than the corresponding surface facing the Sun. 

 Most of the direct heat of the Sun goes through the ice 

 without warming it ; but the reflected heat returns with a 

 different quality from what it had on entering, and endowed 

 with a higher capacity for melting ice and snow. In this 

 respect it will be seen that the solid and the fluid states of 

 HgO behave differently in relation to the Sun. 



In experimenting with the lens of ice, it is important that 

 one should bear in mind that the effect produced will vary 

 materially with the initial character of the heat employed. 

 Luminous heat from a source of great intensity produces less 

 effect upon the side of the ice-lens facing the illuminant, than 



^See especially W. D. Cooley, "Physical Geography," p. 76 (1876). 



