On the Expansion of Gases by increase of Temperature. 271 



ones. In the vast quantities of moraine-matter which cum- 

 ber many of the valleys we have also suggestions as to the 

 magnitude of the erosion which has taken place. This moraine- 

 matter, moreover, is only in part derived from the falling of 

 rocks from the eminences upon the glacier ; it is also in great part 

 derived from the grinding and the ploughing-out of the glacier 

 itself. This accounts for the magnitude of many of these ancient 

 moraines, which date from a period when almost all the moun- 

 tains were covered with ice and snow, and when consequently the 

 quantity of moraine-matter derived from the naked crests cannot 

 have been considerable. The erosion theory ascribes the forma- 

 tion of Alpine valleys to the agencies here briefly referred to. It 

 invokes nothing but true causes. The artificers by which its 

 work is performed are still there, though, it may be, in dimi- 

 nished strength ; and if they are granted sufficient time, it is 

 demonstrable that they are competent to produce the effects 

 ascribed to them. And what does the fracture theory offer in 

 comparison ? From no possible application of this theory, pure 

 and simple, can we obtain the slopes and forms of the moun- 

 tains. Erosion must in the long run be invoked, and its power 

 therefore conceded. The fracture theory infers from the dis- 

 turbances of the Alps the existence of fissures ; and this is a pro- 

 bable inference. But that they were of a magnitude sufficient 

 to determine the conformation of the Alps, and that they fol- 

 lowed, as the Alpine valleys do, the lines of natural drainage of 

 the country, are assumptions which do not appear to me to be 

 justified either by reason or by observation. 

 Royal Institution, 

 September 1864. 



P.S. — The foregoing paper was in the printer's hands before 

 it was my privilege to read the last Anniversary Address to the 

 Geographical Society by its President, Sir Roderick Murchison. 

 I have since considered the arguments, and given, I trust, due 

 weight to the authorities urged and cited in that excellent Address 

 against the theory of erosion, as applied to the valleys of the 

 Alps. But the effect on my mind is not such as to induce me 

 to alter the opinions, based on observed facts, which I have ven- 

 tured to express in these pages. 



XXXI. On the Law of the Expansion of the Gases by increase of 

 Temperature. By Professor Potter, A.M* 



IN the theory of heat, the law of the expansion of the gases 

 by increase of temperature is a most important subject, not 

 only on account of the air- thermometer having been taken as the 

 * Communicated by the Author. 



