76 Professor Tyndall, [March 4, 



crevasses the beds were shown with the veins crossing them. The 

 idea that the veins could be due to the turning up of the strata is plainly 

 irreconcileable with these observations. 



The speaker subsequently visited the Mer de Glace, and its tribu- 

 taries, and found the same key applicable to their phenomena. The 

 transverse structure of the Glacier du Geant is formed at the base of 

 the series ; that of the Talefre branch of the Mer de Glace at the base 

 of the Talefre ice-fall, where the change of inclination and the thrust 

 from behind develop the requisite longitudinal compression. He has 

 already had occasion to remark upon the peculiar dipping of the struc- 

 ture, and the scaling-ofF of the protuberances, which are effects of the 

 same cause. These phenomena are exhibited at the base of all the ice 

 cascades visited by the speaker. 



He divided, finally, the principal kinds of structure into three ; as 

 follows : — 



1st, Marginal structure, developed by pressure due to the swifter 

 motion of the centre of the glacier. 



2nd, Longitudinal structure, due to mutual pressure of two tribu- 

 tary glaciers; the structure here is parallel to the medial moraine 

 which divides the tributaries. 



3rd, Transverse structure, produced by pressure due to change of 

 inclination and to the longitudinal thrust endured by the glacier at the 

 base ©fan ice-fall. 



The speaker then entered upon the physical analysis of the manner 

 in which the pressure produced the veins. When a liquid is heated, 

 the attraction of the molecules operates against the action of heat, 

 which tends to tear them asunder ; at a certain point the heat triumphs, 

 the cohesion is overcome, and the liquid boils. But supposing we 

 assist the attraction of the molecules by the application of an external 

 pressure, the difficulty of tearing them asunder will be increased, more 

 heat will be required for this purpose, and we say that the boiling 

 point of the liquid has been elevated by the pressure. 



Spheres of sulphur were exhibited, which had been cast in a mould ; 

 these, on cooling, contracted so as to leave a large space hollow within 

 each sphere ; the same occurred, though in a less degree, with lead and 

 most other substances. Conceive the sphere replaced in the mould, and 

 the latter heated : to liquefy, it is necessary that the sulphur, or the 

 lead, should swell. Here, as in the former case, the swelling of the 

 substance is opposed by the attraction of its molecules ; but with a cer- 

 tain amount of heat this attraction is overcome ; we reach the fusing 

 point of the solid. But suppose we assist the molecules by external 

 pressure, a greater amount of heat will then be necessary to tear them 

 asunder : and we say the fusing point has been elevated by the pressure. 

 Reference was made to the researches of Mr. Hopkins and Mr. Fair- 

 bairn, in which this reasoning was experimentally verified. 



But the speaker also exhibited a sphere of bismuth, and showed that 

 this metal expanded during its solidification ; hence, in order to liquefy 

 a sphere of bismuth, the substance must contract : the molecules must 

 arrange themselves more closely together. Heat produces tliis con- 



