452 Intelligence and Miscellaneous Articles. 



due to the heating, but corresponds to the strength of the photo- 

 chemical action. According to the author, the velocity of the action 

 of two bodies, whose specific electrical conductivities are A and A,, 

 is proportional to the product A. A, ; and thus in a mixture of 

 gelatine and chloride of silver the action must be nearly propor- 

 tional to the conductivity of chloride of silver, since A must be 

 regarded as tolerably constant for the former. If sensitisers are 

 present, A, the conductivity of the sensitiser, is variable, and a 

 maximum is obtained for the colour which exerts an influence, and 

 which corresponds to a sharp absorption-band, together with the 

 other maximum without sensitiser. — Wiener Berichte, vol. xcvi. 

 p. 831 (1887) ; Beibldtter der Physik, ]S T o. 2 (1888). 



GLACIAL MOTION. 

 To the Editors of the Philosophical Magazine and Journid. 



(xENTLEMEN, 



In your issue of Febuary last (p. 156) Mr. Deeley brings forward 

 what he thinks is a new theory of Glacial Motion ; but, if 1 am not 

 greatly mistaken, it does not differ essentially from that of James 

 Thomson. In regard to this theory, in 1882 I wrote as follows * : — 

 "Some time ago James Thomson brought forward a theory which 

 deserves far more attention than it has yet received. Thomson 

 shows that the fusing-point of ice is lowered by pressure, and 

 therefore that ice at or near its freezing-point (as is the fact in 

 glaciers) is promptly melted by pressure. Now it is obvious that 

 in the differential motion of a glacier, whatever point, at any 

 moment, receives the greatest stress of pressure must melt and give 

 way, and, the stress being relieved, must immediately again refreeze. 

 Meanwhile, by change of relative position of parts, the stress is 

 transferred to some other point which in its turn melts, gives 

 way, and is refrozen ; and again transfers its stress to still another 

 point, and so on. If we compare this theory with Tyndall's, in 

 both cases the ice gives way at the point of greatest stress : in the 

 one case the stress is a stress of tension, in the other of pressure ; 

 in the one case the giving way is by fracture, in the other by 

 melting. The differential motion in the one case is by fracture, 

 change of position and regelation ; in the other by melting, change 

 of position and regelation."' 



These of course are not Thomson's words, but only a condensed 

 statement of my own of the view I got from studying Thomson. 



Very respectfully, 



Berkeley, Cal., Joseph LeConte. 



March 28, 1888. 



* ' Elements of Geology,' p. G03. 



