in Metalliferous Veins. 15 



zinc-copper element ; a proof that it is not the intensity of the 

 current from the electrometers, but the resistance to conduc- 

 tion that exerts the chief influence on the degree of the effects 

 observed. This is probabW the principal cause, too, why with 

 a non-metalUferous point the effect is never very great ; for the 

 resistance to transition from the disc to the bad conducting 

 rock, with proportionally fewer and less perfect points of con- 

 tact, must be much greater than from a mass of ore still firmly 

 imbedded in the rock even when this is small, but then parti- 

 cularly when it is of great dimensions. 



On the Glaciers of the Alps. By M. J. Andre de Luc* 



I HAVE read ^nth interest the notice on glaciers by M. 

 Agassiz, inserted in the preceding number of the Bibliotheque 

 Universelle.f It contains many ingenious ideas, but they re- 

 quire to be confirmed by observations, continued without in- 

 terruption during a long sojourn in the vicinity of glaciers, and 

 conducted with the same care that Saussure bestowed on his, 

 after which, I had supposed, nothing new of any importance 

 could be brought forward on the subject. 



M. Agassiz attributes the progressive movement of glaciers 

 to the dilatation of the water transformed into ice ; but the 

 congelation of water cannot take place except near the sur- 

 face, for, if the glacier be 100 feet in depth, ^ more than nine- 

 tenths of this thickness can undergo no change of temperature, 

 since ice is so bad a conductor of heat, that the water infiltering 

 into its fissures cannot be frozen, whatever may be the season 

 of the year. For this reason, the explanation of the motion 

 of glaciers from the expansion of congealed water, cannot be 

 admitted, with the exception perhaps of three or four feet at 

 tlie surface, although this effect must be very insignificant ; 



■" From Bibliotheque Univcrsellc dc Geneve, 18:J1). 



t A translation of this appoarod in our last Nuiiibt-r, p. ',M\?> ; and a trans- 

 lation of an essay on the same subject, read by M. A;j:assiz to the Helvetic 

 Natural History Society, was published in vol. xxiv. of this Journal, p. 'M'A. 

 —Ed. 



X According to Saussure, the Glucierdes Bois, iutLe valley of Cluuuouni^ 

 is from «0 to mo feet in deptii. 



