373 



present before the decay is in any way visible to the eye M 

 (p. 46). 



The presence of carbonates, indicated by the reaction, 

 must of course be due to the carbonic acid of the water oozing 

 through the rocks. In almost all regions, but the Arctic, this 

 gas will be present in rather considerable quantities produced, 

 as it is, mainly from organic sources, and its tension may rise 

 to 3 per cent or even higher. It cannot be inferred therefore 

 from Bischof's experiments (carried on at a CO^-tension of 

 100*^/0) or observations whether a certain tension of carbonic 

 acid be necessary for the action on sihcate or not, but at first 

 sight it would seem probable that it was so. My determinations 

 of the tension in Disko-waters show however that it is not, 

 and reveal the fact that basaltic rocks will absorb carbonic 

 acid down to a tension of less than 0.5 tenthousand-parts. All 

 the observations 1 have made on the carbonic acid in Disko- 

 fresh waters serve to conlh'm this conclusion. 



Before I invented the shaking-method of tension-determi- 

 nation 1 made some analyses of air-bubbles from the bottom 

 of rivers and small ponds. A glacier-river very often forms a 

 tract of comparatively level ground at its mouths , and by 

 running over this is sometimes shifts its course from hour to 

 hour. Considerable quantities of air become hereby enclosed in 

 the loose sand of the bottom, and after some time tension- 

 equilibrium will be established between the bubbles of air and 

 the surrounding water. If this is at rest the tension may 

 become 0, if it is rapidly renewed from above, the tension 



') Some Authors (Ebelmen: Ann. des Mines, sér. 4, T. 7, 1845 and 

 Hunt: American Journ. of Sc. and Art. ISHO) have held that all 

 silicates must be decomposed by carbonic acid, but this opinion, on 

 which Hunt bases some verv fantastic speculations concerning the 

 carbonic acid in the atmosphere, is entirely hypothetic and was dis- 

 proved, several years before Hunt wrote, by Bischof who found the 

 effervescence of carbonic acid only in such rocks as contained silicate 

 of lime (oligoclase and anorthite) 



