381 



As shown in the preceding chapter basaltic rocks must 

 have the power of precipitating Ca CO.^ from the bicarbonate- 

 solutions, because they combine with the free carbonic acid and 

 constantly diminish the tension. 



In Denmark however, where basaltic rocks are not to be 

 found, the dissolved carbonate of lime is not deposited, until 

 the water arrives at the surface of the ground, Avhere several 

 causes m\l produce a diminution of the tension of carbonic 

 acid and consequently the deposition of a corresponding quantity 

 of lime. The most important of these causes is the atmo- 

 sphere , towards which the surplus of carbonic acid readily 

 diffuses M. Well-known instances of this process are the petri- 

 fying springs which sometimes give rise to extensive layers of 

 amorphous limestone''^), but also the chalk-marl-deposits in many 

 lakes owe their origin mainly to it. 



Not a few investigators have studied the formation of these 

 last-mentioned deposits, but though many valuable observations 

 have been made concerning the special forms of the deposits, 

 the general problem involved seems to have been somewhat 

 overlooked, and some of the papers published have, no doubt, 

 done more to confuse the question than to solve it. 



^) Bischof {Lehrbuch ... I pp. 100 — 108) has made a series of experi- 

 ments in order to investigate this process. He prepared solutions of 

 bicarbonate of lime by saturating water with carbonic acid and car- 

 bonate of lime, but he also experimented upon natural waters from 

 wells. He found that all these waters deposited carbonate of lime, when 

 atmospheric air was allowed to bubble through them for a sufficiently 

 long time. It is remarkable, however, that the greater part of the free 

 carbonic acid was thereby got rid of in a comparatively short time, 

 whereas large quantities of air were often necessary in order to cause 

 precipitation of the lime. In these cases the deposit was sometimes 

 crystalline and continued to appear after the stream of air had ceased. 

 I cannot satisfactorily explain these phenomena, but I think they 

 must be due either to a formation of supersaturated solutions of Ca CO^ 

 or to the probably very small reaction-velocity for the dissociation of 

 CaH^C^Oe as previously mentioned (p. 362). 



*) See Bischof: Op. cit. vol. I p. 545. 



