GYPSUM AND SALT 347 



bility ; but, according to T. Sterry Hunt, the lime is all precipitated 

 in eight to ten days. 



When the water is saturated with carbonic acid, the chlorides of 

 the alkalies and alkali earths will form unstable supersaturated 

 solutions, from wliich the lime crystallizes out at low temperature 

 as the hydrous carbonate. The solution then retains only 0.8 

 gram of carbonate of lime per liter, corresponding to that dis- 

 solved by the CO... If, however, sodium and magnesium sulphates 

 are present in the solution in carbonated waters, the capacity of 

 these waters for carbonate of lime is nearly doubled. Thus car- 

 bonated water holding either of these sulphates in solution in the 

 proportion of i to lOO or less will take into permanent solution, 

 under ordinary temperature and pressure, a quantity of pure car- 

 bonate of lime equal to 1.56 to 2 grams in a liter. Thus only when 

 free CO.. is present in pure or mineral waters will CaCOj be held in 

 permanent solution. The precipitation of this lime may be brought 

 about by an abstraction of the carbonic acid necessary to hold it in 

 permanent solution. The abstraction may be caused by (Weed- 

 68:640) : 



Relief of pressure. 



Diffusion of the CO., by exposure to the atmosphere. 



Evaporation. 



Heating. 



Influence of plant and animal life. 



The first of these is mainly active in nature where springs issue 

 on the surface, while the second occurs on a more extended scale, 

 as in the formation of stalactites and stalagmites and in the encrus- 

 tations formed in petrifying springs. Here diffusion of the CO, is 

 facilitated by increase of surface of exposure, and in such cases 

 evaporation generally accompanies diffusion. The deposition of 

 lime through evaporation is illustrated by the tufa deposits of Lake 

 Lahontan. Deposition through heat, illustrated by the encrustation 

 in boilers, is of little significance in nature. Precipitation through 

 the influence of plant and animal life belongs under the head of 

 deposits of Biogenic type, and will be more fully discussed later. 



Deposits of Marine Gypsum and Salt. 



Gypsum and Salt of Marine Origin. Gypsum and salt 

 are as a rule the products of similar physical conditions of pre- 

 cipitation, though they do not by any means always occur together. 



