PRESSURE IN FORMATION OF ROCKS AND MINERALS 741 



are so much alike chemically as are the silicates are usually com- 

 pletely miscible under all conditions. 



But the effect is of another order of magnitude in presence of a 

 gas phase which may enter into reaction or solution with other com- 

 ponents; and consequently this circumstance must be taken into 

 consideration in any discussion of the behavior of the magma, 

 which undoubtedly contains volatile components of this character. 



TABLE III 



The Solubility* of CaCOj at 16° and Its Dependence upon the Partial 

 Pressure oe CO2 over the Solution 



Partial Pressure of COa 

 in Atmospheres 



Solubility 



Parts CaCOj per Million 



Parts Water 



0.0 



14-3 



0.00037 



46. I 



0.00050 



74.6 



0.0033 



137.2 



0.0139 



223.1 



0.0282 



296.5 



0.0501 



360.0 



0.142 



533 -o 



0.254 



663.0 



0.417 



787.0 



0-5S3 



885.0 



0.730 



972.0 



0.984 



1,086.0 



*Data according to Schloesing {Compt. rend., LXXIV[i872], 1552; LXXV [1872], 70, except the first 

 two, which are due to Kendall [Phil. Mag., XXIII (191 2), 958-76]). 



As a simple example, consider the amount of CaCOj which will 

 dissolve in water inclosed in a vessel with various partial pressures 

 of CO2. In pure water it is 14 parts per million, an amount which 

 is trebled by the presence of as little as o . 00037 atm. CO2 (3 . 7 parts 

 in 10,000, about the proportion of CO2 in ordinary air) and has 

 increased by more than 70 times when the CO2 pressure reaches 

 I atm. (see Table III). This great increase is an example of the 

 very great effect of increase of concentration (partial pressure) of a 

 gaseous component; it is, of course, due to the formation of calcium 

 bicarbonate in the solution, but this circumstance in no wise affects 

 the force of the argument. Indeed, this is an excellent illustration 



