WEAKNESS OF ACIDS COMPENSATED BY QUANTITY. 93 



Ostwald divides the acids into strong, moderately strong, weak, and 

 very weak. The acids H 2 S0 4 , HC1, and HN0 3 , are strong acids. The 

 acids H 2 S0 3 and H 3 P0 4 are moderately strong. The acids H 2 S, H 3 B0 3 , 

 and H 2 C0 3 are weak acids. The acids of silica are very weak." 



The strong acids H 2 S0 4 , HC1, HN0 3 , when present in ground solu- 

 tions, as they sometimes are, of course take possession of the bases in 

 proportion to their quantity. However, in the crust of the earth strong 

 acids are not abundant on the average, although under exceptional con- 

 ditions, as in volcanic districts, they may be rather plentiful. Also the 

 moderately strong acids H,S0 3 and H 3 P0 4 are not abundant, although 

 phosphoric acid is rather widespread. Of the weak acids H 2 S and H 3 B0 3 

 are not plentiful. The two great acids of nature are carbonic and silicic 

 acids, and the major contest in the rocks, so far as the acids are concerned, 

 is between the weak carbonic acid and the very weak silicic acid. These 

 two acids are everywhere very abundant in the rocks. While, therefore, 

 the moderately strong and the strong acids play a relatively important 

 part in proportion to their quantity, one weak and one very weak acid, 

 because of their dominant quantity, under the law of mass action play 

 the greatest part in rock alterations; and in the contest the very weak 

 acid, silicic, holds its own against the weak acid, carbonic, partly because 

 its far greater abundance compensates for its relative weakness. The fact 

 of the formation of carbonates and the simultaneous decomposition of 

 the silicates under surface conditions the world over is well known. 

 (See pp. 163, 473-486.) The partial explanation of the phenomena is the 

 relative abundance of carbonic acid under the conditions in the zone of 

 katamorphism. As shown in another place (see p. 479), the reaction is also 

 one which liberates heat, and this is a favorable factor in the process. 



In the zone of anamorphism, where the pressure is great, the reaction 

 of the upper zone is reversed. (See pp. 173-178, 677-679.) The replace- 

 ment of carbonic by silicic acid results in decrease in volume (see p. 177). 

 Therefore, under the great pressures of the zone of anamorphism, 

 the relative volumes of the original and secondary compounds is a most 

 important, probably dominant, factor in the process. But also it is 

 probable that at the high temperatures and pressures which obtain in the 

 lower zone silicic acid gains strength as compared with carbonic acid. 



a Foundations, cit., p. 55. 



