154 G. F. Becker — Silicic Acids. 



Art. XX. — Silicic Acids ; by George F. Becker. 



The silicic acids usually assumed to be needful to account 

 for the natural silicates are ortho-, meta-, poly-, and disilicic 

 and these are represented by the formulas H 4 Si0 4 , H 2 Si0 3 , 

 H 4 Si 3 8 and H 2 Si 2 0\ Professor P, Groth, however, points 

 out* that polysilicic acid may be regarded as compounded of 

 disilicic and metasilicic acids, since H 2 Si 2 6 -f H 2 Si0 3 =H 4 Si 3 8 . 

 This method of viewing these compounds has the advantage of 

 reducing the acids to three in number. On the other hand 

 polysilicates (orthoclase and albite) are among the most abun- 

 dant substances in the accessible portion of the earth, while 

 disilicates are extremely rare, and are commonly considered as 

 being represented only by petalite and milarite. The latter is 

 known in but one locality, and petalite is very sparsely distrib- 

 uted. If disilicic acid is really a constituent of the alkaline 

 feldspars, it seems strange that it should not more often appear 

 independently. The other hypothetical component of poly- 

 silicic acid is little less common than polysilicic acid itself. 



The series of acids may be simplified in another way to 

 which I have seen no reference. Orthosilicic and polysilicic 

 acids may be conceived as united to form metasilicic acid, for 



H 4 Si0 4 +H 4 Si 3 O e = 4H 2 Si0 8 . 



On this hypothesis also the acids are reduced to three, and, 

 furthermore, the acids supposed to unite are each very abun- 

 dant in nature. This is a distinct advantage. There seem also 

 some grounds in the behavior of the silicates favorable to this 

 view of their constitution. It is known that the metasilicates 

 of a metal are more stable than the orthosilicates. Now if a 

 molecule of an orthosilicate combines with a molecule of a 

 polysilicate, the union of the two must be attended with the 

 liberation of heat and the compound must be more stable than 

 its proximate constituents. In so far the behavior of the meta- 

 silicates accords with the hypothesis. 



Again, the most important metasilicates are the amphiboles 

 and pyroxenes, and, as Groth remarks, these two series afford 

 most remarkable examples of the crystallization of the same 

 substances in forms belonging to three different crystallo- 

 graphic systems, yet presenting extraordinary similarity in 

 their angles. Such relations seem more readily reconcilable 

 with the hypothesis that these ferro-magnesian minerals are 

 double compounds of two acids than with the theory that they 

 are substitution products of a single acid. One can easily im- 



* Tabell. Uebersicht der Min., 1882, p. 75. 



