CLARKE: CHEMICAL- STABILITY 343 



If we study the occurrence of compounds in the mineral king- 

 dom, we shall find many illustrations of the influence of mass 

 equilibrium. Among the oxides, corundum is noted for its 

 stability and inertness towards solvents, and in it aluminum 

 and oxygen are combined in the ratio 54 to 48. Quartz, 28: 

 32, is also remarkably stable. Among sulfides, pyrite, with the 

 ratio 56 to 64, is characteristic, and the fluoride, fluorspar, 40 

 to 38, is equally so. The fluorides of strontium and barium are 

 as yet unknown as natural minerals, and these metals combine 

 preferably to form sulfates in which the ratios of metal to S0 4 

 are simpler. Of the three fluorides, moreover, that of calcium 

 is the least soluble, while calcium sulfate dissolves in water 

 quite perceptibly. 



When silica and alumina are combined magmatically, that is 

 in dry fusion, only one silicate, sillimanite, AlSi0 5 , is formed. 

 This is the one compound, among the several which are theoreti- 

 cally probable, in which the ratio Al to Si + is the simplest, 

 54 to 108 or i to 2. The silicates which contain aluminum with 

 other metals are too complex in their ratios to admit of accurate 

 comparison; but it is noteworthy that the relatively simple 

 anorthite, nephelite and leucite are easily crystallized from 

 fusion, while the more complicated albite and orthoclase are 

 not so readily synthesized. 



There is one more general relation in the mineral kingdom 

 which is most suggestive. The elements of low atomic weight, 

 exclusive of the halogens, are chiefly found as oxidized compounds. 

 Sulfides appear first with calcium, as the mineral oldhamite, 

 found only in meteorites. Sulfides of vanadium and manganese 

 are rare, but from iron upwards they are abundant. Selenium 

 and tellurium are chiefly combined with elements of .high atomic 

 weight, and their few oxidized compounds are merely the products 

 of secondary alterations. Arsenic and antimony, in their primary 

 minerals, also seek companions in the higher portions of the 

 atomic weight scale, that is, from iron upwards. Heavy atoms 

 seem to choose heavy partners, altho the rule is by no means 

 universal. 



