XVII.] CONSTITUTION AND VOLUME OF MINERAL SPECIES. 439 



this method has led, and to show that we must assign to most 

 mineral species much higher equivalent weights than have 

 hitherto been admitted. 



It was further asserted that a relation similar to that ob- 

 served in the formulas of allied hydrocarbonaceous bodies, 

 and designated as chemical homology, exists in the formulas 

 of mineral species. We have said that these formulas, de- 

 duced from the results of analysis, are not to be looked upon 

 as expressing any pre-existing relations in the constitution of 

 the species, which is not to be regarded as a compound, but as 

 an individual, in which the so-called chemical elements have 

 no actual existence. The arrangements of these in our formu- 

 las only serve to make apparent the numerical relations which 

 have been found to govern the transformations of the higher 

 species. 



The formulas of homologous bodies may be represented as 

 series in arithmetical progression.* The first term may be the 

 same as the common difference, and the series is then 



b, 2b, 36... n&, 



as in the hydrocarbons C 2 H 2 , C 4 H 4 , C 6 H 6 , etc. If the first 

 term is unlike the common difference, the series is 



a, a-f- b, a-J-26,...a-|-n&, 



of which the ammonias, NH 3 , NH 3 + C 2 H 2 , NH 3 + 2C 2 H 2 , etc., 

 are examples. Both of these cases are illustrated in the chemi- 

 cal history of mineral species. 



In the paper already referred to it has been shown, from 

 the relations of carbon, sulphur, and oxygen on the one hand, 

 and of hydrogen and the metals on the other, that M 2 S 2 , 

 M 2 2 , and H 2 2 (M representing any metal) may be compared 

 with H 2 C 2 . This view will be applied in extending the appli- 

 cation of the principle of homology. The sesqui-oxides like 

 ferric oxide, chromic oxide, and alumina, will be regarded as 



[* The conception of progressive series in chemical compounds is generally 

 ascribed to Gerhardt, who made it widely known in his Precis de Chimie 

 Organique, but appears to have been first enunciated by Dr. James Schiel of 

 St. Louis, in 1842, in Wohler and Liebig's Annalen, Vol. XLIIL, page 107. 

 See farther the American Journal of Science (2), XXXII. 48, where Dr. Schiel 

 has developed the whole question of series in a very complete manner.] 



