442 CONSTITUTION AND VOLUME OF MINERAL SPECIES. [XVII. 



analyses, and rendered uncertain "by doubts as to the equiva- 

 lent weight of silicon. 



[An important point in the question of homology and homceo- 

 morphism was then referred to in the following language : 



"The similarity in crystallization between species whose 

 formulas differ only in the elements of water has been pointed 

 out by Laurent in certain salts of organic acids, and is seen in 

 several mineral species. The chabazites, for example, give 

 the formula 3RO,Si0 8 . 3Al 2 3 ,2Si0 8 , w ith 15HO and 18HO, 

 while the variety ledererite affords, according to Hayes and 

 to Rammelsberg, but 6HO. The hydrous iolites are also cases 

 in point, as well as aspasiolite, the serpentines, and the talcs, 

 with their varying proportions of water. In the formulas of 

 these species, water appears to replace magnesia, and Scheerer 

 has shown that many different species may be referred to a 

 common chemical type, by admitting 3HO to replace MgO, 

 and 2HO to replace CuO, etc. These cases, to which he has 

 given the name of polymeric isomorphism, are but instances 

 of the partial substitution of water for other bases in homolo- 

 gous genera which differ by nMO." 



[In the continuation of this subject, in 1854, as above re- 

 ferred to, the question of homologies was further illustrated 

 by the neutral and the basic nitrates of lead, represented by 

 a common formula (Pb 2 2 )n . 2s T 2 10 . " These salts vary in 

 solubility and in physical characters, but resemble each other 

 in yielding nitric acid and oxide of lead as results of their de- 

 composition, and are completely analogous to the homologous 

 series of Gerhardt, which differ by n(C 2 H 2 ). From the rela- 

 tion between basic and hydrated salts the same view is to be 

 extended to the latter, and species differing by n(0 2 H 2 ) and 

 n(() 2 M 2 ) may thus be homologous. The above formulas are 

 intended to involve no hypothesis as to the arrangement of the 

 elements, for in the author's view, each species is an individual, 

 in which the pre-existence of different species that may be ob- 

 tained by its decomposition cannot be asserted. He regards 

 silic.it.'> like eudialyte, sodalite, and pyrosmalite as oxychloridrs, 

 (M 8 0j)n . MCI, and nosean, hauyene, and lapis-lazuli as basic 



