444 CONSTITUTION AND VOLUME OF MINERAL SPECIES. [XVII. 



tion that there are mixtures of homologous fatty acids which 

 cannot be separated by crystallization, and have hitherto been 

 regarded as distinct acids. The author insists that the possi- 

 bility of such mixtures of related species should be constantly 

 kept in view in the study of mineral chemistry. The small 

 portions of lime and potash in many albites, and of soda in 

 anorthite, petalite, and orthoclase, are to be ascribed to mix- 

 tures of other feldspar-species." 



[The above extracts are from the author's abstract of his 

 paper in the American Journal of Science for September, 1854. 

 There might be found reason to-day for modifying the formu- 

 las above given for petalite and orthoclase, but I leave them as 

 they were written twenty years since. 



[These views of mine with regard to the triclinic feldspars 

 have since been generally accepted, but by an oversight they 

 are attributed to Tschermak, who, so far as I am aware, first 

 announced them ten years later, namely, in 1864 (K. K. Aca- 

 demic Wissenschaft, Wien). He there stated that with the 

 exception of the baryta-feldspar, hyalophane, and the boric 

 feldspar, danburite, the feldspars were reducible to three spe- 

 cies, namely, adularia (orthoclase), albite, and anorthite, hav- 

 ing a common formula, wh'ich, adopting the equivalent weights 

 used by me above, becomes as follows for the two triclinic 



species : 



Anorthite Ca 4 *al 6 al a si M M 

 Albite Na, a! 8 si 8 si w M 



This, which is but my common formula divided by two, is by 

 Tschermak also assigned to orthoclase. He, while admitting 

 that the potash-soda feldspars are made up of alternations of 

 orthoclase and albite, as Gerhard had shown in the case of 

 perthite, further concludes, as I had already done, " that oligo- 

 clase, andesine, and labradorite appear to be members of a 

 great series, with many transitional forms, and may be regarded 

 as isomorphous mixtures of albite with anorthite, sometimes 

 with small admixtures of orthoclase." 



[My views on the gradation into one another of the triclinic 

 feldspars are again referred to in my Contributions to Lithology, 



