XVIL] CONSTITUTION AND VOLUME OF MINERAL SPECIES. 447 



dipyre By its hardness, its specific gravity, and its 



indifference to acids, jadeite is completely separated from the 

 wernerite group, and takes its place alongside of zoisite or saus- 

 surite, with the garnets, idocrase, and epidotes. The following 

 table will serve to show the relations of the new species : 



Density (about) 

 2.7 3.3 



Oxygen ratio 3:2:1. . . . Meionite . Saussiirite. 

 Oxygen ratio 6:2:1, . . . Dipyre . . Jadeite." 



[The hardness of jadeite, according to Dauiour, is between 

 that of orthoclase and quartz. It is lamellar in structure and 

 exhibits two axes of polarization. Unlike saussurite, it is not 

 attacked by acids after fusion, a fact which is to be ascribed 

 to the large proportion of silica which it contains.] 



[Professor J. P. Cooke described, in 1860 (American Journal 

 of Science (2), XXX. 194), some curious examples in the 

 crystallized alloys of antimony and zinc, of considerable varia- 

 tions in composition without change in crystalline form. These 

 cases, as remarked by him, do not come within the limits of 

 isomorphism as generally understood, and hence he concludes 

 that " the composition of a mineral species may be modified 

 by an actual variation in the proportions of its constituents." 

 These alloys of varying composition are to be regarded in part as 

 examples of a progressive series of isomorphous compounds of 

 antimony and zinc, of high equivalent, differing from each other 

 by nZn 2 , and in part, doubtless, as crystalline mixtures of these 

 isomorphous homologous species. The principle embodied in 

 the conception advanced by Professor Cooke, and rightly re- 

 garded by him of great importance to a correct science of min- 

 eralogy, he has named allomerism. It is evidently a case of 

 homologous and isomorphous relations between members of a 

 progressive series, a general principle upon which I have in- 

 sisted throughout the pages of this paper, and which includes 

 the polymeric isomorphism of Scheerer.] 



