98 Hidden and Hillebrand — MacJcintoshite, a 



tical axis 6. The bisectrix lying nearest this axis is that of 

 least elasticity C. The salt is not, however, properly biaxial, 

 the plane of the optic axes is sometimes parallel, sometimes 

 perpendicular to the edge of the prism. Moreover this direc- 

 tion often changes from place to place in the same plate and 

 at times no bar is seen, but a black dot in the center of the 

 field surrounded by rings. Such behavior can be explained by 

 supposing the crystal to be in a condition of internal strain. 

 Sections parallel to the prism sometimes remain light between 

 crossed nicols, sometimes extinguish at varying angles and 

 show a slight pleochroism, the absorption being e > co ; the color 

 a deep reddish orange. 



In conclusion the author wishes to express his indebtedness 

 to Prof. H. L. Wells for valuable advice in connection with 

 the present investigation and to Prof. S. L. Penfield under 

 whose direction the crystallography of these salts was investi- 

 gated. The author is also indebted to Mr. L. Y. Pirsson for 

 aid in the optical description of these salts. 



Sheffield Scientific School, March, 1893. 



Aet. XIII. — On Mackintoshite, a new thorium and urani/um 

 mineral ; by Wm. Eakl Hidden :■ with analyses by W. F. 

 Hillebrand. 



History. — It is three years since an alteration product* of 

 the new species here described was announced jointly, by the 

 late James B. Mackintosh, f and the writer, under the name of 

 " thoro-gummite." In the interim a most diligent search has 

 been made at the locality in Llano County, Texas, to discover the 

 mineral in quantity, and also, if possible, the parent mineral from 

 which it was derived. In an endeavor to find some fragments of 

 the unaltered mineral, I broke up nearly a kilogram of the thoro- 

 gummite, with the result of finding less than one gram of a 

 black mineral which, from the position it occupied within the 

 thoro-gummite, we concluded must represent the original 

 species. I note here that some irregular cores, of a dark brown 

 translucent mineral, were also found but they proved to be 

 very soluble and not essentially different from the enclosing 

 thoro-gummite. They had a density of 4*50. 



Some chemical tests, then made by the writer, proved the 

 black mineral to be practically unaffected by hydrochloric or 

 sulphuric acids and the density as then determined was 5-361. 



*This Jour., vol. xxxviii, Dec, 1889, p. 480. 



f He died on April 15, 1891 ; see this Journal, xli, 444. 



