Organic Acids in the Examination of Minerals. 15 



of many small crystals arranged on a straight rib or spine 

 somewhat like a feather. The spear-headed forms are deeply 

 striated or furrowed, and inclose large fluid-cavities ; they 

 give beautiful colors also in polarized light. A few simple 

 crystals, as in Fig. 14, are apparently monoclinic or triclinic ; 

 and the edges or sides of the feather-like forms generally 

 terminate in similar small, distinctly outlined crystals, placed 

 on the extremity of the lateral ribs. 



Yery striking were the forms (Figs. 17, 18) obtained from 

 treating baryto-calcite with oxalic acid. They consisted, first, 

 of the large spear-headed or feather-shaped crystal groups, as 

 shown in the sketch, in which the mode of formation is very 

 easily seen, especially the simple crystals terminating the 

 lateral ribs. The latter are curved upward a little, but seem 

 to be nearly at right angles to the main spine or axis. Besides 

 these forms, amorphous calcium oxalate was observed, and 

 also some long, columnar crystals of oxalic acid. By allowing 

 the calcium oxalate to settle, and decanting and crystallizing 

 the clear solution, the best and cleanest crystals of hydro- 

 barium oxalate can be obtained, of tolerably large size. 



Hydro-strontium oxalate (Figs. 19, 20), from strontianite 

 and oxalic acid, gave very small crystals, somewhat resem- 

 bling those of hydro-calcium tartrate, but apparently with 

 much flatter angles. One form, Fig. 20, seemed to be made 

 up of prism, pinacoid, and domes, probably orthorhombic or 

 monoclinic. Other crystals, probably pyramidal, appeared to 

 be minute rhombs (Fig. 19), sometimes in groups. 



Ferrous oxalate (Fig. 21), from siderite and oxalic acid, 

 gave yellow crystals, exceedingly minute, but nevertheless 

 quite distinctly outlined. Their form was prismatic with a 

 flat or basal termination. 



The same crystals were abundantly obtained from ankerite 

 also ; but the crystalline mass was of a lighter yellow than 

 that formed from siderite, owing to admixture of calcium 

 oxalate. 



Formate of lead, from cerussite and formic acid, crystallized 

 in quite large acicular forms, sharp and transparent, but 

 terminated with rounded faces, the exact nature of which 



