14 Organic Acids in the Examination of Minerals. 



Dolomite, treated as above, also afforded some good crys- 

 tals of hydro-calcium tartrate, very easily recognized among 

 other irregular fragments or masses of no particular form. 



Neutral calcium tartrate, obtained by treating a concen- 

 trated solution of tartaric acid with an excess of calcite, gave 

 very minute needles, which in almost every case were grouped 

 in small radiated circular masses, much resembling the spher- 

 ulites of volcanic rocks. The outlines of these groups are 

 sharp and distinct, always perfectly round, sometimes having 

 the central part transparent, but no particular crystalline form 

 could be made out. 



Hydro-barium tartrate (Fig. 13), from witherite and tartaric 

 acid, crystallizes in very delicate, long, almost capillary, acicu- 

 lar crystals, which are often grouped in bundles, the needles 

 lying parallel to each other. A few characteristic crystals, 

 probably hydro-calcium tartrate, having the form and faces 

 (as previously mentioned) of prism and dome, were easily 

 recognized ; they doubtless proceeded from the calcium con- 

 tained in the witherite. It is interesting to be able so easily, 

 by the aid of the microscope, to distinguish a small percentage 

 of calcium in the presence of a large quantity of barium. 

 The needles of bydro-barium tartrate give fine colors in polar- 

 ized light. 



Oxalic acid forms long prismatic crystals, quite stout, and 

 generally terminated with domes, similar to those faces in 

 hydro-calcium tartrate ; but apparently of much flatter angles. 

 When carefully made, the crystals are sharp and distinct in 

 outline and terminations, but the mass much oftener crystal- 

 lizes in indistinctly radiated groups, in which the forms can 

 scarcely be distinguished. Oxalic acid, as is well known, gives 

 beautiful colors in polarized light. 



Calcium oxalate is an amorphous powder, and, even under 

 the microscope, appears to be in minute masses, almost opaque 

 and with no distinguishable crystalline characteristics. 



Hydro-barium oxalate, from witherite and oxalic acid (Figs. 

 14-16), crystallizes in large, beautiful forms, very character- 

 istic, and different from anything before noticed. The crystals 

 have the shape of a spear-head or arrow-head, being built up 



