Organic Acids in the Examination of Minerals. 13 



observed were triangular-shaped crystals, of very beautifully 

 sharp and distinct outline. The smaller ones appeared to be 

 without any modifications, as in Plate I, Fig. 4, but some of 

 the larger and thicker ones had faces and planes on the angles 

 and edges similar to those in Fig. 2. The same crystals 

 viewed in a different position appeared as in Fig. 5, which 

 forms were often grouped, together with the triangles, into 

 stellate masses, Fig. 6. Flat, tabular crystals, apparently 

 with domes and prismatic faces, were also observed. They 

 were remarkable, owing to the terminations being different on 

 the same crystal, thus occasioning a somewhat wedge-shaped 

 form, as in Figs. 1-3. These tabular crystals are quite large, 

 and often have inclosures of small acicular crystals and cavi- 

 ties. Very small needle-like crystals were also observed in 

 the same slide as those already mentioned. 



Hydro-calcium tartrate (Figs. 7 to 12). — From a solution of 

 calcite in an excess of tartaric acid, crystals were obtained, 

 strongly resembling orthorhombic or monoclinic forms, consist- 

 ing of prism and dome, as shown in the figures. These crystals 

 were quite small, but very beautifully sharp and distinct, and 

 gave bright colors in polarized light. The figures show forms 

 almost identical, but seen in different positions. The crystals 

 were then twice boiled with water, to remove any excess of 

 tartaric acid, the only effect being to mass the crystals in groups 

 and to round their angles somewhat, although some good indi- 

 viduals were seen. The water solution was then evaporated 

 and some very sharp and good crystals obtained, having the 

 same characteristic face and angles as in the figure, but gen- 

 erally larger and sharper than those before observed. These 

 forms, then, belong undoubtedly to hydro-calcium tartrate, 

 tartaric acid being entirely too soluble to have furnished 

 crystals in this way. Sometimes the crystals are in long, 

 slender prisms, but always with the characteristic dome-like 

 termination ; and in some of the larger ones the piuacoid is 

 present. 



Gurhofite, treated with tartaric acid, gave the forms 

 already described, and also some transparent masses of irregu- 

 lar shape, having no crystalline characteristics. 



