

410 OBSERVATIONS IN CHEMISTRY AND MINERALOGY. 



well-developed praseosalt-forms come up in astonishing abundance. This 

 experience was gained in working with mohawkite and the kindred arsenids from 

 Mohawk Mine, Mich. 



Forms of the Praseocobaltioctaminosulfate. — The single crystals assume 

 oftenest the block-form (Plate XXXVI, fig. 1, a, 6, c, d, e) or the staff-form (fig. 

 l,h); the book-form (g, fig. 1) and the -paddle-form, f (which is a combination of 

 staff and block), are much less often seen. Owing to the rather deep color of 

 even very small crystals it is not possible to determine the system of crystallization 

 with certainty, but such tabular forms as d, fig. 1, leave little room for doubt that 

 the system is monoclinic. Among the numerous twin forms the saw-buck or chi 

 type is frequently seen. It is represented much magnified at a and b, fig. 3. 

 Right angle or lateen crosses are not uncommon. A modification of the chi we 

 find in the tongs-type, f, /, fig. 3, as well as in the large, ragged shape at d, fig. 2. 

 The very bizarre composite forms which may be named the bird, bug or butterfly 

 types, b, c, fig. 2, have undoubtedly the triplet saw-buck, a, fig. 3, as crystallo- 

 graphic base. The apparent curved lines as in the body portion of the butterfly, 

 b, fig. 2, which look so very ungeometrical, or rather uncrystallographical, result 

 from very rapid and interrupted growth. In the bug-form, c, fig. 2, the body 

 represents a conical shape brought about by same interruption of growth and 

 especially the legs are due to this same cause. The strange digitate type form e, 

 fig. 3, as well as the grass type, g, h, fig. 3, are referable to combinations with 

 the angle-templet type c, fig. 3, as base. The burr type is very common, d, fig. 

 3 ; it is a variation or rather combination of several groups of type c, fig. 3. 



When cobalt-ions preponderate in test solution there form swarms of very 

 dark-green, almost black, seemingly cubic, crystals. These probably are the 

 products of nearly rectangular twinning, the base of which seems to be repre- 

 sented by a, fig. 2. Such prismatic forms as e, g, fig. 2, are incomplete terminally 

 with strangely curved lines and are not understood by me. 



Fig. 4 shows the microscopic picture which is obtained by dissolving 5 mg. of 

 roasted mohawkite in 2 drops of hydrochloric acid and one drop of nitric acid; 

 by evaporating the solution to dryness; by adding 5 mg. of ammonium chlond 

 and sufficient water to dissolve all; again evaporating to dryness, adding three 

 drops of strong ammonia solution, stirring with capillary pipette, and transferring 

 one drop to slide with about 3 mg. of ammonium persulfate. Liquid was in- 

 tensely blue with an indefinable precipitate. Soon the lozenge and prismat 

 crystals of cupritetraminosulfate developed, intensely purplish-blue m co o , 

 they grew ultimately so large that they became quite visible to the nakea^ y^- 

 In measure as the crystals grew the field became more and more c °|° r ^ o ' d 

 rather lightly colored. When at last the preparation dried, the field sn^ 

 pale greenish patches of nickel salt, much ammonium sulfate and amn \ ible 

 chloride forming an arborescent veil over all. No praseo crystals were^ ^ 



A drop of strong ammonia water was allowed to run over the sur ace. ^ 

 mmonia redissolved all but the copper salt. As evaporation proeee 



