Formation of Minerals by Igneous Action, 325 



lustre. Chromite of Mang anese vfs^s obtained by igniting together 

 oxide of chromium, oxide of manganese, and boric acid. The crystals 

 were regular octahedrons, of a semi-metallic iron-gray colour, of 

 density 4*87, and of a hardness sufficient to scratch quartz. Chro- 

 mite of Zinc was procured in the same manner, and obtained in the 

 form of small, very brilliant, regular octahedrons of a greenish-black 

 colour, and of density 5'309, at 11° C, The atomic volumes of the 

 chromites were found somewhat higher than those of the aluminites, 

 that of chromite of zinc being 27' 5 ; of chromite of magnesia 28*7 ; 

 of chromite of iron 28-3. Ferritc of Zinc was obtained in the form 

 of small black brilliant octahedrons, of density 5*132. Its atomic 

 volume is 29'3. -. ! n tuic? 



The formation and properties of artificial Chrysoheryl, the alumi- 

 nate of glucina, were described by the author in a memoir, in the year 

 1847. By repeating the experiment, with the addition of carbonate 

 of lime to the mixture of alumina, glucina, and boric acid, a perfectly 

 pure Chrysoberyl was obtained in transparent crystals 5 or 6 milli- 

 metres in length. These crystals have a slightly greenish tint, are 

 hard enough to scratch topaz, and of density 2*759 at 12° C. The 

 crystalline form was found to correspond perfectly with the deter- 

 minations of Descloizeaux, made with the natural mineral ; many of 

 the crystals were similar to those found in North America, in Brazil, 

 and the Ural. Chrysolite or Peridot was obtained by fusing toge- 

 ther silica, magnesia, and boric acid ; the crystals were several milli- 

 metres in length, perfectly transparent, but slightly yellow ; their 

 form was an octahedron, with a rectangular base deeply truncated 

 upon its two summits. The angles measured agreed perfectly with 

 those of the mineral species. A borate of alumina, having the for- 

 mula (Al^ 0^) BO^ was prepared by igniting alumina with borax, 

 or by igniting alumina, oxide of cadmium, and boric acid. The 

 crystals were rectangular prisms from six to ten millimetres in 

 length, transparent and colourless, and hard enough to scratch 

 quartz. The addition of silica to the mixture prevented the forma- 

 tion of a borate, and the alumina then crystallised in large hexa- 

 gonal crystals of a very high lustre. These crystals are double six- 

 sided pyramids, deeply truncated upon their two summits; tho 

 measured angles exactly corresponded with those of Corundum. 

 The density of the crystals was found to bo 3*928 at 20° C. ; their 

 hardness was that of Corundum, and they easily scratched topaz. 

 In this process the silica may bo replaced by other substances ; 

 carbonate of baryta gave large crystals of the same form ; carbonate 

 of soda produced the same effect, and several other bodies were 

 employed without essentially modifying the result. Butile was 

 obtained in long acicular prisms by igniting a mixture of titanic 

 acid with the phosphate of soda and ammonia; the crystals were 

 transparent, and of a golden-yellow colour ; their density was 4-283, 

 which agrees with that of rutile. It will be remembered that 



