316 Intelligence and Miscellaneous Articles. 



are often grouped perpendicularly to each other, like the prisms of 

 staurolite. In the same way, with magnesia, peridote is obtained in 

 rectangular prisms. 



Alumina gives a silicate in elongated prisms, with oblique bases, 

 which are not attacked by acids, are infusible, and possess all the 

 characters of disthene. It is very interesting in this case to see 

 chloride of aluminium formed at the expense of the silicium. 



To form a double or multiple silicate, it is necessary not only to 

 mix the bases in suitable proportions, but also to furnish the oxygen 

 necessary for the formation of the silicic acid, by the addition of an 

 excess of one of the bases or of lime. 



A mixture of lime and magnesia furnishes crystals of diopside 

 perfectly colourless and transparent ; they present the broad trun- 

 cation and the bevel usual with augite. Seven equivs. of potash 

 or soda and one equiv. of alumina, or one equiv. of alkali, one equiv. 

 of alumina, and six equivs. of lime, produce by the action of chloride 

 of silicium, crystals in oblique prisms with obtuse bevelments, which 

 are scarcely acted upon by acids, and fuse before the blowpipe, pre- 

 senting, in fact, all the characters of the feldspaths. 



By the same process, varying the proportions and the nature of 

 the bases submitted to the chloride of silicium, I obtained silicates 

 presenting the crystallographic characters of willemite, idocrase, gar- 

 net, phenakite, emerald, euclase and zircon. 



By mixing the elements corresponding with the compositions 

 recently given by Rammelsberg for the magnesian and ferromag- 

 nesian tourmalines, adding an excess of magnesia or lime to furnish 

 oxygen to the silicium, I obtained in the midst of crystals of quartz 

 some very distinct hexagonal prisms, presenting in other respects 

 all the external and chemical characters of tourmaline. 



Chloride of aluminium may be used in the same manner as chlo- 

 ride of silicium. When passed over lime at a red heat, it produces 

 chloride of calcium and aluminium, in crystals belonging to two 

 types proper to corundum — the prism, and the acute double pyramid. 



The same reaction takes place with magnesia ; and besides, in this 

 latter case, a portion of the regenerated alumina may combine with 

 the excess of magnesia, so as to produce spinel, recognizable by its 

 crystals in regular octahedra truncated at the edges. Nevertheless 

 it is preferable, in order to obtain spinel, to submit a mixture of 

 chlorides of aluminium and magnesium in contact with lime to a red 

 heat. With chlorides of zinc and aluminium, we obtain zinciferous 

 spinel or gahnite. 



Chloride of titanium conveyed over lime, furnishes, along with 

 some other crystals which will be studied hereafter, oxide of titanium 

 in the form of brookitc. Oxide of tin, obtained in a similar manner, 

 is in crystals of the same form as that which I had previously pro- 

 duced by the action of aqueous vapour. Thus the rectangular pris- 

 matic form is persistent for the acids of titanium and tin, produced 

 by the decomposition of the chlorides of these metals, at tempera- 

 tures included between 572° and 1652° F. 



