54 MINERALOGY. 
bringing 180° on the disk directly opposite to the 0° of the vernier a, and 
establishing a perfect parallelism between the edge of our crystal and the 
standard window-bar. The image of the window-bar, seen at this time, 
must be on a line with the black lie drawn on the paper already referred 
to. The whole apparatus, clamped together, is next to be turned until the 
same bar is seen in the other face, and on a line with the line on the paper. 
The angle through which the apparatus has been rotated, as read off on the 
vernier, will be the angle of inclination of the two faces. For further 
remarks on the subject of the reflecting goniometer see Optics. 
For very large crystals, in which the preceding form of reflecting 
goniometer cannot be used, we may advantageously employ the goniometer 
of Gambay (pl. 32, fig. 9). It consists principally of a horizontal and 
rotating disk, AB, supported by a stand, C. The crystal being fixed in the 
centre of the smaller disk, @b, the telescope, D, is now directed towards the 
crystal, and this turned until some distant object, as a distant tower, shall be 
seen in the cross-hair of the former. The angle through which the disk, ab, 
bearing the crystal, must be turned to see this same image crossed by the 
cross-hair in the other face of the crystal, will be the angle of inclination of 
the two faces. 
5. Seectra, Mrinerapoey. 
The succeeding descriptions of the minerals figured in plates 33, 34, 35, 
and 36, will allow of a brief survey of the application of crystallography ; as 
also of the manner of mineralogical examinations. The number of single 
minerals already determined, with great exactness, is so great, that we can 
no more omit system in mineralogy, than we can in botany or zoology. 
The minerals figured in our plates have therefore been, to a certain extent, 
arranged according to the system of Hausmann, which is the one we have 
deemed best for our purpose. 
~ We thus divide minerals into ten classes: 1, Metalloids; 2, Native 
Metals; 3, Tellurids; 4, Antimonids; 5, Arsenids; 6, Selenids ; 7, Sulphu- 
rids; 8, Oxydes ; 9, Silicates ; 10, Salts. 
Class 1. Metalloids —HausmMann. 
Simple non-metallic bodies, which disappear when heated in the air or in 
oxygen, forming a combination with the latter. | 
1. Sulphur. 
This is a substance occurring in great abundance. It is found both 
simple or native, and combined with other bodies. The combinations most 
generally met with are those with the metals, and with oxygen; less abun- 
dantly with hydrogen. It is frequently, although in small quantities, found 
as an essential ingredient in organic substances. Native sulphur is of a 
yellow color, and has a specific gravity of 2.07. It is a non-conductor 
of heat, and becomes negatively electric by friction. It is one of those 
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