32 MINERALS AND GEOLOGY 



dition. The operation is most readily performed as follows. A small 

 fragment of the mineral is reduced to powder. Some of this is made 

 into a paste by moistening with a drop of water, and is spread over 

 the surface of a piece of charcoal, or broken fragment of a porcelain 

 evaporating-dish or thin crucible. It is then ignited before the point 

 of an oxidating flame (Fig. 32), the heat being kept low, at first, to 

 prevent fusion. It is sometimes necessary to remove the ignited 

 paste to the mortar, and to break it up again with a fine steel spatula 

 (the end of a flattened wire, or knife-point), and renew the opera- 

 tion. When the roasting is terminated, the powder will present a 

 dull earthy aspect, and cease to omit fumes or odour. It is then 

 ready for operations 5 and 6, described below. By reducing the 

 substance to powder before roasting, the risk of decrepitation and 

 fusion is prevented, and the process itself is more efficiently performed. 



Roasting is sometimes effected in a piece of open glass tubing as 

 in Fig. 35 only the test object is placed near one end of the tube, 

 and the tube itself is held in a more inclined position. Sulphur 

 eliminated from bodies by this treatment, is converted into sulphur- 

 ous acid (a compound of sulphur and oxygen, the latter taken up 

 from the atmosphere) ; and arsenic forms arsenious acid, which de- 

 posits itself in the shape of numerous microscopic octahedrons on the 

 cool sides of the glass near the upper part of the tube. Sulphurous 

 acid in escaping from the open end of the tube is easily recognized 

 by its odour (identical with that emitted by an ignited match), as 

 well as by its property of changing the blue colour of a slip of 

 moistened limus paper to red. Antimonial compounds form a dense 

 white uncrystalline sublimate. 



(5.) Formation of glasses on platinum wire or charcoal : This 

 operation is one of constant utility in the determination of the con- 

 stituents of minerals. The glasses, in question, are formed by the 

 fusion of small portions of borax, phosphor salt, or carbonate of soda : 

 the latter reagent, however, being only occasionally used. Most sub- 

 stances, dissolve in one or the other of these glasses before the blow- 

 pipe, and many communicate to them peculiar colours by which the 

 nature of the test-matter is made known. If the matter to be tested 

 contain sulphur or arsenic, it should be roasted before being subjected 

 to the action of these fluxes. Metals and metallic alloys, as well as 

 metallic oxides, chlorides, &c , of very easy reduction, must be exami- 





