SILVER ORES. 333 



merce, however small the proportion. The weight of the globule, es- 

 pecially when quite minute, may be also ascertained by measurement, 

 according to a scale given by Prof. W. W. Mather, in the American 

 Journal of Science, volume iii, second series, page 414. Much thai 

 has been mentioned in the preceding pages on the American mines 01 

 Silver, has been derived from an article by Prof. Mather, in volume xxiv 

 of the same Journal. 



Other modes of reducing silver ores without, quicksilver, have been 

 proposed. According to one, the ore is calcined with common salt, m 

 in Mexico, and converted thus to a chlorid. It is then removed to some 

 proper vessel, and a hot solution of salt poured over it ; ihis takes up 

 the chlorid of silver and holds it in solution. The liquid is transferred 

 to another vessel, and by means of metallic copper the silver is de- 

 posited. 



Another process consists in roasting the sulphurets and converting 

 them in a reverberatory furnace to sulphates ; then by boiling water, 

 dissolving the sulphates in a proper vessel, and finally precipitating as 

 above by copper. This process requires the presence of a good deal ot 

 sulphur, and is the best when there is much iron and copper pyrites 

 present. 



In the assay to separate copper from silver, the alloy is dissolved in 

 nitric acid, and the silver precipitated in the state of a chlorid by com- 

 mon salt. The amount of silver may then be ascertained by weighing 

 the precipitated chlorid, and observing that 75"33 per cent, of the chlo- 

 rid is pure silver. 



SUPPLEMENT. 



Forms of Gems. — Gems are cut either by cleaving, by sawing with 

 a wire armed with diamond dust, or by grinding. Some remarks on 

 the cutting of the diamond are given on page 83. The harder stones, 

 as the sapphire and topaz, are cut on a copper wheel with diamond 

 powder soaked with olive oil, and are afterwards polished with tripoli. 

 For other gems, less hard, a lead wheel with emery and water is first 

 used, and then a tin or zinc wheel with putty of tin or rotten stone 

 and water. 



The following are some of the common forms. It will be remem- 

 bered that the upper truncated pyramid is called the table, the lower 

 part or pyramid, the collet, and the line of junction between the two 

 parts, the girdle. Figures 1 and 2 represent the brilliant, the best form 

 of the diamond, used also for other stones, as well as pastes. Figs. 3 

 and 4 are views of a, variety of the rose diamond. Fig6. 5 and 6 the 

 same of an emerald. The cut in steps is called the pavilion cut. Fig. 

 7 is an upper view of a mode of cutting the sapphire. A side view 

 would be nearly like figure 6, except that the collet is more like that of 

 figure 8. Fig. 8 represents a side view of an oriental topaz. The 

 table has the brilliant cut, like figs. 1 and 2. Figure 9 represents a 

 Bohemian garnet, which is made thin because its color is deep. The 

 common topaz is cut like figure 8 ; often also like figure 9, but much 

 thicker, and frequently having the table bordered by two or more rows 

 of triangular facets. Figure 10 is a very simple table. Figures 11 and 

 12 represent the form "en cabochon" given the opal; and figures 12 

 »nd 13, " en cabochon" with facets, a mode of cutting the chrysobervl 



