230 ANALYTICAL CHEMISTRY. 



silver will be stained black in consequence of the formation of silver 

 sulphide. The presence of the alkali sulphide may also be demon- 

 strated by the addition of a few drops of hydrochloric acid to the 

 fused mass, when hydrogen sulphide is evolved and may be recog- 

 nized by its odor. 



6. Flame tests. Many substances impart a characteristic color 

 to a non-luminous flame. The best mode of performing this test is 

 as follows : A platinum wire is cleaned by washing in hydrochloric 

 acid and water, and heating it in the flame until the latter is no 

 longer colored. One end of the wire is fused in a short piece of 

 glass tubing (see Fig. 25), the other end is bent so as to form a small 



FIG. 25. 



loop, which is heated, dipped into the substance to be examined, and 

 again held in the lower part of the flame, which then becomes colored. 



Some substances show the color-test after being moistened with 

 hydrochloric or sulphuric acid. 



A second method of showing flame reactions is to mix the substance 

 with alcohol in a small dish ; the alcohol, upon being ignited, shows 

 a colored flame, especially in the dark. 



7. Colored borax beads. The compounds of some metals when 

 fused with glass, impart to it characteristic colors. For analytical 

 purposes not the silica-glass, but borax-glass is generally used. This 

 latter is made by dipping the loop of a platinum wire in powdered 

 borax and heating it in the flame (directly, or by means of the blow- 

 pipe) until all water has been expelled and a colorless, transparent 

 bead has been formed. To this colorless bead a little of the finely 

 powdered substance is added and the bead strongly heated. The 

 metallic compound is chemically acted upon by the boric acid, aborate 

 being formed which colors the bead more or less intensely, according 

 to the quantity of the metallic compound used. 



Some metals (copper, for instance) forming two series of compounds, 

 give different colors to the bead when present in either the higher or 

 lower state of oxidation. 



By modifying the blowpipe flame so as either to oxidize (by 

 supplying an excess of atmospheric oxygen) or deoxidize (by allowing 

 some unburnt carbon to remain in the flame), the metallic compound 



