When the substance is heated in an inclined tube, open at both ends, similar indications are to be observed; modified somewhat, however, by the fact -that oxygen can now pass up through the tube and come in contact with the specimen under examination. Thus sulphides are com monly oxidized in the open tube, arsenic will sublime as the trioxide and not as the metal, and selenium gives a sublimate that may be gray or red, and also a strong odor of horse radish.
The color that the specimen communicates to the non-luminous part of the flame is like wise of great service in determinations by the blowpipe. A piece of platinum wire, bent at the end into a small loop, is dipped in hydrochloric acid and held in the flame, this process being re peated several times until the analyst is con fident that the wire itself is free from any sub stance that can color the flame. The little loop at the end is then brought into contact with some of the finely pulverized specimen, and introduced into the flame again. Sodium gives a strong tasting yellow ; calcium an orange red; lithium and strontium a crimson; potassium a lavender; barium an apple green; thallium, cop per and boracic acid a brighter green; lead and antimony a pale blue; selenium a deep blue. The yellow due to sodium is so powerful, even when that metal is present only in slight amounts, that the colors due to the other metals present are sometimes difficult to observe by the unaided eye. Hence . colored glasses are often used, through which to take note of the flame color; the tint of the glass being selected so as to cut off the yellow light of the sodium, while allowing the particular color that is sought to pass through unobstructed. Cobalt
blue glass, for example, is used in this way in testing, by flame coloration, for potassium.
When a sample of the specimen to be ana lyzed is heated upon charcoal, it is often possible to obtain some of the elements that are present, in the form of a metallic bead, by the reduction of their oxides or of the other compounds in which they were originally contained. Lead, tin and silver give beads that are white and mal leable; copper gives a malleable red bead; anti mony and bismuth give brittle beads; and iron, cobalt and nickel may often be obtained in the form of gray, magnetic powders.
While the substance is being heated upon charcoal, an incrustation commonly forms on the charcoal, from the character of which useful inferences can be drawn. Thus antimony gives a white incrustation; bismuth, an incrustation that is deep yellow when hot and lighter yellow when cold; lead, one that is light yellow when hot and deep yellow when cold, and is sur rounded by a white border; arsenic gives a white incrustation that is very volatile; and with zinc the color is yellow when hot and white when cold.
Many metallic oxides are soluble in melted borax, and valuable color indications are ob tained by heating small quantities of the sub stance in little beads of melted borax, that are held in the flame upon tiny loops of platinum wire. The phenomena that are observed in this way are quite complicated, however, and for an account of them the manuals on blowpipe analysis should be consulted. Consult Corn wall, 'Manual of Blowpipe Analysis' ; Moses and Parsons, 'Elements of Mineralogy, Crystal lography, and Blowpipe Analysis' ; Dana, 'Min erals and How to Study Them.)