FIRST M.B. EXAMINATION.

189

Benzoic and hippuric acids would now be precipitated, though they might have escaped precipitation by acetic acid.

If no clue has yet been obtained, examine the substance for an organic acid by Table K, and for an organic alkaloid by Table O; and, as a last resource, for a metal by Table A, and for an inorganic acid by Table G, in the hope that some light may thus be thrown upon the nature of the substance.

EXERCISE XVI. (Seee (282) for Examples for Practice.)

* In examining substances which are entirely unknown, it will be found advantageous to make a preliminary experiment without adding the carbonate of soda (242a).

REDUCTION ON CHARCOAL.

191

EXPLANATIONS AND INSTRUCTIONS ON
TABLE R.

241. To reduce metals on charcoal before the blowpipe.Select a piece of hard thoroughly carbonized charcoal, free from crevices, not less than four inches long and one or two inches in diameter; grind down one of its sides to a flat surface (fig. 73) on the hearthstone. Scoop a very shallow

FIG. 74.

cavity at a with the blade of a knife, making it smooth and round. Place in this a grain or two of the substance to be examined, previously reduced to powder (124), and cover it with dry powdered carbonate of soda. Hold the charcoal and the blowpipe in the positions represented in fig. 74. Direct the point of the inner (reducing) flame upon the specimen in the cavity, blowing gently at first lest the powder

Reduction on charcoal.

should be scattered, and allow the outer (oxidizing flame) to play over the flat surface of the charcoal.

Observe very closely the appearances presented by the mass under the influence of heat, especially noticing whether any minute metallic globules are to be seen in the fused

substance. If this be the case, try to fuse them together into larger globules.

Should an infusible mass be left after the first application of the blowpipe-flame, add more carbonate of soda, and again heat intensely, since the binoxide of tin often requires repeated additions of carbonate of soda to bring it into fusion and reduce it to the metallic state.

Watch the appearance of the mass after withdrawing it from the flame, noting any changes of color which may occur in cooling.

The surface of the charcoal is generally covered, for some distance beyond the cavity, with a deposit or incrustation which sometimes consists of a thin white film of ash left after the charcoal has burnt away, and sometimes of a more opaque coating of some metallic oxide formed by the combustion of metallic vapor in passing through the outer flame.

Observe very carefully the color and general appearance of this incrustation, comparing the results with Table S.

If any globule of metal is visible, detach it carefully from the fused mass with the point of a knife, place it upon a hard surface, such as a porcelain slab or the bottom of an inverted mortar, and press it with a knife-blade, to ascertain whether it is malleable or brittle. Compare the results with Table S. When no metallic globule is visible, or when the metallic globule has been removed, scrape the mass, together with the

FIG. 75.

particles of charcoal in contact with it, into a small agate mortar (fig. 75), moisten it with one or two drops of water, and grind it into a paste. Stir this paste up with more water, then fill the mortar with water, allow it to rest for a few seconds, in order that any metallic particles may subside, and carefully pour off the water, carrying with it the lighter particles of charcoal and

Agate mortar.

LEVIGATION. BROWN SLAG.

193

slag (fig. 76). Repeat this grinding and levigation until metallic particles are distinctly visible at the bottom of the mortar, or until the whole has been washed away without showing any metal.

FIG. 76.

Levigation in blowpipe analysis.

The metals which are generally detected in this way,

are

Copper, which gives characteristic red spangles. Tin, in white silvery spangles of considerable size. Iron, in gray metallic powder, attracted by the magnet. 242. Fused carbonate of soda is absorbed into the pores of the charcoal, but very frequently a slag is formed which refuses to sink into the charcoal, and remains on the surface.* This is the case with silicate and borate of soda, formed when silicic and boracic acids are present. Sulphide of sodium also generally remains on the surface of the charcoal as a brown mass, the formation of which renders it highly probable that the substance under examination is a sulphide.

Although most metallic oxides would be reduced to the metallic state by the combined action of the blowpipe-flame and the charcoal support, it is necessary to add carbonate of soda for the following reasons:

(1) The carbonate of soda removes any acid (silicic acid,

* Cyanide of potassium will occasionally assist in getting rid of such slags.