TABLE OF

Al2 [NH+] Sb As Ba Bi Cd Ca Cra Co Cu Au Fe" [Fe2 ]'

Ala [NH4] Sb As Ba Bi Cd Ca Cr2 Co Cu Au Fei [Fe2]

In this table W signifies that the substance is readily soluble in water, (W) that the substance is soluble with difficulty in water; A and (A) that the substance is readily or with difficulty dissolved by acids; I that the substance is insoluble in water or acids; U signifies that the compound is either unknown or so uncommon as rarely to be met with.

Pb Mg Mn (Hg2 ]" Hg Ni Pt K Ag Na Sr Sni Sniv Zn

U W A W A

U W (W) Wa (W) A U

(W) W W W (A) W W W W W Bromide

A W (W) A

W U W (W) W W W U U

W2 (W) W

Pb Mg Mn (Hg2 ]" Hg" Ni Pt K Ag Na Sr Snü Sn▾ Zn

who is analyzing comparatively simple substances. In the case of complex mixtures the table is of little service. Thus, chloride of silver is designated as I, but it would be easy to mix chloride of silver and chloride of sodium in such proportions that on treatment with water the whole of the mixture would go into solution: so too the chlorides of potassium and platinum are readily soluble in water, each by itself, but the double chloride of these two elements is so insoluble that it is used as a test for potassium (§ 42) and for platinum (§ 96).

D. TREATMENT OF INSOLUBLE SUBSTANCES.

88. The substances of common occurrence which are practically insoluble in water and acids are:

The sulphates of barium, strontium and lead.

Chloride of silver.

The anhydrous sesquioxides of aluminum, chromium and iron, either native, or the result of intense ignition. Chrome-iron-ore, a native mineral.

Some aluminates.

Binoxide of tin, native, or the result of ignition.
Silica and many silicates.

Fluoride of calcium (fluor-spar).

Besides the substances included in this list, sulphur and carbon, or graphite, should, perhaps, be mentioned, because they are insoluble; but they will have been detected during the preliminary blowpipe examination, and their presence allowed for. Bromide, iodide and cyanide of silver are decomposed by boiling with aqua regia, and converted into the chloride, so that these substances never appear in their proper form in the final insoluble residue.

89. Substances which resist solution in liquids are generally liquefied by the action of fluxes at a high temperature; they are fused in contact with some powerful decomposing agent, like the carbonate or acid sulphate of an alkali-metal,

or the hydrate or carbonate of an alkaline-earth metal. Certain preliminary experiments should precede the fusion.

The insoluble powder is first examined carefully (with the help of a lens, if convenient) to ascertain if it is a homogeneous substance of the same color throughout, or a mixture composed of dissimilar, variously-colored particles. The following blowpipe experiments sometimes give decisive indications, particularly with homogeneous substances.

a. The reduction test (§ 81) is repeated with great care, looking especially for silver, lead and tin, and applying to the globule, if any is obtained, the test for distinguishing between these three white metals. This test has already been applied to the original substance; but if this substance was a complex mixture containing soluble ingredients, it is quite possible that the test should give a more satisfactory result, now that all substances soluble in water and acids have been removed, than it yielded before. If, however, decided indications of the presence of a reducible metal were obtained in the first instance, the repetition of the test is, of course, unnecessary. If any reducible metal is detected, it is necessary to use a porcelain crucible for the fusion which it may be desirable to make (§.90) in order to convert the insoluble substance into a more manageable form. A platinum crucible, which is employed for most fusions, cannot be used with safety when the substance to be fused contains any reducible metal; for many of the alloys of platinum are extremely fusible.

Sometimes, when the substance under examination contains but a small proportion of metal, some metal may be reduced during the blowpipe experiment on charcoal, but the detached particles may not run together into a single conspicuous globule. Since a mistake as to the presence of a reducible metal may involve the destruction of a platinum crucible, it is best in doubtful cases to operate in a more delicate fashion. To ascertain, beyond question, whether any reduced metal has been separated in this experiment, moisten the cavity in the

charcoal with water after the fusion has been finished, cut the charcoal out for a little distance, both around and below the cavity, and transfer the contents of the cavity and the scraps of charcoal to an agate or porcelain mortar. Pulverize the whole mass, and then carefully wash away the powdered charcoal and all the lighter portion of the mixture. Any malleable metal that may have been reduced remains in the mortar in little flattened grains or spangles, in which the peculiar color and lustre of the metal or alloy are generally visible. Sometimes metallic streaks are produced on the mortar or pestle by little particles of metal ground between them. The student must not mistake glistening particles of wet charcoal sticking to the mortar or pestle for metallic spangles, and the metal should be thoroughly removed from both mortar and pestle by the use of a few drops of warm aqua regia, immediately after the experiment is finished, in order to avoid errors on a subsequent occasion.

b. Prepare another pellet of a mixture of equal parts of the insoluble powder and carbonate of sodium, adding a little charcoal powder to the paste. Fuse this mixture upon charcoal in the reducing flame of the blowpipe. Scoop out the fused mass and the surrounding charcoal with a penknife, place the dry mass upon a bright surface of silver (coin or foil), and wet it with a drop of water. If a brown stain be produced on the silver, it is evidence of the presence of sulphide of sodium in the fused mass. This sulphide results from the reduction of a sulphate, and is evidence of the presence of a sulphate in the substance tested. The odor of sulphuretted hydrogen is often perceptible when the fused mass is moistened. The silver coin or foil may be replaced by a piece of lead paper, if care be taken not to mistake the mere dirtying of the paper for a stain of sulphide the silver is, however, to be preferred; it may be cleaned after use by treating it with a solution of cyanide of potassium and then washing with water. It is obvious that the carbonate of sodium used in this test must be so free from sulphate of