the colour, which should be pure white, is yellowish, it is owing to the presence of iron, which is easily got rid of by washing the filter after, weighing, with water acidulated with sulphuric acid.

If there is reason to suspect the presence of bismuth, treat a small quantity of the weighed precipitate with a little nitric acid. A few drops of iodide of potassium in the liquid will detect the presence of bismuth by the formation of a brown precipitate, or yellow-brown if there is bismuth and lead. The latter metal, when present alone, gives a pure yellow precipitate.

DETERMINATION OF LEAD BY OXALIC ACID.- In estimating lead by carbonate of ammonia, in presence of an excess of ammonia, two or three thousandths of this metal can be determined. By operating, under the same conditions, with oxalic acid, it has been found impossible to determine it to less than 1 per cent. Writers have, indeed, observed, that the precipitation of lead by oxalic acid should be effected in neutral liquids; but this necessity but ill agrees with the most ordinary instances of the analysis of metallic substances, where the presence of an excess of ammonia is indispensable for maintaining in solution certain substances from which the lead should be separated,

BLOWPIPE REACTIONS OF LEAD.

When plumbiferous compounds which are met with in nature, and furnace products, are treated on charcoal, in the oxidising flame, they give a sublimate which is very easily recognised. Other easily volatilised metals, which may be in combination with the lead, either fume away entirely, or else deposit an oxide upon the support. The oxide of lead sublimate, which is dark lemon-yellow while hot, and sulphuryellow when cold, deposits nearer to the assay than the sublimates of some other metallic oxides, namely, those of tellurium, selenium, antimony, and arsenic, and is by this means distinguished. Should zinc also be an ingredient, the sublimate of oxide of lead will probably be contaminated with a quantity of the oxide of this metal, but the sulphur-yellow

colour of the lead deposit cannot be mistaken, when the assay has perfectly cooled.

OXIDE OF LEAD.-Alone, minium blackens when heated, and is transformed into the yellow oxide. It forms by fusion a fine orange glass, which is reduced with effervescence on charcoal.

With borax it fuses readily on the platinum wire, and gives a transparent glass, which, when saturated and hot, is yellowish, but which becomes colourless on cooling. It is reduced on charcoal.

With microcosmic salt it readily fuses into a transparent and colourless glass.

With soda, oxide of lead readily fuses on the platinum wire, forming a transparent glass, which becomes yellowish and opaque by cooling. Its reduction takes place instantaneously on charcoal.

ORES OF LEAD.

SULPHIDE OF LEAD, GALENA.-Alone, on charcoal, it does not fuse until after disengagement of sulphur; globules of lead then form on the surface, and finally a bead of lead is obtained. By cupelling this, the presence of silver may be ascertained. After cupellation, the bone-ash indicates by its colour whether the lead were pure or not; if it were, when cold the cupel would be pure yellow; copper renders it green, and iron brown or blackish.

In the tube, galena gives off sulphur, and a white sublimate of sulphate of lead.

OXIDE OF LEAD.—Its action with fluxes has been already shown.

SULPHATE OF LEAD decrepitates, and fuses on charcoal in the outer flame into a transparent bead, which becomes milky by cooling. In the reducing flame it effervesces, giving a button of lead.

CARBONATE OF LEAD behaves like oxide of lead.

PHOSPHATE OF LEAD.-Alone, on charcoal, it fuses, the bead crystallising as it cools. The crystals have large facets, and a pearly whiteness.

With fluxes it behaves like oxide of lead.

407

CHAPTER XI.

THE ASSAY OF TIN.

THIS metal is always found by the assayer in the state of oxide.

Oxide of Tin (SnO2).—The appearance of this mineral gives no indication, excepting to an experienced eye, that metallic matter enters largely into its composition; yet its great density would lead one to suppose such to be the case. Its colour varies from limpid yellowish white to brownish black and opaque, passing from one to the other by all intermediate shades. It usually possesses a peculiar kind of lustre which cannot be readily described, but once seen can scarcely be mistaken. It occurs crystallised in square prisms, terminated by more or less complicated pyramids. These crystals, derived from the octahedron, are often macled or hemitropic, so that they often possess reentrant angles, which is to a certain extent characteristic. The principal varieties are the following:

1. Crystallised Oxide of Tin is found in more or less voluminous crystals of the colour and form as above.

2. Disseminated Oxide of Tin.-This variety occurs in grains of various sizes, sometimes so small as not to be visible to the naked eye. It is found in the primitive

rocks.

3. Sandy Oxide of Tin forms pulverulent masses often of great extent; in appearance it is merely a brown sand.

4. Concretionary Oxide of Tin, Wood Tin.-This variety occurs in small mamellated masses, the fibrous texture of which resembles that of wood: hence the name.

The following is an analysis of a sample of oxide of tin from Cornwall:

Assay of Pure Oxide of Tin.-Pure oxide of tin may be very readily assayed in the following manner :-Weigh off 400 grains, place them in either a black-lead or charcoal-lined crucible, cement on a cover by means of Stourbridge clay, and subject to the fire. The heat should for the first quarter of an hour be a dull red, after which it may be raised to a full bright red for ten minutes, and the crucible removed with care so as not to agitate or disturb the contents; tapping in this case must not be resorted to. When the crucible is cold, remove the cover, and a button of pure tin will result: this weighed and divided by four gives the percentage. If the operation has not been carefully conducted, it sometimes happens the tin is not in one button, but disseminated in globules either on the charcoal lining or on the sides of the black-lead pot; in this case the charcoal on the one hand, or the black-lead crucible on the other, must be pulverised in the mortar and passed through a sieve; the flattened particles of tin will be retained by the sieve, and can be collected and weighed. If any small particles escape the sieve, they may be separated from the lining or crucible by vanning.

If a charcoal or black-lead crucible be not at hand, an ordinary clay pot may be used, but not so successfully, excepting under certain circumstances to be hereafter described. Indeed, in Cornwall the ordinary mode of conducting this assay is in a naked crucible, thus: About 2 ounces of the ore are mixed with a small quantity of culm, and projected into a red-hot crucible. If the ore seems to fuse or work sluggishly, a little fluor-spar is added, and after about a quarter of an hour's fusing at a good high temperature, the reduced and fused tin is poured into a small ingot mould, and the slag examined for metal by pounding and vanning. This method never gives the whole of the metal. To effect

this, without fear of mischance in the assay sometimes occurring as already described, with both black-lead and charcoal lined crucibles, it may be thus conducted; always supposing the oxide to be pure, or nearly so, or at least containing little or no siliceous matter.

To 400 grains of ore add 100 grains of argol, 300 grains of carbonate of soda, and 50 grains of lime; mix well together, place in a crucible which the mixture half fills, cover with a small quantity of carbonate of soda and 200 grains of borax. Place the whole in the furnace with the necessary precautions, raise the heat very gently, and keep it at or below a dull red heat for at least twenty minutes; then gradually increase until the whole flows freely. Remove the crucible, tap it as for copper assay, and allow to cool. When cold, break it, and a button of pure metallic tin will be found at the bottom, and a flux perfectly free from globules and containing no tin.

There is yet another process, which is more easy of execution; but the reagent employed is more expensive, not so readily obtainable, and more difficult to keep without decomposing than any of the substances above employed. The reagent now to be discussed has been introduced to the notice of the student, in another part of this volume, as a blowpipe flux, and in the assay of copper ores by standard solutions, as cyanide of potassium. This is the most effective reducing flux for tin ores yet known. It acts by absorbing oxygen to form a compound known as cyanate of potash: thus

The

SnO2+KCy=Sn+KO,CyO.

assay, by means of this substance, may be made in ten minutes.

This method of estimating the value of tin-stone has been frequently practised by the writer during the last nine years, and has uniformly furnished correct results with but little expenditure of time and labour. The method of operating is as follows:-The sample having been carefully selected, is first crushed by the hammer in a steel mortar, and then further reduced to powder in an agate