silver were employed, the gold would be left in a very finely divided state, and could not readily be weighed.

This process, though not absolutely correct, is sufficiently so for all commercial purposes.

For the analysis of alloys of gold, silver, and copper, see Quantitative Analysis, Special Methods.

The quantity of gold contained in auriferous sand may be determined roughly by washing a large quantity, and weighing the residual gold, but more accurately by extracting the metal with nitro-hydrochloric acid, and reducing the gold with oxalic acid, with the precautions mentioned above.

Assay of Auriferous Ores.-If the ore to be examined contains oxide of lead, it is only necessary to fuse it in an earthen crucible with a sufficient quantity of black flux.

If the ore contain no oxide of lead, it should be fused with a mixture of litharge and black flux.

When considerable quantities of deoxidizing agents are present, such as pyrites, &c., litharge only may be used.

Sometimes, when the quantity of reducing minerals present is very large, so that too much lead would be reduced if enough litharge were added to oxidize and remove the whole of the sulphur, a little nitre is added, together with the litharge.

If 400 grains of ore are employed, they should be mixed with about an equal weight of litharge, and about 300 grains of black flux.

The button of lead obtained should never weigh more than 200 grains.

The fusion is conducted in a wind furnace. The button, which consists of lead, silver, gold, copper, &c., is then cupelled and parted as in the analysis of alloys of gold (p. 401).

When the amount of gold is required in a mixture of gold and quartz free from other minerals, it may be very readily calculated from the specific gravities of the constituents as compared with that of the mixture. If possible, a piece of pure quartz may be separated from the mass, and its actual specific gravity determined; but if this cannot be done, the number 2.6 may be taken as sufficiently accurate for practical purposes.

The calculation is effected as follows:

Let m = specific gravity of the gold.

A result expressed by saying that the quantity of gold per cent. in a mixture of gold and quartz, is the quotient obtained by dividing, by the difference of the

products of the specific gravities of gold and quartz multiplied with that of the mixture, 100 times the difference between the products of the specific gravity of gold into that of quartz, and of the specific gravity of gold into that of the mixture.

PLATINUM.

Sym. Pt. Eq. 98.7. Sp. Gr. 21.5.

§ 272. This somewhat rare metal was not applied to any useful purpose before the commencement of the present century. Its name is derived from the Spanish word platina, signifying little silver.

Preparation. The extraction of this metal from its ores will be described hereafter; the platinum of commerce is nearly pure, containing merely a little iridium, which renders it harder, but somewhat less malleable than pure platinum; in order to purify the ordinary metal, it is dissolved in nitro-hydrochloric acid, the solution evaporated to expel excess of acid, the residue redissolved in water, and mixed with solution of chloride of potassium, which produces a yellow crystalline precipitate containing the double chlorides of platinum and potassium, and of iridium and potassium. This is dried, mixed with carbonate of potassa, and heated to redness in an earthen crucible; the bichloride of platinum is decomposed according to the equation :

PtCl2+2(KO.CO„)=Pt+2KCl+0 ̧+2CO„;

the iridium is converted into oxide, which is left behind, together with the platinum, when the fused mass is washed with water; this residue is heated with dilute nitro-hydrochloric acid, which dissolves only the platinum; the solution is mixed with chloride of ammonium, and the platinum thus precipitated as a double chloride of platinum and ammonium, which may be washed and heated to redness, when the chloride of ammonium and chlorine are expelled, and the metal left as a spongy mass, the conversion of which into malleable platinum will be presently described.

Properties-Platinum is a nearly white metal, capable of a high lustre; it is the heaviest substance known, its specific gravity being 21.5. It is very malleable and ductile, standing next to gold and silver with respect to the latter quality. Platinum is remarkable for its tenacity, in which it is only surpassed by iron and copper. A wire of this metal, of inch in diameter, will sustain a weight of 361 lbs.

Platinum is rather a soft metal; it is harder than copper, but not so hard as silver. It is less expansive by heat than any other metal, and conducts it very slowly. Platinum is employed in three different forms, viz., malleable platinum, spongy platinum, and platinum black, which is in an extremely divided state. We have already mentioned that spongy platinum is obtained by igniting the double chloride of platinum and ammonium. The preparation of platinum-black will be presently described.

In either of these forms, platinum possesses the remarkable property of condensing gases in its pores, and thus seeming to absorb a certain quantity of them; even malleable platinum exhibits this property in a high degree; hence, a perfectly clean plate of this metal is capable of inducing the combination of hydrogen and oxygen at a comparatively low temperature. This property may also be exhibited by suspending a helix of platinum-wire in the flame of a spirit-lamp, which is allowed to heat the wire to redness, and then suddenly blown out; the

wire will then continue for any length of time at a red heat, in consequence of the energetic combination of the vapor of alcohol with the atmospheric oxygen induced by the peculiar surface-action of the metal. In this case the alcohol is not completely burnt; acrid odors, due to the imperfect oxidation of this body, are evolved. The experiment is even more satisfactory when a platinum crucible is heated to bright redness in the flame of a gauze gas-burner, and the gas extinguished, and rapidly turned on again, when the cold stream of gas will maintain the temperature of the crucible.

This interesting property of platinum will be reverted to in the description of platinum-black.

Platinum is not in the least affected by exposure to air, even at high temperatures; it is very infusible, and does not undergo a change of state in the hottest furnaces; it may, however, be easily fused in the oxyhydrogen blowpipe-flame, or between the charcoal points of a powerful galvanic battery. A platinum wire, held in the flame of the oxyhydrogen blowpipe, emits brilliant white sparks (due to the combustion of the metal?). A very high temperature appears to volatilize it slightly.

Platinum is very easily welded, especially at a high temperature.

This metal, like gold, is unalterable by many chemical agents, and hence is largely employed in the forms of crucibles, spatulas, dishes, wire, and foil, in chemical operations; since, however, it is very costly, it is necessary that the chemist be fully acquainted with the circumstances under which platinum vessels are acted upon, in order that he may avoid accidents.

Platinum does not decompose water under any circumstances; it is not attacked by nitric, hydrochloric, or sulphuric acid; but a mixture of nitric and hydrochloric acids dissolves it in the form of bichloride. Potassa and soda, in the fused state, act upon platinum, probably in the same way as upon gold, by inducing the action of the oxygen of the air upon the metal.

Lithia acts upon platinum much more rapidly than the above alkalies.

A mixture of potassa and nitrate of potassa (which is produced by the action of heat upon the latter), attacks this metal very readily.

Several other oxides, which are not reduced by heat, also affect platinum. Chlorine slowly attacks platinum; iodine and bromine have no effect upon it. In the nascent state, however, these elements are capable of acting upon the metal.

Fluorine appears to combine readily with platinum.

Malleable platinum is scarcely affected by sulphur, even at a high temperature; spongy platinum, however, combines with it.

Phosphorus and arsenic act upon platinum at a high temperature, forming a very fusible phosphide, or arsenide; hence, substances containing phosphorus in an unoxidized state (brain, blood, &c.), should never be burnt in a crucible of this metal, neither should phosphates be heated in contact with reducing agents in a platinum crucible.

Silicon in the nascent state converts platinum into a brittle silicide; hence, silicic acid and carbon should never be allowed to come simultaneously in contact with platinum at a high temperature; for this reason, platinum vessels are never directly heated by a coal, coke, or charcoal fire, but are imbedded in a layer of magnesia, and inclosed in a Hessian crucible.

Most metals are capable of combining directly with platinum, if the temperature be sufficiently high, so that we should avoid the contact of this metal with easily reducible metallic oxides in the presence of reducing agents (filter-paper, for instance).

When platinum is alloyed with silver, it becomes soluble in nitric acid; this is often cited as an example of catalytic action.

Platinum-black. This form of platinum, which derives importance from the

interesting processes of oxidation which it is capable of inducing, may be prepared by several methods:

I. (Proto-) chloride of platinum is dissolved in a concentrated solution of potassa, with the aid of heat, and alcohol is added, by degrees, with constant stirring, to the hot liquid; the platinum is precipitated as a black powder, which is boiled, successively, with alcohol, hydrochloric acid, potassa, and water.

II. A solution of bichloride of platinum is boiled, with constant agitation, with carbonate of soda and sugar, when the sodium takes the chlorine of the bichloride of platinum, and the metal is precipitated, the oxygen of the soda serving to convert the carbon and hydrogen of the sugar into carbonic acid and water.

III. By boiling a solution of (proto-) sulphate of platinum with alcohol. By whichever method the platinum-black is prepared, it must be very carefully washed, and dried between blotting-paper.

It will be remembered that when platinum-black (or spongy platinum) is thrown into a mixture of hydrogen and oxygen, it causes combination, attended by explosion, and that a jet of hydrogen may be inflamed by directing it upon platinum in one of these states, air being present (see p. 119).

If a mixture of sulphurous acid and oxygen be passed over platinum-black, or sponge, anhydrous sulphuric acid is produced.

A mixture of binoxide of nitrogen, or ammonia, with an excess of oxygen, passed over heated spongy platinum, or platinum-black, gives rise to nitric acid:

NH,+0,=HO.NO,+2HO.

On the other hand, the oxides of nitrogen, when mixed with an excess of hydrogen, and exposed to the action of the heated platinum, yield ammonia and water:NO,+H, NH,+2HO.1

3

Both spongy platinum and platinum-black lose these peculiar properties after some time, but they may be revived by heating with nitric acid, washing with water, drying at a gentle heat, and, in the case of spongy platinum, heating to redness.

§ 273. This oxide is known only in the form of hydrate, which is obtained, as a black powder, by decomposing the (proto-) chloride of platinum with potassa.

It is in the department of organic chemistry that we find the most interesting examples of the action of platinum-black.

If a drop of alcohol be projected upon a little platinum-black, the latter becomes vividly incandescent, and the excess of alcohol takes fire; again, when platinum-black is placed in a capsule and covered with a bell-jar, the sides of which are wetted with alcohol, the latter is gradually oxidized, and converted into acetic acid:

Formic acid, moreover, is immediately converted into carbonic acid and water, by contact with platinum-black:

C2HO, HO+0=2CO2+2HO.

Formic acid.

The hydrated oxide, when freshly precipitated, dissolves in excess of potassa. If the hydrate be heated, it parts with water and oxygen, leaving metallic platinum; in fact, this oxide is very unstable; most reducing agents decompose it with facility. Oxide of platinum possesses feeble basic characters; it dissolves slowly in sulphuric, nitric, and acetic acids, yielding brown solutions.

When this oxide is heated with hydrochloric acid, the metal is separated, and bichloride of platinum formed :

2PtO+2HCl=PtCl,+Pt+2HO.

The salts of (prot-) oxide of platinum are little known, and, for the most part, do not crystallize; the oxalate has been obtained in copper-red needles by heating hydrated binoxide of platinum with oxalic acid.1

Solutions of (prot-) oxide of platinum are not precipitated by chloride of ammonium, and may thus be distinguished from those of the binoxide.

BINOXIDE OF PLATINUM, PLATINIC ACID.

Ptog

Anhydrous binoxide of platinum may be obtained by gently heating the hydrate; it is a black powder, which does not dissolve in acids or alkalies.

The hydrate (PtO, 2HO), is prepared by mixing a solution of bichloride of platinum with an excess of potassa, when a double chloride of platinum and potassium is first precipitated, but redissolves on gently heating, being converted into platinate of potassa; if the solution be now mixed with excess of acetic acid, the hydrated binoxide of platinum is precipitated.

This hydrate forms a red-brown precipitate, somewhat resembling the sesquioxide of iron; when heated, it loses first its water, and then the oxygen. It is easily decomposed by reducing agents.

The hydrate dissolves in acids and alkalies, yielding brown solutions; hence, binoxide of platinum is another example of an oxide which plays, indifferently, the part of a weak base and that of a weak acid.

Fulminating Platinum.-This substance, which is probably a compound of binoxide of platinum with ammonia, is obtained by adding the latter reagent to a solution of sulphate of binoxide of platinum, and digesting the basic doublesalt then precipitated, with a dilute solution of soda.

It is a brown powder, insoluble in water, nitric and hydrochloric acids, but soluble in sulphuric acid. It is not exploded by friction or percussion, but detonates violently when heated to about 400° F. (204° C.) Care should therefore be taken, lest it should be formed, unexpectedly, in examining the compounds of platinum.

The platinates of potassa and soda may be crystallized; they are decomposed by acids, platinic acid (hydrated binoxide of platinum) being separated.

Nitrate of binoxide of platinum is formed when an alloy of platinum and silver is dissolved in nitric acid; it may be prepared by dissolving the hydrated binoxide in nitric acid, or by decomposing bichloride of platinum with nitrate of silver.

This nitrate does not crystallize; it has a dark-brown color, and leaves metallic platinum when exposed to heat.

Nitrate of binoxide of platinum forms double-salts with the alkaline nitrates. In order to prepare the Sulphate of Binoxide of Platinum, the bisulphide is oxidized with fuming nitric acid, and the liquid evaporated to dryness with a little sulphuric acid. The sulphate has a dark-brown color, and is very soluble

1 A double-salt of the formula 2(PtO.2SO,,3(KO.SO,))+5Aq, is deposited as a white precipitate, when the double chloride of platinum and potassium (KĆI.PtCl ̧) is heated with sulphide of potassa, and the colorless solution concentrated by evaporation.