scales; the supernatant fluid is then filtered off from the gold, and the latter washed, dried, and ignited.

c. By precipitation as sulphuret of gold.

Sulphuretted hydrogen gas is transmitted in excess through the dilute solution of the compound under examination; the precipitate formed is speedily filtered off, washed, dried, and ignited in a porcelain or platinum crucible. For the properties of the precipitate, vide § 62. The results are accurate.

a. Solution.

$94.

2. PEROXIDE OF PLATINUM.

Metallic platinum, and those of the platinum compounds which are insoluble in water, are dissolved by digestion with aqua regia, at a gentle heat.

b. Determination.

Platinum is invariably weighed in the pure metallic state to which its compounds are reduced, either by direct ignition or by precipitation as CHLORIDE OF PLATINUM AND AMMONIUM, CHLO

RIDE OF PLATINUM AND POTASSIUM, or SULPHURET OF PLATINUM,

and subsequent ignition. All the compounds of platinum without exception may, in most cases, be reduced to the metallic state by either of these methods. Which is the most advantageous process to be pursued in special instances, depends entirely upon the circumstances of particular cases. The reduction of platinum compounds to the metallic state by simple ignition, is preferable to the other methods, in all cases where its application is admissible. The method of converting into sulphuret of platinum is used exclusively to separate platinum from other metals.

Determination as metallic platinum.

a. By precipitation as chloride of platinum and ammonium. (Ammonio-chloride of platinum.)

The solution of the compound under examination is introduced

into a beaker-glass, and mixed with ammonia, until the excess of acid-if there be any present-is nearly neutralized; sal ammoniac is then added in excess, and the fluid mixed with a considerable amount of absolute alcohol. Should the solution of the platinum compound be very dilute, it is to be concentrated previously to the addition of the alcohol. The vessel is then covered with a glass plate, and allowed to stand for the space of twentyfour hours, when the precipitated double chloride will be found to have completely subsided; the fluid is now filtered off from the precipitate, collected upon a filter, (which need not be tared,) thoroughly washed with spirits of wine of 80°, carefully dried, ignited, and weighed.

The process of ignition is conducted in the following manner : the dry precipitate, wrapped up in the filter, is introduced into a weighed platinum crucible, which is then covered, and exposed to a gentle heat, until the disengagement of fumes of sal ammoniac ceases completely; the lid is then removed, the crucible placed obliquely, (§ 32,) and the filter incinerated, which latter operation may be aided by moistening with a few drops of nitric acid. The process is finally concluded by applying an intense heat for some time. For the properties of the precipitate and residue, vide § 63. The results are very satisfactory, although a trifling loss is generally incurred, since ammonio-chloride of platinum is not entirely insoluble in spirits of wine. (Experiment No. 11.) If the operator be not particularly careful to apply only a moderate degree of heat during the first stage of the process of ignition, it may also happen that traces of the yet undecomposed double chloride are carried off with the fumes of sal ammoniac.

Were the precipitated ammonio-chloride of platinum weighed as such, the results would be inaccurate, since (as I have convinced myself by direct experiments) it is impossible to wash the double chloride with spirits of wine sufficiently to free it entirely from the sal ammoniac thrown down in conjunction with it, without dissolving, at the same time, a perceptible portion of the ammoniochloride of platinum.

b. By precipitation as chloride of platinum and potassium, (potassio-chloride of platinum.)

The solution of the compound under examination is introduced into a beaker-glass, and mixed with potass, until the greater part of the excess of acid (if there be any) is neutralized; chloride of potassium is then added slightly in excess, and finally a considerable portion of absolute alcohol. The mixture is allowed to stand for the space of twenty-four hours, when the precipitated double chloride will be found to have completely subsided. The fluid is then filtered off, and the precipitate collected upon a weighed filter, washed with spirits of wine of 70°, thoroughly dried at 212°, and weighed. A portion of the dried precipitate is now introduced into a weighed bulbous tube, the ends of which are subsequently cleaned with the beard of a quill; the tube is weighed again, to ascertain the exact amount of potassiochloride of platinum which it contains. The tube is now connected with an apparatus evolving dry hydrogen gas, (Plate XXXI.), and its contents are heated to redness, until the ensuing evolution of hydrochloric acid vapor ceases completely; (this may be readily ascertained by approaching the end of the tube with a glass rod, moistened with ammonia). The contents of the bulb are then allowed to cool, the tube is removed from the apparatus, and filled with water; the solution of chloride. of potassium which is formed is carefully decanted, and the residuary platinum thoroughly washed. The tube is again connected with the apparatus, and heated in a stream of hydrogen gas; when perfectly dry, it is weighed. By subtracting the original weight of the empty tube from that which it is now found to possess, the exact weight of the platinum contained in it will be ascertained; and this is then to be calculated upon the whole of the precipitated potassio-chloride of platinum.

For the properties of the precipitate and residue, vide § 63. The results are more accurate than those obtained by method a., since, on the one hand, the potassio-chloride of platinum is more insoluble in spirits of wine than the ammonio-chloride; and, on

the other hand, because loss of substance is less likely to arise during ignition, than in method a. The results would be less accurate were the ignition effected in a crucible, since in that case complete decomposition would not ensue, at least not if the amount of substance operated upon be considerable. Were the precipitated potassio-chloride of platinum weighed as such, the results obtained would be inaccurate, since washing with spirits of wine fails to free this double chloride entirely from the chloride of potassium thrown down in conjunction with it.

c. By precipitation as sulphuret of platinum.

The solution of the compound under examination is precipitated (according to its greater or less amount)—either with sulphuretted hydrogen gas, or with sulphuretted hydrogen water; the mixture is heated to incipient ebullition, the fluid filtered off from the precipitate, and the latter collected upon the filter, washed, dried, and ignited. (§ 32.) For the properties of the precipitate and residue, vide § 63. The results are accurate. d. By ignition.

The compound under examination is heated, in a porcelain or platinum crucible, gently at first, finally to ignition, and the residuary pure platinum weighed. For the properties of the latter, vide § 63. The results are exceedingly accurate.

§ 95.

3. OXIDE OF ANTIMONY.

a. Solution.

Oxide of antimony, and those of its compounds which are insoluble in, or decomposable by water, are dissolved in hydrochloric acid of a greater or less degree of concentration, according to circumstances; metallic antimony is most advantageously dissolved in aqua regia.

b. Determination.

Antimony is weighed either as SULPHURET or in the METALLIC state, or as ANTIMONIOUS ACID.

We may convert into

1. SULPHURET OF ANTIMONY. All compounds of antimony without exception.

2. METALLIC ANTIMONY.

All compounds of oxide of antimony without exception.

3. ANTIMONIOUS ACID.

The oxides of antimony and their salts, with readily volatile or decomposable oxygen acids.

The method of determining antimony by reduction to the metallic state, is generally only had recourse to for the purpose of separating antimony from tin, and its detailed description belongs therefore more properly to the fifth section, where it will be found, at § 130.

1. Precipitation as sulphuret of antimony.

a. The solution contains oxide of antimony or the corresponding chloride, and besides this, no other substance possessing the property of decomposing sulphuretted hydrogen.*

The solution of the compound under examination is mixed with some tartaric acid, and (if necessary) diluted with water. Sulphuretted hydrogen gas is then transmitted in excess through the clear solution. The beaker-glass is now covered with a glass plate, and kept standing at rest in a moderately warm spot, for the space of half an hour; the fluid is then speedily filtered off from the precipitate, and the latter collected upon a tared filter, thoroughly washed, dried at 212°, and weighed. For the properties of the precipitate, vide § 64. The results are generally a trifle too high, owing to traces of free sulphur precipitating in conjunction with the sulphuret of antimony. As a precautionary measure, it is invariably advisable to dissolve a small portion of the weighed precipitate in hydrochloric acid, at

This method is applicable in all cases where we have a salt of the oxide of antimony, or protosulphate of antimony, in hydrochloric solution, but never in presence of free nitric acid.