fully saturated, set aside in a covered vessel for twentyfour hours. The sulphuretted hydrogen is then expelled by a gentle heat, and the precipitate filtered off. The precipitate contains arsenic, antimony, copper, and the rest of the lead and bismuth.

The solution is then heated to ebullition, and fuming nitric acid gradually added to the boiling liquid, until all the protoxide of iron is reconverted into sesquioxide, and the solution has acquired a pure yellow color. It is then precipitated by an excess of ammonia, and the yellowish-brown precipitate filtered off.

Part of the nickel, cobalt, zinc, lime and magnesia remain in solution, but the remainder is precipitated together with the sesquioxides of uranium and iron.

The washed precipitate is treated with a hot, pretty strong solution of carbonate of ammonia containing free ammonia, with which the precipitate is digested, at a moderate heat, until it has the appearance of hydrated sesquioxide of iron. The solution of uranium is rapidly filtered off, while hot, and the residue of hydrated sesquioxide of iron (still containing uranium) is washed, the washings being received apart from the filtrate.

The solution (which is yellow, or colored reddish by the cobalt) deposits on cooling, if sufficiently concentrated, crystals of the pure double carbonate of ammonia and sesquioxide of uranium, which may be collected, and washed several times with cold water. When ignited, this salt leaves pure dark green protosesquioxide of uranium.

The mother-liquor is mixed with the washings and sulphide of ammonium carefully added, drop by drop, as long as it produces a dark brown precipitate, which is immediately filtered off.

The precipitate consists of the sulphides of cobalt, nickel and zinc.

The yellow filtrate is then boiled till the greater part

of ammoniacal salt is volatilized, and all the sesquioxide of uranium precipitated.

The pure yellow precipitate, uranate of ammonia is filtered off, and, when the filtrate begins to pass through turbid, washed with solution of sal-ammoniac.

When ignited, it leaves dark green proto-sesquioxide of uranium. By digesting this with dilute hydrochloric acid, any lime and magnesia may be extracted.

In order to prepare protoxide of uranium from the uranate of ammonia, it is dissolved in hydrochloric acid, the solution mixed with an excess of pure salammoniac, and about an equal quantity of pure common salt; it is then evaporated to dryness, and the mass heated in a covered crucible until the sal-ammoniac is volatilized, and lastly, until the common salt fuses. On dissolving it in water, the protoxide of uranium is left as a heavy crystalline powder. The common salt only serves to shield the oxide from the action of air.

When the quantity is very small, the uranate of ammonia is calcined and the proto-sesquioxide is dissolved in hydrochloric acid with a few drops of nitric acid; the addition of chloride of potassium forms a substance U, O, Cl + R Cl. It is evaporated to dryness and the yellow salt ignited in a current of hydrogen.

2 2

In order to extract from the hydrated sesquioxide of iron the small quantity of sesquioxide of uranium which is chemically combined with it, it is dissolved in the smallest possible quantity of hydrochloric acid, the solution neutralized with carbonate of ammonia, and added, drop by drop, with constant stirring, to a mixture of carbonate of ammonia and sulphide of ammonium, when all the iron is separated as sulphide, and the sesquioxide of uranium remains in solution; the latter may be precipitated by boiling the filtrate.

Or the sesquioxide of iron may be reduced in a

stream of hydrogen, and the reduced pyrophoric mass allowed to fall, immediately, into dilute hydrochloric acid, which dissolves the iron, leaving the uranium a protoxide.

In order to detect selenium, arsenic, and vanadium in pitch-blende, it is ignited with of its weight of a mixture of carbonate of soda and nitre. The selenates, vanadates and arsenates of the alkalies may then be extracted with water.

105. SELENIFEROUS DEPOSIT FROM SULPHURIC ACID CHAMBERS.

(Sulphate of lead, selenium, selenide of mercury, selenates selenites, &c.)

The dry mass is rubbed to a thin paste with a mixture of about equal parts of sulphuric acid and water, and boiled for a long time, concentrated nitric acid or chlorate of potassa being added at intervals, to oxidize the free selenium, until all the reddish color has disappeared.

The mixture is then diluted with water and filtered. The solution contains, besides iron, copper, mercury and a little lead, all the selenium as selenious and selenic acids. It is mixed either with about as much common salt as amounts to half the weight of the deposit originally employed, or with of its volume crude of fuming hydrochloric acid, and boiled down to about of its original bulk. The hydrochloric acid reduces the selenic acid to selenious acid.

On cooling, the solution is poured off from any sulphate of potassa and common salt which may have been deposited; these are washed several times with water, and the solution saturated with sulphurous acid gas, evolved from a mixture of powdered charcoal and concentrated sulphuric acid.

The selenium is thus precipitated of a fine red color. Its separation is promoted by digestion, and ultimately by boiling for a quarter of an hour, when it becomes black, and collects into a dense hard mass. It is well washed and dried.

The filtered liquid is boiled once more with hydrochloric acid, and again treated with sulphurous acid, in case it should still contain selenium.

The selenium thus obtained contains still small quantities of lead, copper, and iron, and especially mercury. On distilling it in a small retort or bent tube closed at one end, the first-mentioned impurities are left behind as selenides.

In order to free it from mercury, the distilled selenium is dissolved in aqua-regia, the greater excess of acid evaporated, so that no nitric acid may remain, the solution mixed with excess of carbonate of soda, evaporated to dryness, and the saline mass ignited to expel the mercury.

The mass is redissolved in water, the solution boiled with hydrochloric acid, and the selenium again precipitated by sulphurous acid.

Or the ignited mass may be mixed with about an equal weight of chloride of ammonium, and heated in a retort till the greater part of that salt has sublimed, when the selenium is reduced, and remains behind on dissolving the saline mass in water.

The selenium may also be at once extracted, and obtained free from mercury, by fusing the deposit, with an equal weight of carbonate of soda and about

of nitre, in a crucible. When the mass is in a state of tranquil fusion, it is poured out, so as to leave the oxide of lead, as far as possible, at the bottom of the crucible. It is then dissolved in water, the solution acidulated with sulphuric acid, the precipitated sulphate of lead filtered off, and the filtrate treated, as above, with hydrochloric acid and sulphurous acid.

It is necessary in this process that all the nitric acid from the nitre should either be expelled or decomposed, for otherwise part of the selenium will escape precipitation.

Or the solution of the fused saline mass is saturated with hydrochloric acid, chloride of ammonium added, evaporated to dryness, and the mass heated in a retort until the chloride of ammonium begins to sublime, when all the selenium is reduced.

106. SELENIUM SOOT.*

(Selenium with Selenides, Coal, Sand, &c.)

The black mass is moistened with sulphuric acid, thoroughly washed, fully dried and distilled from a porcelain or hard glass retort, with a strong heat, until most of the selenium passed over nearly pure.

The residue consisting of selenides, coal and other impurities, is dissolved in hydrochloric acid with gradual addition of nitric acid, and while hot the copper and iron are precipitated by caustic soda, the solution filtered, and the selenium precipitated by saturating with sulphurous acid, or reduced by adding an excess of chloride of ammonium, evaporating to dryness and heating until the chloride of ammonium begins to sublime, when the alkaline salt is washed out. If the selenium is precipitated directly from the solution containing copper more or less of this metal is thrown down.

In order to detect and separate the sulphur in the selenium, it is dissolved in very strong nitric acid, the

* It collects in the chimneys where the copper ores are roasted at Mansfeld. It contains from 30 to 40 per cent. of selenium after it is washed and dried.