remainder of the lead, arsenic, and copper, and the filtered liquid evaporated to dryness; the residue is treated with water (which leaves a little sesquioxide of iron undissolved), and the solution evaporated to crystallization. In order to purify the crystals thus obtained, they are dissolved in ether, crystallized by spontaneous evaporation, and recrystallized from water.

This salt forms fine yellow crystals, of the formula U,O,.NO,+6Aq. When heated, it undergoes the aqueous fusion, loses water, then parts with its acid, and subsequently with more or less oxygen. It is very soluble in water.

Uranate of potassa (KO.2U,O,) is obtained as a yellow powder when a salt of sesquioxide of uranium is precipitated with an excess of potassa, or when sesquioxide of uranium is fused with carbonate of potassa. A corresponding compound of soda is obtained in a similar manner.

Sulphate of Sesquioxide of Uranium crystallizes in small prisms, having the formula U,O,.SO,+3Aq. When heated to 212° F., they lose 2 eqs. of water, and become anhydrous at 572° F. (300° C.); the anhydrous salt absorbs 3Aq. when exposed to air. According to Ebelmen, a solution of this salt in alcohol, when exposed to the sun's rays, deposits the whole of the uranium as sulphate of the (prot-) oxide. It combines with sulphate of potassa, forming a doublesalt (not an alum) of the formula KO.SO,,U,O,.SO,+2Aq.

CHLORIDE (OR PROTOCHLORIDE) OF URANIUM, UCl.-This compound is prepared by passing chlorine, at a red heat, over an intimate mixture of (prot-) oxide of uranium and charcoal, when it condenses in the cool part of the tube in dark green octohedra, possessing a metallic lustre. It is deliquescent, and forms a green aqueous solution.

Oxychloride of Uranium or Chloride of Uranyle (U,O,Cl) is formed as a yellow, deliquescent, crystalline compound, when chlorine is passed over (prot-) oxide of uranium at a red heat; its vapor has an orange-yellow color. Chloride of uranyle combines with the chlorides of potassium and ammonium, forming the crystalline compounds U,O,Cl. KCl+2Aq, and U,O,CI.NH,Cl+2Aq.

Subchloride of Uranium (UCI) is formed when the (proto-) chloride is heated in a current of hydrogen. It dissolves in water, yielding a purple solution, which, after a time, becomes green, hydrogen being disengaged, and (proto-) chloride of uranium reproduced.

When sesquioxide of uranium is treated with hydrochloric acid, the compound U,O,CI is produced, which speaks strongly in favor of the existence of uranyle:

Little is known of the sulphides of uranium; the sesquisulphide appears to be a sulphur-acid, forming sulphur-salts with the sulphur-bases.

REACTIONS OF SESQUIOXIDE OF URANIUM.-Potassa, soda, and ammonia; yellow precipitates, insoluble in excess.

Alkaline Carbonates; yellow precipitates, soluble in excess, and reprecipitated on boiling.

Another method of extracting uranium from its ores, applicable on a large scale, consists in mixing the powdered pitchblende with half its weight of quicklime, and roasting for several hours in a reverberatory furnace. The mass is treated with dilute sulphuric acid, the copper and antimony precipitated by metallic iron, and the solution mixed with a large quantity of water, which precipitates the basic sulphate of sesquioxide of uranium.

Or, the powdered ore may be treated with a mixture of sulphuric and nitric acids, the excess of acid expelled by heat, and the perfectly dry mass treated with water, which extracts the whole of the sulphate of uranium, leaving the silica and insoluble sulphates. The clear liquid is poured into a hot solution of carbonate of soda, with constant agitation, until the alkaline reaction is nearly destroyed. The solution is filtered and boiled. when carbonate of lime, magnesia, and copper are precipitated. The sesquioxide of uranium is now separated from the solution, by slightly acidifying the boiling solution with hydrochloric or sulphuric acid.

Phosphate of Soda; white precipitate.

Ferrocyanide of Potassium; red-brown precipitate.

Sulphide of Ammonium; black sulphide of uranium.

With borax, in the outer flame, a greenish-yellow bead, which becomes green in the inner flame.

Organic matter interferes with the precipitation of sesquioxide of uranium by alkalies.

IRON.

Sym. Fe. Eq. 28.

§ 225. In describing this most important of metals, we shall adopt a course which it is our intention to pursue with all metals in common use, viz., that of giving first a purely chemical history of the metal and its compounds, reserving for subsequent consideration the smelting of its ores, and the various forms in which the metal is found in commerce, as well as those properties which belong rather to a technical than to a chemical work.

Preparation. In order to obtain iron in a state of purity, a quantity of pianowire (which is contaminated only with traces of carbon) is made up into small bundles, which are oxidized at the surface by heating to redness in a current of steam; these bundles are introduced into a porcelain crucible, and covered with a quantity of powdered green glass (free from lead); the porcelain crucible is now inclosed within a Hessian crucible, and exposed to the highest temperature of a wind furnace; the carbon contained in the iron wire is oxidized at the expense of the superficial coating of oxide, and the excess of the latter dissolves in the fused glass, leaving the pure iron in the form of a button at the bottom of the crucible. For the ordinary purposes of the chemist, however, pure iron is best prepared by reducing the sesquioxide of iron by means of hydrogen.

This oxide is obtained by precipitating the sesquichloride by excess of ammonia, heating, and washing the precipitate, first by decantation, and subsequently upon a filter, until the washings are free from chlorine; the precipitated hydrate is then dried at a sand-heat, reduced to powder, and introduced into a tube of hard glass drawn out to a point at one extremity, and connected at the other with an apparatus for the disengagement of pure hydrogen; when the apparatus is filled with the latter gas, the tube is heated with a spirit-lamp as long as any vapor of water is disengaged; the metal is thus obtained as a dark gray powder, which is pyrophoric, but if the reduction be effected at a very high temperature, this is not the case.

The button of pure iron is white, and possesses a silvery lustre.

General Properties of Iron.-Iron generally presents a dusky-gray color and a rather feeble lustre, which is, however, greatly increased by polishing, for iron is possessed of considerable hardness.

Bar-iron varies in specific gravity between 7.7 and 7.9.

Iron is a malleable metal, and exceeds all others in tenacity; an iron wire of inch in diameter is capable of supporting 705 lbs.

Iron is eminently magnetic at ordinary temperatures, but loses this character entirely at a very high temperature.

This metal is not affected by dry air or oxygen at the ordinary temperature; when heated in air, it is covered with a film of oxide, which presents an irides

1 Its pyrophoric properties are much enhanced by the presence of alumina ; a mixture of this description constitutes the pyrophoric iron of Magnus.

cent appearance, and changes in color as it increases in thickness; the oxidation takes place rapidly at a red heat, and a compound of (prot-) oxide and sesquioxide of iron is produced. Iron undergoes a rapid combustion when heated to whiteness in a forge, and we have already become acquainted with its combustion in oxygen gas; in both these cases, the above-mentioned proto sesquioxide is produced. The combustion of iron in air is also witnessed when a piece of this metal is violently struck with a flint, whereby small particles of metal are detached and raised, by the heat evolved in the stroke, to the temperature at which they burn in the air.

Iron is rapidly oxidized when exposed to moist air; it becomes covered with a film of red-brown, hydrated sesquioxide of iron, commonly termed rust. Iron does not rust under water containing minute quantities of the alkalies or their carbonates. It has been observed that the rusting of iron proceeds much more rapidly after the first spot of rust is formed, since this forms the negative pole of a voltaic couple, of which the iron is the positive pole, and which is capable of decomposing water, eliminating hydrogen, which, in its nascent state, is also said to combine with the nitrogen of the air, forming the ammonia which is always contained in the rust of iron, and may be elicited by heating the latter with potassa. Iron is more rapidly oxidized in air containing carbonic acid than in pure air.

A very high temperature is requisite for the fusion of iron; its fusing point, in fact, can only be attained in a good wind-furnace, but it becomes soft long before it fuses, and is then capable of being easily welded. If iron be allowed to cool gradually from a state of fusion, it deposits cubical, or octobedral crystals. Iron combines directly with most of the non-metallic elements. We have already seen that this metal is capable of decomposing steam at a red heat.

The strongest nitric acid acts but feebly upon iron, but when it is somewhat diluted, the metal is oxidized and dissolved very rapidly, nitrate of sesquioxide of iron being formed, and an inferior oxide of nitrogen disengaged. In nitric acid of a certain strength, iron dissolves without apparent evolution of hydrogen, since this gas, in its nascent state, is capable of decomposing the nitric acid, yielding nitrate of ammonia.

Iron which has been plunged into very strong nitric acid, is found to be unaffected by the dilute acid, and the same is observed with an iron wire, one end of which has been heated to redness. In both these cases, the iron is said to have assumed the passive state; and if, when in this condition, it be made the positive pole of a galvanic battery, it will be found that it does not combine with the oxygen which is liberated at its surface.1 Hydrochloric acid acts energetically upon iron, chloride of this metal being formed, and hydrogen liberated. The other strong hydrogen-acids have the same effect.

When iron is heated with concentrated sulphuric acid, it is oxidized at the expense of the latter, sulphurous acid being evolved, and sulphate of (prot-) oxide of iron produced. Dilute sulphuric acid dissolves iron with disengagement of hydrogen.

This curious phenomenon has been attributed in a plausible manner to the formation of a protecting film of oxide upon the surface of the metal, but this explanation does not account for all the properties of iron in its passive state.

2 Or, according to Levol, sulphate of sesquioxide.

The last of these is inclosed in parentheses, because it is not an independent oxide.

OXIDE, OR PROTOXIDE OF IRON.

FeO. Eq. 36.

§ 226. This oxide has never been obtained in the pure state; it is precipitated as a white hydrate, when solution of potassa is added to solution of (proto-) sulphate of iron; the precipitate, however, very readily absorbs oxygen from the air, being converted, first into the green hydrated magnetic oxide, and ultimately into red-brown bydrated sesquioxide of iron. If the two solutions be mixed in a state of ebullition, and the mixture be then boiled, a black precipitate is formed, which appears to be the anhydrous oxide of iron, but it is impossible to collect it without its suffering oxidation; it even decomposes water at the boiling point, eliminating hydrogen.

Hydrated (prot) oxide of iron dissolves in ammonia, but the solution rapidly deposits the sesquioxide when exposed to air.

This oxide is a very powerful base, and forms well-defined salts.

NITRATE OF (PROT-) OXIDE OF IRON, PROTONITRAte of Iron.

FeO.NO,

When iron is dissolved in cold dilute nitric acid, and the solution carefully concentrated, crystals of a double nitrate of oxide of iron and oxide of ammonium are deposited.

The nitrate of oxide of iron is best prepared by decomposing the sulphate with nitrate of baryta.

It is of a green color, and crystallizes with difficulty. Its aqueous solution is decomposed by ebullition, depositing a basic salt of the sesquioxide; the solution must therefore be concentrated in vacuo.

SULPHATE OF (PROT-)OXIDE OF IRON, PROTOSULPHATE OF IRON, commonly called SULPHATE of Iron, CoppERAS, and GREEN VITRIOL.

FeO.SO.

§ 227. This salt is found when iron, or sulphide of iron, is dissolved in dilute sulphuric acid :

Fe+HO.SO=FeO.SO,+H.

Preparation. It is generally prepared from the mineral known as ironpyrites, FeS,. This mineral is strongly heated in a retort, in order to separate part of the sulphur which it contains; the residue is then exposed to the action of air and moisture, when it absorbs oxygen, and is converted into sulphate of oxide of iron; the mass is exhausted with water, and the solution evaporated to crystallization.1

It will be remembered that a considerable quantity of this salt is obtained in the manufacture of alum (see § 211).

Properties.-Pure sulphate of iron has a slightly bluish-green color; it forms oblique rhomboidal prisms, which are transparent, and have the composition FeO.SO. HO+6Aq. When exposed to air, these soon become covered with an ochreous crust of a basic sulphate of sesquioxide of iron (2Fe,O,.SO,) from absorption of oxygen:

10(FeO.SO)+0,=3(Fe ̧0 ̧.3S0 ̧)+2Fe,O,.SO. The crystals lose 6 eqs. water of crystallization at 212° F. (100° C.), and fall to

Any copper which may be present in the crude lye is precipitated by means of metallic iron.

a grayish-white powder (Ferri sulphas exsiccatum), which dissolves gradually when treated with water; at a higher temperature, the last equivalent of (constitutional) water begins to go off, but cannot be thoroughly expelled without partial decomposition of the salt. After strongly heating, the sulphate is almost insoluble in water, but dissolves in nitric acid.

At a pretty strong red heat, sulphate of iron is completely decomposed, sesquioxide of iron (colcothar) being left, and sulphurous acid, together with anhydrous sulphuric acid, passing off:

since a little water is always present, the sulphuric acid is obtained in the receiver as a hydrate (see Nordhausen Sulphuric Acid, p. 159).

The crystals of sulphate of iron dissolve in 2 parts of cold and in part of boiling water; the solution has a pale green color; it is perfectly neutral, and of a nauseous metallic taste. When exposed to the air, solution of sulphate of iron deposits a dirty-brown basic sulphate of sesquioxide (2Fe,O, SO,), the neutral sulphate of sesquioxide (Fe,O, 3SO,) remaining in solution (see the above equation). If a solution of sulphate of iron be mixed with a slight excess of sulphuric acid, and crystallized at a temperature of 176° F. (80° C.), the formula of the crystals is FeO.SO,.HO+3Aq. If a large excess of sulphuric acid be present, the crystals are FeO.SO.HO+2Aq. On adding alcohol to solution of sulphate of iron, a white precipitate is obtained, which is FeO.SO,,HO; the same salt is obtained with concentrated sulphuric acid. The sulphate of iron is easily converted into sulphate of sesquioxide by oxidizing agents (e. g. chlorine, nitric acid). Sulphate of iron in solution (like all other protosalts of iron) is capable of absorbing the binoxide of nitrogen, forming a brown solution, which contains just so much of the binoxide of nitrogen that its oxygen would suffice to convert into sesquioxide all the (prot-) oxide of iron present.

Three varieties of sulphate of iron are met with in commerce; viz. 1. The pale green crystals which have been deposited from neutral solutions. 2. The bluish-green crystals obtained when the solution is acid; and 3. The bright green crystals formed in liquids which have been long exposed to the air; the difference of color appears to be due to the sulphate of sesquioxide of iron which is present.

The sulphate of iron prepared on the large scale is generally contaminated with sulphates of copper, zinc, manganese, alumina, magnesia, and lime; the manufacturer generally separates the copper by digesting the iron-liquor with scraps of metallic iron.

Cses. This salt is very largely employed by dyers and calico-printers, for it is the basis of several mordants and colors, and serves, moreover, for the preparation of others; thus, it is employed for the preparation of Prussian blue, and of white or reduced indigo, where the deoxidizing properties of the salt are turned to advantage.

The use of sulphate of iron for the preparation of fuming sulphuric acid and of colcothar, has already been alluded to. A considerable quantity is consumed in the preparation of ink. It is also used in the laboratory as a reducing agent.

Sulphate of iron is also an important medicinal agent. For this use it is highly important that it should contain no sulphate of copper, which may be shown by adding a few drops of dilute sulphuric acid, and immersing a bright steel knifeblade in it, which would become covered with a film of metallic copper after the lapse of a few minutes.

The sulphate of (prot-) oxide of iron is capable of forming double-salts, in which the water of constitution in the sulphate is replaced by sulphate of potassa, soda, ammonia, or oxide of manganese; these salts crystallize with 4 equivalents of water; the potassa-salt, however, contains 6 equivalents, unless deposited from