air, has a cooling taste, and is soluble in ten times its weight of cold water.

Carbonate of cosium (Cs¤Ð ̧=326) is deliquescent: it requires five times its weight of boiling alcohol for solution. An acid carbonate (CsHЄO=194) may be obtained in brilliant prismatic crystals, which are permanent in the air.

The salts of cœsium are not easily distinguished from those of potassium and rubidium, except by the characteristic lines in the spectrum of their flame (Part I. p. 172).

§ VI. AMMONIUм: H2N=18 (hypothetical).

(610) Action of Oxyacid Anhydrides on Ammonia.-When dry gaseous ammonia (H ̧N) is presented to the oxans, or anhydrides of the oxyacids, such as sulphuric (SO), sulphurous (SO), or carbonic (E) anhydride, the gas enters into combination with the anhydride, and a peculiar compound is formed, in which it is maintained by Laurent and Gerhardt that one-half of the ammonia only exists in the form of an ordinary ammoniacal salt, the other half having entered into combination with the elements of the anhydride, to form a compound termed an amidated acid; the product obtained differs, therefore, in many important particulars from the compound which would be obtained by neutralizing with ammonia a solution of the same acid in water. In the latter case one of the ordinary 'salts of ammonia,' as they are usually termed, is produced; in the former case an ammoniacal salt of new amidated acid would be the result; but the preparation of these amidated compounds is difficult, and their true nature is not as yet thoroughly ascertained.*

The general properties of these bodies may be illustrated by examining the several combinations formed between the sulphuric and sulphurous anhydrides and dry ammoniacal gas.

(611) Sulphuric Ammonide, Sulphat-ammon (H ̧N),SØ ̧.—At

*These compounds of ammonia with the anhydrides are often incorrectly spoken of as amides. The amides of monobasic acids are, properly speaking, salts of ammonium which have been deprived of 1 atom of water. Benzoate of ammonium (H,NE,HO), for example, when deprived of He, furnishes a white fusible volatile solid known as benzamide (H2NE,H,0). The amides of the dibasic acids are salts of ammonium which have been deprived of 2 atoms of water. Sulphamide would be (H,N),Se, and would contain an atom of water less than sulphuric ammonide. The ammonides, or ammons, as these compounds of ammonia with the anhydrides of dibasic acids have been termed, contain only one atom of water less than the ordinary salts of ammonium. Sulphate of ammonium, for instance, may be represented as (H,N),SO, while sulphuric ammonide contains (H,N),SO,. The different varieties of compounds obtained from the salts of ammonia by dehydration will be considered amongst the products of organic chemistry.

least three distinct compounds may be obtained by the action of dry ammonia on sulphuric anhydride. When a current of dry ammoniacal gas is transmitted over sulphuric anhydride, placed in a flask, and maintained at a low temperature, taking care to leave the anhydride somewhat in excess, a hard gummy mass is produced, which when exposed to the air absorbs moisture and gradually deliquesces. The liquid thus obtained is saturated with carbonate of barium, in order to remove the excess of acid, and is then evaporated; it yields large transparent crystals derived from an octohedron with a square base. This compound is the parasulphat-ammon of Rose, and consists, according to this chemist, of (H,N),SO. It is freely soluble in water, but insoluble in alcohol. Its solution has a bitter taste, and gives no precipitate with salts of barium, and none with perchloride of platinum. By long boiling with water, or with a solution of tartaric acid, it is slowly changed into ordinary sulphate of ammonium; but if heated with a free alkali, sulphate of the alkaline metal is speedily produced, and ammonia is expelled.

If ammoniacal gas in excess be made to act upon sulphuric anhydride, another compound, isomeric with the former, termed sulphat-ammon by Rose, is obtained. It does not crystallize, and is quickly transformed when in solution into sulphate of ammonium.

A third compound, which may be procured in beautiful transparent crystals, is prepared by transmitting the vapour of sulphuric anhydride into ammoniacal gas in excess; the solid compound thus obtained is fused in a current of dry ammonia, and dissolved in water. The crystals obtained on evaporation, according to Jacquelain, consist of (H,N), 2 S. Although the solution of this compound has an acid reaction, it gives no precipitate with salts of barium.

(612) Sulphit-Ammon (H ̧N),SO,.-If dry gaseous sulphurous anhydride be mixed with an excess of perfectly dry ammoniacal gas, 1 volume of the anhydride and 2 of ammonia combine and form a yellow amorphous, volatile, deliquescent compound, which when dissolved in water undergoes gradual decomposition (Rose).

If the sulphurous anhydride be in excess, a different compound is formed (H,NSO), corresponding in composition to acidsulphite of ammonium from which I atom of water has been abstracted: H,NSO2+H2O=H ̧NHSO. It is a reddish-yellow, crystalline, volatile substance, freely soluble in water: when in solution, it is speedily decomposed into sulphate and trithionate of ammonium ; 4 (H ̧NSO2) +2 H2O=(H ̧N),SO,+(H ̧N),SO. No

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454

THEORY OF AMMONIUM.

such decomposition occurs when the ordinary acid sulphite of ammonium is dissolved in water.

Phosphoric and carbonic anhydrides also form ammonides, which are analogous to those which have just been described.

(613) Action of Anhydrous Hydracids on Ammonia.-Dry ammoniacal gas likewise unites with facility with the anhydrous hydracids, but the compounds which are produced do not correspond in properties to the ammonides, but, on the contrary, form ordinary salts of ammonium. For example, dry hydrochloric acid and dry ammoniacal gases unite with avidity, and a white solid compound is produced, which is ordinary sal ammoniac; when dissolved in water it gives with solution of nitrate of silver the usual curdy precipitate indicative of chlorine, and with perchloride of platinum the usual yellow double salt characteristic of salts of ammonium is deposited.

(614) Theory of Ammonium.-The differences between the characters of the compounds which dry ammonia forms with the oxyacid anhydrides, and those which it produces with the anhydrous hydracids were explained by Berzelius with the aid of a hypothesis originally suggested by Ampère, which has been termed the ammonium theory, by the adoption of which the salts of ammonia admit of being considered as possessing a constitution analogous to that of the metallic salts.

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According to this view, the compounds which are frequently spoken of as salts of ammonia with the oxyacids do not contain ammonia at all, but a compound in which the elements of an atom of water have been added to those of ammonia [H,N+H,O]: in consequence of the assimilation of this atom of water, the substance which is formed may be regarded as (HN) HO, hydrated oxide of ammonium, analogous to hydrate of potash (KHO), and the basis of the salts which it yields would be the compound body ammonium (HN), which takes the place of a metal. Anhydrous ammonia, when it unites with the oxyacid anhydrides, must therefore produce bodies very different from those obtained by the combination of hydrated ammonia with the compounds formed by the action of the same anhydrides upon water, as may be seen, for instance, by comparing the formula of the compound with sulphuric anhydride, and with sulphuric acid when water is present:

It is likewise easy to see why, by the combination of anhydrous ammonia with a hydracid equally free from water, a true salt of

AMALGAM OF AMMONIUM.

455

ammonium should be formed: for instance, hydrochloric acid and ammonia by their union yield chloride of ammonium, a salt which obviously presents the closest analogy with the metallic chlorides; H2N+HCl=(H1N)Cl.

With the oxyacids, then, ammonia forms two classes of compounds; the more important class constitutes the normal salts of the alkali in which the elements of water are necessarily present; the other class consists of the ammonides already described.

The theory of ammonium is supposed to derive considerable support from the following remarkable experiment:—If an amalgam of potassium or of sodium be moistened with a concentrated solution of sal ammoniac (HNCI), the amalgam immediately begins to increase in bulk, and ultimately swells up till it acquires 8 or 10 times its original volume, and it at the same time assumes a pasty consistence, but still preserves its metallic lustre. This substance, if exposed to a temperature of o° F., crystallizes in cubes. It begins to undergo spontaneous decomposition immediately after its production, and the same effect occurs still more rapidly if it be placed in water: hydrogen gas is given off in minute bubbles, and ammonia is found in the solution. It is generally supposed that this remarkable amalgam consists of a combination of HN (or ammonium) with mercury.

On attempting to expel the mercury by heat, however, the compound is decomposed, metallic mercury is sublimed, and a mixture of hydrogen and ammoniacal gas evolved: all other attempts to isolate the ammonium have been equally unsuccessful. The proportion of ammonium present in the amalgam, notwithstanding the great change in bulk and in properties experienced by the mercury, is extremely minute, amounting, according to GayLussac and Thénard, to little more than one two-thousandth of the weight of the mercury.

(615) Solution of Ammonia.—The preparation of ammoniacal gas and of its aqueous solution have been already described (369). The solution in water has an intensely alkaline reaction, and may be regarded as a solution of hydrated oxide of ammonium (H,NHO); but when heated, pure ammoniacal gas (H,N) alone is expelled, and by passing the gas through a tube filled with quicklime, to absorb the water which it carries over with it in suspension, ammonia may be obtained in a state of purity. The solution in water, when neutralized by acids and evaporated, yields the ordinary salts of ammonium.

The compounds of ammonium, are continually receiving fresh applications. The sulphate and the chloride are extensively used

456

SULPHIDES OF AMMONIUM.

in the preparation of ammonia-alum, as a substitute for the corresponding compounds of potassium; for agricultural purposes the sulphate is largely used, as a manure for cereal crops. Caustic ammonia is employed by the dyer as a solvent for cochineal; it is essential in the preparation of orchil and the lichen colours. A remarkable application of ammonia has also lately been made in Carré's refrigerating machines (note, Part I., p 298), which are already extensively used in chemical operations on a large scale.

(616) SULPHIDES OF AMMONIUM.-Ammonium forms several sulphides which are freely soluble in water. The protosulphide [(HN)2S], if it exist at all, cannot be procured in a solid form: it may be prepared in solution by dividing a quantity of solution of ammonia into two equal portions, through one of which sulphuretted hydrogen is transmitted so long as it is absorbed; the saturated liquid is then added to the second portion of the solution. It is, however, possible, though not probable, that the two solutions H NHS and H ̧N,HO may remain uncombined, instead of uniting to form (H,N),S+H,. This liquid dissolves many of the sulphides of the metals which furnish acids with oxygen, and forms double sulphides with them (535, 551). Many of these double sulphides may be obtained in crystals; this, for example, is the case with those which contain the pentasulphide of antimony and of arsensic, and the tersulphide of molybdenum.

Bisulphide of ammonium (H4N),S2, or (HNS) may be obtained in large yellow, transparent, very deliquescent crystals, by passing sulphur vapour and dry ammonia through a redhot porcelain tube. In the hydrated form it has been long known as Boyle's fuming liquor, and is prepared by calcining 3 parts of slaked lime with 2 of sulphur, and distilling 3 parts of this mixture with 2 of sal ammoniac and 1 part of sulphur: a yellow, oily, fœtid liquor passes over which fumes in the air, and on cooling, deposits deliquescent yellow lamellar crystals; acids disengage hydrosulphuric acid from it, and cause a deposit of sulphur. Its aqueous solution dissolves a large quantity of sulphur, forming a pentasulphide, (H4N),S, which crystallizes from its solution in long orange-yellow oblique rhombic prisms. Fritsche has also obtained a crystallized compound containing (H ̧N),S,.

Sulphide of ammonium and hydrogen, or sulphhydrate of ammonium (H1NHS=51) is the liquid commonly used as a reagent under the name of hydrosulphate of ammonia: it is formed by transmitting sulphuretted hydrogen through a solution of ammonia to saturation. This liquid, when newly prepared, is colourless, but it absorbs oxygen rapidly from the air, and becomes yellow from