The chlorate of potassium being less soluble in water than the chloride, separates out as white tabular crystals. Chlorate of potassium is the only important salt of chloric acid. It is often used as an oxidizing agent, on account of the readiness with which it parts with oxygen. Thrown into warm strong sulphuric acid it explodes with violence, because the liberated chloric acid can only exist without water at a low temperature. A dilute solution

of chloric acid may be got by adding sulphuric acid to a solution of chlorate of barium. The sulphate of barium which is thereby formed is insoluble and sinks down; the solution of chloric acid may be decanted therefrom. All chlorates are soluble in water.

§ 190. Perchloric acid, HClO4, is got, in combination with oxide of potassium, by heating chlorate of potassium until only a portion (or) of its oxygen is expelled; thus:

2KC10, = KCl + KC10, + 20.

The residue, consisting of a mixture of chloride of potassium and perchlorate of potassium, is treated with a little cold water, which dissolves the chloride of potassium. The perchlorate is finally purified by recrystallizing it from boiling water. Perchloric acid in combination with water may be prepared by treating perchlorate of barium with sulphuric acid. It is much more stable than chloric acid.

Combination of Chlorine with Hydrogen.

§ 191. Hydrochloric acid, HCl, also called muriatic acid or chlorhydric acid.-This important body may be formed by the direct union of its elements. If equal volumes of chlorine and hydrogen be heated red-hot or exposed to the direct sunlight (sunshine), they combine completely and with explosive violence, giving rise to hydrochloric acid, HCl, which, when cool, occupies the same volume as the original gases. The density of hydro35.5+1 chloric acid is therefore or 18-25 (compared with air

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1.258). Even diffuse daylight and artificial light effect, according to their intensity, the more or less rapid formation of hydro

chloric acid out of its elements, and a mixture of equal volumes of the gases has been used as a photometer* or light-measurer.

§ 192. Hydrochloric acid, however, is more conveniently prepared by treating chloride of sodium with sulphuric acid; the reaction which takes place varies according to the relative quantities of the substances employed.

Or, if half the quantity of chloride of sodium is employed,

The second of these reactions is the one generally used in the laboratory; it requires little or no aid from heat. The first is employed in manufacture; it requires a high temperature, but economizes the sulphuric acid.

Sulphate of sodium is extensively employed in the manufacture of carbonate of sodium; and it is from this manufacture that the hydrochloric acid of commerce is chiefly derived, according to the above equation.

§ 193. Hydrochloric acid is a colourless gas, of suffocating acid smell; it fumes in moist air, on account of its combining with the water therein. The gas is eagerly absorbed by water; and the hydrochloric acid, or "muriatic acid," or "spirit of salt" of commerce is a solution of the gas in water. Water at 4° C. absorbs

about 470 times its volume of the gas; 100 volumes of water dissolve 47,000 volumes of HCl. If this solution be boiled, it at first gives off the pure gas, but afterwards distils at about 112° C. The composition of the acid distilled depends upon the pressure to which it is subject during distillation.

§ 194. Hydrochloric acid in solution attacks a large number of the metals (as iron, zinc, tin, aluminium), forming chlorides and evolving hydrogen. It also attacks, and generally dissolves,

*Or more properly actinometer, "the actinic rays" being the name given to that part of light which in this and many other cases affects the chemical force.

H

metallic oxides, forming the corresponding chlorides and water,

On this account hydrochloric acid is often used in order to bring metals or their oxides into a state of solution previously to their examination.

§ 195. Hydrochloric acid and soluble chlorides are detected by the formation of the insoluble chloride of silver when they are mixed with a soluble salt of that metal (§ 184). The chloride of silver is insoluble in all acids, but soluble in ammonia.

Compounds of Chlorine with Carbon.

§ 196. There will probably be found as many chlorides of carbon as there are hydrocarbons. They are usually got by the action of chlorine upon hydrocarbons, assisted by thelight of the sun. CH, +2nCl = CmCl2 + nHCl.

One part of the chlorine in such cases replaces the hydrogen, and another part of the chlorine unites with it. One may very often obtain an intermediate product in which only a part of the hydrogen is replaced; thus :

It is clear that there may be a very great many more of such chloro-hydrocarbons than there are of hydrocarbons alone.

§ 197. Chlorine combines with carbonic oxide in the direct sunlight, and produces a gas called phosgene gas, of which the composition is COCI. Equal volumes of the gases unite, and they contract in doing so to one volume. The density of this gas is therefore 49.5 (compared with air 3.414). It is called also chlorocarbonic acid. When in contact with water it is decomposed into carbonic acid and hydrochloric acid.

Compounds of Chlorine with Nitrogen.

§ 198. Chloride of ammonium, NH Cl, which consists of the

three elements, has been already sufficiently described under ammonia (§§ 133–145). If this body be submitted to the action of chlorine, some, perhaps all the hydrogen of the NH, may be replaced by chlorine, and an oily, highly explosive body be obtained, the so-called chloride of nitrogen.

CHAPTER X.

BROMINE AND IODINE, AND THEIR COMPOUNDS WITH THE

PRECEDING ELEMENTS.

Bromine.

Symbol, Br. Equivalent, 80. Density of vapour, 80 (compared with air 5.517).

§ 199. Bromine is a widely distributed but by no means abundant element. It is found in nature as an insoluble bromide of silver, AgBr. But its chief source is the sea (which contains a very small quantity of it) and certain mineral springs (from which it is also sometimes extracted). In both cases it is found as bromide of magnesium, MgBr,, a very soluble salt; so that when the less soluble salts, chloride of sodium, sulphate of magnesium, &c., have been separated by concentrating the brine, the bromine is found in the "mother-liquor" or liquid residue.

§ 200. A current of Cl is passed through the "mother-liquor" of sea-water; the chlorine forms chloride of magnesium with the magnesium, expelling the bromine, which dissolves in the water, deepening it in colour. The aqueous solution is then agitated with ether, which dissolves the bromine and rises with it to the surface of the liquid. The etherial solution is then decanted and shaken with a solution of hydrate of potassium. The bromine combines with the potassium partly as bromide, and partly as bromate of potassium,

6Br + 6KHO = 5KBr + KBr0, + 3H ̧0,

both of which dissolve in the water, while the ether is decolourized, and may be again employed for a similar purpose. On evaporating the aqueous solution of KBr and KBrO, to dryness, and heating, the KBRO, parts with the whole of its oxygen,

KBr0, = KBr + 0,,

so that the residue consists entirely of KBr. This salt, the bromide of potassium, is used extensively in photography, and also in medicine. From bromide of potassium, bromine may be prepared by a method quite similar to that employed (§ 181) in the preparation of chlorine, thus:

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2KBr + MnO2 + 2(H2SO) K2SO1 + MnSO ̧ + 2H ̧0 + 2Br. The bromine passes over as a reddish-black liquid of extremely pungent smell. Its density is about 2.96. It boils at 63° C., and freezes at -7° C. Exposed to the air, it evaporates, giving a red vapour.

Combination of Bromine with Oxygen.

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§ 201. Bromic acid, Br0, (only known in combination).—The most important oxide of bromine is bromic acid, Br2O̟, corresponding with chloric acid. Bromate of potassium is prepared in a way precisely similar to that used to make chlorate of potassium (§ 189), which body it closely resembles-namely, by treating a hot solution of hydrate of potassium with bromine. The free acid dissolved in water is formed by treating bromate of barium with sulphuric acid. Its formula is HBrO,.

Combination of Bromine with Hydrogen.

$202. Hydrobromic acid, HBr.-This body cannot be formed. similarly to hydrochloric acid, because, if a bromide is treated with strong sulphuric acid, bromine is set free, and part of the sulphuric acid is reduced to sulphurous acid; thus:

2KBr+ 2H,SO1 = K2SO, + 2H,0 + 2Br + SO2.

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It may be prepared in solution by precipitating the barium out of bromide of barium by means of dilute sulphuric acid.

BaBr2+ H2SO = BaSO, + 2HBr.

2

(insoluble.)