CHLORIDE OF NITROGEN.

115

Chloride of nitrogen is a volatile yellow oil, of peculiar, penetrating odor; it is insoluble in water, and does not congeal when exposed to cold. Its specific gravity is 1.653. It decomposes very easily. Upon being heated to nearly 100°, or touched with any fat or oil, with turpentine, or with various other substances, it explodes with extreme violence; indeed it often explodes spontaneously, without any apparent cause. A single drop of it, exploded upon a glass or porcelain dish, shatters the vessel to atoms. The preparation and handling of this body require the greatest caution; it should never be prepared by the novice in chemistry.

118. We have heretofore adduced experimental proof of every proposition and statement so far as was possible at such a stage of the student's progress. The chemical properties of the four elements, oxygen, nitrogen, hydrogen, and chlorine, have been exhibited by experiment, the composition of many of their most important compounds has been demonstrated by analysis or by synthesis, or by both these methods, and the chemical properties of these compounds have been illustrated by actual experiment. Several objects have been thus attained: first, experimental methods of research have been illustrated by tangible examples; secondly, the foundation, scope, and application of important laws of chemical combination have been explained; thirdly, four leading elements have been minutely studied-hydrogen (the standard atom and the unity of specific gravity for gases), oxygen, nitrogen, and chlorine (three widely diffused elements, each of which is the first member and prototype of an important group of elements, many of whose properties we shall hereafter find we have already become acquainted with in studying the prototypes); fourthly, three compounds of these elements have been carefully studied-chlorhydric acid, water, and ammonia -compounds which are not only interesting in themselves, but of great significance as types, or models, of three large groups of compounds whose properties we have really been studying while we studied their types.

From this point forward the student will be asked to accept on trust many facts, drawn from the accumulated stores of the science and resting on satisfactory evidence, the full exposition

of which would be both tedious and inappropriate. The subjectmatter of chemistry is so vast and various that it will be necessary to select from the great mass of material only the most valuable portions, and to dwell only on those elements and compounds which are of practical importance in the useful arts, or which are of interest in connexion with instructive theories or recognized laws of the science.

119. Compounds of Chlorine and Oxygen.-Free chlorine does ot combine directly with free oxygen. But by resorting to indirect methods several compounds of the two elements can be obtained. As many as five different oxides of chlorine, enumerated below, have been described, though as yet some of them are known only in combination with water or other substances, and not in the free condition :

120. Hypochlorous Acid (ClO).-If a small quantity of slaked lime (hydrate of calcium) be thrown into a bottle of chlorine gas, and the mixture be then left to itself during several hours, the chlorine will be completely absorbed, and there will be formed two compounds, one of which will be found to be hypochlorite of calcium, the other chloride of calcium. The reaction may be thus formulated:

2(CaO,H,O) + 4C1 = CaO,C1,0

CaCl, + 2H,O.

This mixture is a substance much used in the arts under the technical names "chloride of lime," and "bleaching-powder;" it will be again referred to hereafter.

121. Hydrated hypochlorous acid may be prepared from "bleaching-powder;" the solution has a yellowish color, an acrid taste, and a peculiar sweet odor. When concentrated it decomposes rapidly, even if kept upon ice. Dilute solutions are

CHLOROUS, HYPOCHLORIC, AND CHLORIC ACIDS.

117

more stable, but they decompose slowly upon being boiled. Hypochlorous acid is a powerful oxidizing and bleaching agent. Its solution produces at once effects which are only slowly obtained when chlorine-water is employed.

Anhydrous hypochlorous acid, which may be obtained by removing the water from the aqueous solution, is a gas of pale yellow color and offensive odor, somewhat resembling that of chlorine. It decomposes very easily into 2 volumes of chlorine and 1 volume of oxygen; even the warmth of the hand is sufficient to decompose it; and it is difficult to preserve it unchanged even for a few hours. At low temperatures, such as are produced by a mixture of ice and salt, the gas condenses to a dark orange-colored liquid, heavier than water and very explosive.

122. Chlorous Acid, Cl,O,, may be obtained by deoxidizing chloric acid by means of nitrous acid. When in the anhydrous condition it is a gas of a yellowish-green color, liquefiable by extreme cold. It is a dangerous compound to prepare, since at temperatures above 57° it decomposes, with explosion, into chlorine and oxygen. It is readily soluble in water, and the solution possesses strong bleaching and oxidizing properties. It is a weaker acid than chloric acid, § 124, but resembles it in many respects. With metallic oxides it unites to form compounds

called chlorites.

123. Hypochloric Acid, C10,.-This very explosive compound may be prepared by gently heating a mixture of chlorate of potassium and concentrated sulphuric acid. The gas is of a bright yellow color and aromatic odor. Upon being exposed to daylight or to a temperature somewhat below the boiling-point of water, it decomposes into oxygen and chlorine, the decomposition being usually attended with explosion. The preparation of the gas is dangerous, and should never be attempted unless upon a very small scale. At the temperature of a mixture of ice and salt, the gas condenses to a yellow, highly explosive liquid.

124. Chloric Acid, Cl,O,.—In the present state of science this is the most important of the compounds of oxygen and chlorine. is not known in the free state, and in the hydrated condition tas never been obtained with less than 1 molecule of water, но, со..

When a current of chlorine is made to flow into a cold dilute aqueous solution of caustic potash, a mixture of hypochlorite and of chloride of potassium is produced:

2(K,O,H,O) + 4Cl = K,O,C1,0 + 2KCl + 2H20,—

the reaction being analogous to that between lime and chlorine, described in § 120. But if the conditions as to the concentration and temperature of the solution of potash be changed-if, instead of using a dilute solution, chlorine be passed into a moderately strong hot solution of caustic potash, or of carbonate of potassium, hypochlorous acid will no longer be formed, but instead of it chloric acid. The reaction may be expressed as follows:

6(K2O,H,O) + 12C1 = K,O,C1,0, +10KC1 + 6H2O. Chloride of potassium is formed as before, but the remainder of the chlorine is now more highly oxidized. Chlorate of potassium is less soluble than chloride of potassium; it separates in flat tabular crystals after the liquid has been concentrated by evaporation and cooled. It is the substance which was employed for making oxygen in Exp. 7.

Chloric acid could be prepared directly from chlorate of potassium by boiling a solution of this substance with fluosilicic acid. An almost insoluble fluosilicate of potassium would be formed, and chloric acid set free. But an easier method is to first convert the chlorate of potassium into chlorate of barium, and to liberate the chloric acid from this salt by means of sulphuric acid, with which barium forms a remarkably insoluble compound:

BaO,Cl.0, + H,O,SO,

=

BaO,SO, + H,O,C1,O.

The solution of chloric acid is separated from the insoluble sulphate of barium by filtration, and concentrated by evaporation over sulphuric acid in the exhausted receiver of an air-pump. By cautious evaporation the acid may be brought to a syrupy consistence, but is then rather easily decomposed, especially if it be heated or exposed to light. At the temperature of boiling it is rapidly converted into perchloric acid, water, chlorine, and oxygen. It is a strong acid, and a powerful oxidizing and bleaching agent.

125. Perchloric Acid, Cl,O,, is formed, as above stated, when an aqueous solution of chloric acid is boiled; being volatile it may be distilled off and collected. A compound of this acid and potassium, perchlorate of potassium, can be obtained by heating chlorate of potassium to a certain temperature. Perchloric acid is a more stable compound than either of the other oxides of chlorine. The dilute aqueous solution may be concentrated by

evaporation over fire, even at high temperatures. The hydrated acid H,O,C10, is a colorless, oily liquid, which boils at 203°, and has a specific gravity of 1.782. It is a powerful oxidizing agent.

The student will do well to compare this series of oxides of chlorine with that of the oxides of nitrogen, and to note the points in which the two series resemble and those in which they differ from each other.

CHAPTER IX.

BROMINE.

126. Bromine is an element closely allied to chlorine. It is found in small quantities in sea-water, and in the water of many saline springs. 1 litre of sea-water contains from 0.0143 to 0-1005 grm. of it. As it exists in nature it is combined with metals, bromide of magnesium being the compound most commonly met with. Bromide of magnesium is a constituent of the uncrystallizable residue, called bittern, which remains after the chloride of sodium has been crystallized out from the natural brines; at several saline springs this bittern contains so large a proportion of the bromide that bromine can be profitably extracted from it. Most of the bromine of commerce is thus obtained.

In order to obtain bromine from the bittern, the latter is mixed with black oxide of manganese and chlorhydric acid, and heated in a retort. Chlorine is of course evolved from these materials in the midst of the liquid; it reacts upon the bromide of magnesium and sets free bromine, which distils over into the receiver as a dark-red, very heavy liquid :— MgBr,+2C1 = MgCl2 + 2Br.

All the metallic bromides are readily decomposed by chlorine, bromine being, as a rule, a less energetic chemical agent than chlorine.

127. At the ordinary temperature bromine is a liquid of dark brown-red color, about three times as heavy as water, and highly poisonous. Its odor is irritating and disagreeable, whence the name bromine, derived from a Greek word and signifying a