one branch of which shall be long enough to reach to the bottom of the bottle, while the other arm, made much shorter than the first, dips into an open beaker glass half full of water.

Place the apparatus in such a position that it shall be exposed to as much direct sunlight as possible. After a time oxygen gas will begin to collect at the top of the bottle, and in the course of several hours, or days, so much will have collected that it can be tested by removing the cork from the bottle and thrusting in a glowing splinter. The liquid displaced by the oxygen flows over, through the tube, into the beaker glass. The chlorhydric acid of course remains dissolved in the water of the bottle.

114. The applications of chlorine in the arts depend upon that readiness to combine with hydrogen which has just been exemplified. By virtue of this affinity for hydrogen, chlorine acts indirectly as a powerful oxidizing agent. It acts as a purveyor of nascent oxygen, and is hence a much more efficient agent than free oxygen, such as exists in the air. Its chief uses are for bleaching cotton goods, paper stock, and so forth, and for destroying foul and unhealthy emanations.

Exp. 62.-Pour into a test-glass a quantity of chlorine-water (Exp. 52), drop into it a small quantity of a solution of indigo, and stir the mixture with a glass rod. The blue color of the indigo will be immediately destroyed.

In the same way the color of litmus, cochineal, aniline-purple, or of flowers, calico, and the like, can be readily destroyed by immersion in chlorine-water or in moist chlorine gas. The presence of water is essential; perfectly dry chlorine will not bleach.

Exp. 63.-Fill a glass tube, No. 1, about 20 c.m. long, with scraps of coloured calico and bits of paper which have been written upon with ink. Take care that the tube and its contents are perfectly dry, and that the tube is closed at either end with a cork, through which passes a short piece of tubing, No. 6. Place the tube in a vertical position, and pass into it, from below, chlorine gas which has been thoroughly dried by means of chloride of calcium (Appendix, § 15). The coloring-matters will not be destroyed so long as they remain dry; but if, after the dry chlorine has been allowed to act for a few minutes, a little water be poured in at the top of the tube, so that its contents may be wetted, they will be bleached at once.

115. Those coloring-matters which are of vegetable or animal origin are for the most part complex compounds of carbon, hy

HOW CHLORINE BLEACHES AND DISINFECTS.

113

drogen, nitrogen, and oxygen. When moist chlorine is brought into contact with them, a somewhat complicated reaction occurs; a portion of their hydrogen is no doubt taken out by the chlorine, but at the same time some of the water which is present is decomposed, and its oxygen assists the disorganization of the compound which is to be destroyed.

Of the hydrogenized or carburetted compounds exposed to the action of the nascent oxygen in the foregoing experiment, those which are most complex, and of which the elements are held together least firmly, will of course be acted upon, burned up, and destroyed. As a rule, the coloring-matters are far more easily oxidized than the cotton cloth; hence they can readily be removed by the action of chlorine, without injury to the cloth. But if the action of the chlorine were to be continued after the coloring-matter had been destroyed, the cloth itself would gradually be burned up.

In actual practice, where the duration of the exposure of the cloth to the chlorine is carefully regulated, and the portions of bleaching liquor which at first remain adhering to the cloth are completely removed by washing and by chemical treatment, the process is perfectly safe and trustworthy as regards cotton or even linen; but the animal fibres, such as wool and silk, are of more complex composition than cotton and linen; they cannot be bleached by chlorine, since this gas would attack and disorganize them.

116. In destroying noxious effluvia, chlorine either acts upon them as upon coloring-matters, or it simply takes away hydrogen, as in the case of sulphuretted hydrogen hereafter to be studied. Putrid animal matter may be rendered comparatively odorless, by sprinkling it copiously with chlorine-water; hence a solution of chlorine finds some application in inquests and judicial investigations.

The energy with which chlorine seizes upon hydrogen may be further illustrated by causing chlorine to act upon ammonia-water.

Exp. 64.-Into a glass tube, No. 1, about a metre long, pour enough chlorine-water to fill it nine-tenths full, and then ammonia-water enough to fill the remaining space. Close the tube with the thumb, invert it and place it in an upright position upon the water-pan. The

I

114

ACTION OF CHLORINE ON AMMONIA.

ammonia-water, being specifically lighter than the solution of chlorine, will flow upwards and become mixed with the latter; a reaction will immediately ensue; some of the chlorine will unite with the hydrogen of a portion of the ammonia, to form chlorhydric acid, and nitrogen will be set free. Numberless little bubbles of this gas will escape from the liquor and collect at the top of the tube, and may be subsequently tested with a burning match. The chlorhydric acid formed unites with the remainder of the ammonia to form chloride of ammonium :—

4NH, + 3C1 = N + 3NH,CI.

By modifying the apparatus employed in the foregoing experiment, so that a current of chlorine can be passed into a vessel containing ammonia-water, the evolution of nitrogen can readily be made continuous, and large quantities of the gas may be collected. It would be an excellent and easy method of preparing nitrogen for use in the laboratory, were it not that care must be taken that the ammonia shall always be present in considerable excess. If this precaution were neglected, there might be formed, by the action of the chlorine upon the chloride of ammonium, a very dangerous compound called chloride of nitrogen. As prepared by this method, the nitrogen is always contaminated with a certain amount of oxygen.

In the foregoing experiment, the chloride of ammonium which is produced remains dissolved in the water. It may be recovered by evaporating the water, or a new portion of it may be prepared by mixing chlorhydric acid with ammonia.

Exp. 65.-Fill one half-litre bottle with dry ammonia-gas, and another with dry chlorhydric acid gas. Invert the latter, and place it over the former, so that the mouth of the upper bottle shall rest upon that of the lower. The gases will immediately unite to form solid chloride of ammonium, a dense white cloud of which will fill the bottles :—

NH, + HCl = NHẠC.

One volume of ammonia unites with one volume of chlorhydric acid, and the gases are completely condensed to a white solid.

117. Chloride of nitrogen, the dangerous compound of chlorine and nitrogen which has been alluded to above, is formed when chlorine is brought into contact with a weak solution of chloride or nitrate of ammonium at the temperature of 15° or 20°. As the chlorine is gradually absorbed, yellow oily drops of chloride of nitrogen form upon the surface of the liquid, and soon fall to the bottom:

NH,C1 + 6C1 = 4HC1 + NC1.

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 :

NAMES.

COMPOSITION.

FORMULE.

By volume.

By weight.

Chlorine. Oxygen.

2

35.5x2=71

16 Cl2O

?

2

35.5

2 vols. +5 vols.

?

.....

Hypochlorous acid. 2 vols. +1 vol. Chlorous acid........ 2 vols. +3 vols. 355x2=71 16x3= 48 Cl2O, Hypochloric acid... 1 vol. +2 vols. 16x2= 32 CIO, Chloric acid 355x2=71 16x5= 80 ClO Perchloric acid.. 2 vols. +7 vols. ? 355x2=71 16x7=112 C12O

120. Hypochlorous Acid (Cl ̧O).—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,C1O + CaC1, + 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