112

SOLUTION OF AMMONIA.

the gas. By heating the liquid for some time, the whole of the ammonia may be driven off, so that nothing but water is left in the retort.

Solution of ammonia is prepared on the large scale by mixing together in a capacious retort equal weights of well-burned quicklime and sal ammoniac; the lime is slaked and made into a paste with water before mixture. The retort is then connected with a series of bottles similar to those used for condensing nitric acid. If the operation be conducted on the small scale in the laboratory, the arrangement shown in fig. 293 may be adopted. The three

B

necked bottles, B, C, D, E, are known by the name of Woulfe's bottles; in the globe, A, a small quantity of water is placed, to retain any solid particles which may be mechanically carried over by the gas; in the first bottle, в (which may be kept cool by immersion in cold water), a quantity of water equal in weight to that of the sal ammoniac used is introduced, taking care that it shall not fill more than half the capacity of the bottle, whilst the second contains water to condense any gas that may escape through the first. Each bottle is provided with a safety tube open at both ends, so that if the gas were absorbed in B, for example, more rapidly than it was supplied, instead of the liquid being driven back from the bottle, c, air would enter by the safety tube, and the equilibrium would be restored. The tube which delivers the gas passes down through the safety tube, and projects a little beyond its lower opening, so that the gas rises in bubbles through the liquid and collects in the bottle; an air-tight joint, which can be mounted and dismounted immediately, is thus obtained.

Solution of ammonia, if pure, should, when evaporated, leave

WOULFE'S BOTTLES-AMIDOGEN-AMMONIUM.

113

no solid residue; the presence of carbonic acid may be detected by lime-water, which it renders milky; that of chlorine by acidulating slightly with pure nitric acid, and adding nitrate of silver, when it gives a white cloud; that of sulphuric acid by a white precipitate with nitrate of barium after dilution and saturation with nitric acid; that of lime by a white precipitate on adding oxalate of ammonium; and that of copper or lead derived from the apparatus, by a black or brown precipitate or cloud with sulphuretted hydrogen. Lead in small quantity is a very frequent impurity in the commercial solution; it is usually derived from the action of the ammonia on the flint-glass bottles in which it is often improperly kept.

Alcohol also dissolves ammonia in abundance.

The salts of ammonia will be described with those of the other alkalies.

(371) AMIDOGEN (H,N=16).-Ammonia is the only compound of hydrogen and nitrogen that has been obtained in the isolated form. When, however, potassium is heated gently in perfectly dry ammoniacal gas, the ammonia disappears, half its volume of hydrogen is liberated, and a fusible, olive-green compound is formed, consisting of KH,N. The ammonia is decomposed by the potassium in the following manner: 2 H,N+K, becomes 2 KH,N +H. The compound HN has received the name of amidogen, and is supposed by some chemists to be capable of existing in combination with several metals and with a variety of bodies derived from the organic kingdom, though late researches have rendered it more probable that all these bodies are to be regarded as substitution-products formed upon the type of ammonia. Compounds of this class have received the name of amides; they will be more conveniently examined hereafter.

AMMONIUM (H1N=18).—This compound, as is the case with amidogen, has not been obtained in a separate form. All the usual so-called salts of ammonia, however, appear to contain it. Nitrate of ammonium, for example, consists not simply of H,N,NO, but of H,N,HO,NO,, that is, it, in addition, contains the elements of water, which cannot be expelled by heat without the entire decomposition of the salt; this nitrate is therefore looked upon as a nitrate of ammonium, H,NO,NO,, or H1NNе. Sal ammoniac is on this view regarded as chloride of ammonium H1N,Cl. The full discussion of the grounds upon which this theory rests will be best postponed till we enter upon a description of the salts of ammonia (610 et seq.).

CHAPTER VI.

THE HALOGENS.

(372) BEFORE proceeding to notice some other compounds of the four elements already described, it will be desirable to examine the other non-metallic simple substances. We pass on, therefore, to a group of four closely allied bodies, viz., chlorine, bromine, iodine, and fluorine. These elements are characterized by the powerful activity of their chemical attraction for other substances at the ordinary temperature of the air; and consequently none of them are found in an uncombined state. They form with the metals compounds analogous to sea salt, and have been termed halogens, or salt-producers (from aλç, sea salt).

§ I. CHLORINE: Cl=35'5; Theoretic Sp. Gr. 2453; Observed Sp. Gr. 247; Atomic Vol. *

(373) CHLORINE, the most important member of the group of halogens, was discovered by Scheele in 1774. It is abundantly met with in combination with sodium, with which it constitutes ordinary table salt. This necessary of life occurs plentifully in beds in various parts of the world, and is the most abundant of the saline bodies contained in the waters of the ocean. It contains rather more than 60 per cent. of chlorine.

Properties.-Chlorine is a transparent gas of a greenishyellow colour (whence the name is derived, from xλwpòs, green,) and of a powerful suffocating odour, producing, if breathed, even when largely diluted with air, distressing irritation of the air passages, attended with coughing. It is much heavier than air, 100 cubic inches weighing between 77 and 78 grains. Under a pressure of 4 atmospheres at 60°, it is condensed to a yellow, limpid liquid, of sp. gr. about 133; it does not conduct electricity, and remains unfrozen even at the cold of -220°. Chlorine is soluble in about half its bulk of cold water; this solution,

* Many chemists regard pure chlorine as chloride of chlorine, and consequently its molecular volume would be (CICI) = 2 vols. or [. The molecular volume of all the halogens in their free state will be 2, if that of chlorine be so regarded; the free substances being viewed respectively as chloride of chlorine, bromide of bromine, iodide of iodine.

PROPERTIES OF CHLORINE-BLEACHING ACTION.

115

which is readily formed by agitating the gas and water together, has the colour, odour, and astringent taste of the gas. According to Schönfeld, water at 50° dissolves 2.585 times its bulk of the gas; at 59°, 2368, and at 104°, 1365 times its bulk. Chlorine, in consequence of this solubility, cannot be advantageously collected over cold water. Mercury is acted upon by the gas with great rapidity. It is necessary, therefore, either to use warm water in the pneumatic trough, or to receive the gas by the process of displacement in dry bottles. With water, chlorine forms a definite hydrate (Cl, 5 H,O), which crystallizes at 32°; if it be enclosed in hermetically sealed tubes, it furnishes a ready method of obtaining liquefied chlorine, since it is easily decomposed by a gentle heat into water and free chlorine; the latter amounts to about one-fourth of the volume of the liquid.

Chlorine is not combustible, and it does not combine directly with oxygen. A taper burns in it with a reddish, smoky flame, the hydrogen of the combustible vapour of the wax combining with the chlorine, whilst part of the carbon, for which its chemical attraction is but small, is deposited. Many bodies, however, take fire spontaneously when introduced into chlorine; this is the case with phosphorus: many of the metals in a finely divided state do the same; among them are copper leaf, finely powdered antimony and arsenicum. A great number of organic substances rich in hydrogen are decomposed by chlorine, sometimes with such rapidity as to inflame them; a bit of paper dipped into oil of turpentine and plunged into the gas bursts into flame, and deposits an abundance of a black carbonaceous compound.

The action of chlorine upon bodies containing hydrogen is often of a very peculiar kind. It combines with part of the hydrogen and withdraws it from the combination; each atom of hydrogen uniting with an equivalent of chlorine, and forming a powerful acid, the hydrochloric (HCl); whilst at the same time for each atom of hydrogen so withdrawn from the original compound, an equivalent of chlorine is substituted. It is in this way that chlorine exerts those bleaching powers which have rendered so essential a service to the calico-printer and the paper-maker. Most of the vegetable colouring matters contain hydrogen, and are decomposed by chlorine, whilst colourless, or nearly colourless, compounds containing chlorine are formed instead of the coloured compounds with hydrogen. If a solution of chlorine be mixed with some of the blue liquid formed by dissolving indigo in sulphuric acid, or with ordinary writing ink, or with tincture of litmus, the colour will in each case be immediately and

116

MANUFACTURE OF CHLORINE.

almost completely discharged, and it cannot be subsequently restored.

Another property of chlorine of great value is its disinfecting power, by which is meant its power of destroying noxious vapours and miasmata; with this view it is frequently employed for fumigating buildings after the occurrence of contagious diseases.

Preparation.-1. Chlorine may be easily prepared from a mixture of 10 parts by weight of oil of vitriol, previously diluted with 7 parts of water, and allowed to cool, and 4 parts of pounded chloride of sodium mixed intimately with 3 parts of finely pulverized black oxide of manganese. The decomposition may be represented as follows:

Oxide mang.

Chlor. sodium.

Sulph. acid.

Sulph. mangan.

Sulphate of sodium. Water. Chlorine.

4

Mne,+2 NaCl + 3 H ̧§→1 = MnSO4 + 2 NaHSO, + 2 H2O + Cl2. The gas comes off slowly in the cold, but freely on the application of a gentle heat. A little hydrochloric acid is always formed in the reaction; this acid is easily removed from the chlorine by allowing the gas to bubble up through a vessel containing water, in the manner shown in fig. 289, where a similar apparatus is employed for carbonic oxide.

2. The preparation of chlorine is practised on an enormous scale in the manufacture of bleaching powder, or chloride of lime. It is generally prepared in capacious stills, sufficiently large to hold 200 gallons of liquid; these are usually made of Yorkshire flags clamped together with ironwork, and the joints rendered tight by vulcanized caoutchouc. The lower part of these stills is enclosed in a case through which a current of steam is transmitted. Hydrochloric acid in solution, of specific gravity from 1160 to 1170 (which is obtained as a waste product in the manufacture of carbonate of sodium from sea salt), is run through a curved funnel into the stills, which are charged with oxide of manganese in small lumps. Chloride of manganese is formed, and free chlorine is liberated in abundance; the reaction is illustrated by the following symbols :

Mno, + 4 HCI =

MnCl, + 2 H2O + Cl2. This process may also often be resorted to on the small scale in the laboratory with advantage. Three ounces of powdered oxide of manganese with half a pint of the commercial muriatic acid diluted with 3 ounces of water, will yield between 3 and 4 gallons Care must be taken not to use an acid more dilute

of the gas.