ture of coal-gas. The plants which when dead form coal, when living contain nitrogen. Some of this remains in the coal. When coal is heated in closed vessels, as in the manufacture of illuminating gas, the nitrogen unites with the hydrogen of the coal and escapes as ammonia. When the gas is washed by being passed through water, the water absorbs the ammonia and forms what is called the " ammoniacal liquor" of the gas-works.

§ 137. The solution of ammonia in water possesses many of the properties of solutions of metallic oxides in the same liquid. It is caustic to the taste, turns vegetable colours blue, and completely neutralizes acids, forming with them salts very much resembling in colour, crystalline shape, solubility, &c. the corresponding salts of the alkaline metals. Hence the solution of ammonia in water, which is also called ammonia*, is conveniently considered also as an oxide, namely the oxide of NH,, which is therefore called ammonium. Thus :NH2.

Ammonia

Solution of ammonia

in water, also 2NH,+H, ̧0=2NH,, 0=

called ammonia .

{

oxide of ammonium.

§ 138. If oxide of sodium be treated with HCl (see Chapter IX.), common salt or chloride of sodium is formed. Thus ::

Na,O + 2HCl = 2NaCl + H ̧0.

Similarly, if oxide of ammonium be treated with the same body, chloride of ammonium is formed.

2NH, O+ 2HCl = 2NHCl + H2O.

The chloride of ammonium greatly resembles the chloride of sodium in appearance, taste, &c. Similarly, other salts of ammonium can be formed by neutralizing acids with ammonia (oxide of ammonium). Thus :

2NH,, 0 + 2HNO,

=

2NH ̧NO, + H ̧0,—

3

the products in this case being water and nitrate of ammonium, the latter body being the salt we have already seen employed * See note †, p. 123.

(§ 129) as the source of nitrous oxide, NO. On account of its volatility and the alkaline nature of its solution, ammonia is sometimes called the volatile alkali.

§ 139. Chloride of ammonium is the salt of ammonium from which the gas ammonia, and the other salts of ammonium, are most usually prepared. Chloride of ammonium, NH ̧Cl, is obtained by neutralizing the ammoniacal liquor of the gas-works with hydrochloric acid, and evaporating to dryness, whereupon the chloride of ammonium crystallizes out in small cubic crystals. The same salt was formerly prepared at Ammonia (whence the name) in Asia Minor, by heating camels' dung (containing niFig. 59.

trogen) with common salt (containing chlorine). The chloride of ammonium so formed volatilizes and condenses on the cold surface of the upper part of the oven. This salt, NH Cl, bears sometimes the commercial name of "sal-ammoniac."

§ 140. When chloride of ammonium is heated with lime

(oxide of calcium) or oxide of potassium &c., the metal withdraws the chlorine, water is formed, and the gas ammonia, NH,, is expelled. The arrangement is seen in fig. 59. Slaked lime (hydrate of calcium, CaOH,O) is mixed with powdered NH Cl and a little water in the flask F. On applying heat, the following decomposition takes place :CaOH2O + 2NHC1

Slaked lime.

3

Chloride of ammonium.

=

CaCl2 + 2H20 +

2NH..

Ammonia.

The gas NH, escapes through the tube T, and may be collected (X) by displacement. The end of the tube T is turned up and reaches to the top of the vessel V. Ammonia being lighter than air (§ 133) collects in the tube V, and gradually displaces the air. Or (Y) it is collected over mercury; or (Z) it may be dissolved in water.

§ 141. The composition of the gas ammonia may be shown by leading it repeatedly through a white-hot tube containing copper or iron, or by subjecting it to a series of powerful electric sparks. Every two volumes of ammonia then give rise to 4 volumes of gas, of which 3 are H and 1 is N. Hence (compare § 133) gaseous ammonia is 8.5 times as heavy as hydrogen, or 0.586 the density of air.

§ 142. Ammonia is exceedingly soluble in water. At 0° C. 1 vol. of water dissolves about 1100 vols. of NH,, and about 730 vols. at 15° C. Ammonia is recognized when in the free state by its peculiar pungent smell (the same smell as that of "salvolatile," which is the carbonate of ammonia), by its power of turning red vegetable colours blue when moist, and by the dense white clouds which it produces when it meets in the gaseous state with volatile acids.

§ 143. All salts of ammonium are decomposed when heated with oxide of calcium, potassium, or sodium, or the hydrates of those metals, CaOH,O, KHO, or NaHO. Hence, in order to see whether a body contains any compound of ammonium, it may be gently heated with water and hydrate of potassium, whereupon the ammonia, if present, will be given off as a gas, and can

be recognized by its smell, power of bluing vegetable colours, and fuming when a rod dipped in hydrochloric acid, HCl, or nitric acid, HNO,, is held near it. The NH, as it meets with the HCl or HNO, combines with it, and forms minute particles of solid salt in the air; these constitute the clouds; thus :

Ammonium in combination is also recognized by the fact that chloride of ammonium, NH Cl, forms an insoluble compound with chloride of platinum, PtCl. If any salt of ammonium is mixed with hydrochloric acid and chloride of platinum, a heavy orangeyellow crystalline precipitate is formed called the platinochloride of ammonium, and having the composition

2NHCl, PtC.

This body is so insoluble, especially in alcohol, that it is employed for determining the quantity of ammonium present. With this exception, and the tartrate, almost all salts of ammonium are soluble in water.

§ 144. Gaseous ammonia, NH,, may be condensed by great cold or considerable pressure to a colourless liquid; and the liquid may be solidified by a further withdrawal of heat. Under a pressure of 1 atmosphere, NH, becomes liquid at

Thus :

:

-40° C.

$145. Ammonium, NH,.-The body which has been above called ammonium (that is, NH,) cannot be got in the separate state. But a substance having that composition may be readily obtained mixed with mercury on warming a solution of chloride of ammonium with an amalgam of sodium.

The mercury swells up, increasing to many times its original volume, but preserving its metallic lustre. This substance, which is called "ammonium amalgam," is very instable at ordinary temperatures, breaking up into Hg, H, and NH ̧.

CHAPTER VIII.

CARBON AND ITS COMPOUNDS WITH THE PRECEDING ELEMENTS. Symbol, C. Equivalent, 12. (Hypothetical) density of vapour, 12 (compared with air 0.828).

§ 146. Carbon occurs in nature in nearly the pure state as crystallized carbon or diamond, a well-known precious stone, found chiefly in the East Indies and in Brazil. Its crystalline forms are the octahedron and forms derived from it. The specific gravity of the diamond is 3.5. It is possessed of a great refractive power for light; this and its rarity make it valued as a gem. In the arts it is chiefly used for cutting glass and abrading hard substances.

Carbon also occurs nearly pure as graphite, plumbago, or blacklead. Its origin in this form is probably due to the action of the internal heat of the earth upon the vegetable matter buried in its crust. Graphite may be prepared artificially by dissolving carbon in hot iron, and letting it separate out as the iron cools.

Carbon may also be got in a state of great purity by decomposing by heat some substances which contain it. Such a form of carbon is lampblack.

It is therefore seen that the same element, carbon, may appear under three entirely different modifications-diamond, graphite, and lampblack.

By no mere physical examination could we conclude that these bodies were the same chemical element, carbon. One similar example we have had in the case of oxygen and ozone; others will appear in the sequel.

Diamond may be converted into graphite, and graphite into soot or lampblack; soot or lampblack may be converted into graphite, but neither lampblack nor graphite has been converted into diamond possessing all the properties of the natural gem.

§ 147. The most familiar form of carbon, in a state of considerable impurity, is coke or charcoal. Wood consists almost exclusively of oxygen, carbon, hydrogen, and metals.

When