37. Ammonia. - We have seen (3) that ammonia is a compound of hydrogen and nitrogen. It is a colorless gas, which is greedily absorbed by water, forming the ordinary aqua ammonia. The eagerness with which it is taken up by water may be shown by filling a jar with the gas and opening it under water. The gas is instantly absorbed, and the water rushes in and fills the bottle. At a temperature of 32°, water will absorb more than one thousand times its bulk of ammonia.

Ammonia takes its name from the fact that it was first obtained from a salt found in Libya, near the temple of

Fig. 12.

It

Jupiter Ammon.
is now prepared from
the same salt, sal-am-
moniac or ammonic
chloride, which is
made in large quan-
tities from the waste
liquor of gas-works.
The sal-ammoniac is
powdered and mixed
with lime, and the
mixture is heated in
a flask arranged as
shown in Figure 12.

The gas is washed by passing it through a wash-bottle, and is then conducted into water.

38. Ammonium. — The symbol of ammonic chloride is (HN)Cl; that of ammonic nitrate is (H ̧N)NO。. If we compare these with the symbols of the potassic chloride and nitrate, KCl and KNO„, we shall see that the group of atoms HN plays the same part in the compounds as the metal potassium; and this is the case in a large class of salts. Hence this group of atoms has come to be considered as a metal, and has received the

name of ammonium. It has not, however, been obtained in a separate form, since it at once breaks up into ammonia, H,N, and hydrogen.

39. Cyanogen. - When carbon and nitrogen are heated together in the presence of potash, a remarkable compound called potassic cyanide (cyanide of potassium), KCN, is formed. From this a large number of substances can be prepared, all of which contain the group of atoms CN. To this group the name cyanogen (blue-maker) is given, from its forming a number of blue compounds. It is remarkable as entering into composition exactly like a non-metallic element. It can be obtained in a free state as a colorless gas by heating mercuric cyanide (cyanide of mercury.) It forms with hydrogen an acid, HCN, analogous to muriatic acid, HCl. This acid is called hydrocyanic or prussic acid.

The most important of the blue compounds of cyanogen is Prussian blue, which may be formed by mixing a solution of either the red or the yellow prussiate of potash with a solution of green vitriol.

Groups of atoms, like HN and CN, which play the part of elements, are called radicals.

40. The Characteristics of Nitrogen. - The most marked features of nitrogen are its inertness (29); the indirect way in which it enters into combination; and the great variety, the remarkable nature, and the instability of its compounds.

The great variety of its compounds is shown in the five which it forms with oxygen:

These compounds, as we have seen, are widely unlike in their properties; but in composition they differ only in the amount of oxygen which they contain.

The remarkable nature of the compounds is illustrated by ammonium and cyanogen, one of which acts like a metallic and the other as a non-metallic element.

The instability of the compounds is seen in the readiness with which the oxides give up their oxygen. Nitric iodide (iodide of nitrogen), which may be prepared by the action of ammonia on iodine, is so unstable that it explodes even when touched with a feather. Nitric chloride (chloride of nitrogen) is even more explosive, and is one of the most dangerous compounds known.

CARBON.

41. Properties of Carbon.-Carbon differs from the elements already described in being a solid when in a free state. It exists in three forms, which in outward appearance have nothing in common, but which are identical in their chemical relations. These three allotropic forms are diamond, graphite or plumbago, and charcoal. These differ in hardness, color, specific gravity, and the like; but when burnt in oxygen, the same weight of each produces the same weight of carbonic acid.

The diamond was first found to be pure carbon by Lavoisier in 1775-76. It occurs crystallized in certain rocks and gravels in India, Borneo, and Brazil. It is the hardest of all known bodies; and when cut, has a brilliant lustre and a high refractive power.

Graphite, plumbago, or black lead, is the substance used in the manufacture of the so-called "lead pencils.” Although very brittle, its particles are very hard, so that a saw or other instrument used in cutting it is soon dulled. Mixed with oil it is extensively used to diminish

the friction of machinery. It is the basis of most kinds of stove-polish, and of certain paints used to protect ironwork. It is also made into crucibles, which, on account of their infusibility, are much prized by the chemist. Carbon, in all its forms, is the most infusible substance known. Its infusibility and its allotropic states are its

'chief characteristics.

42. Carbonic Anhydride (Carbonic Acid). — The most important compound of carbon is CO2, carbonic anhydride, commonly called carbonic acid. This gas is readily prepared by the action of dilute muriatic acid on

Fig. 13.

chalk or marble (Figure 13). The reaction is as follows: :

CaCO3 + 2HCl = CaCl2 + H2O + CO2

CaCO3, calcic carbonate (chalk, or marble), is broken up by the action of the acid. The calcium combines with the chlorine of the acid, forming calcic chloride (chloride of calcium), CaCl2; one atom of the oxygen combines with the hydrogen of the acid, forming water; and CO2 is set free.

We have already seen (11) that lime-water readily shows the presence of carbonic acid, by becoming milky white. The lime-water contains lime, CaO, which com

bines with the carbonic acid, CO,, to form CaCO again.

If a lighted taper be plunged into a jar of carbonic acid, it goes out. We have seen that the same thing takes place in nitrogen; but the two gases can be distinguished by the lime-water test.

Carbonic acid is about 1.5 times as heavy as air, so that, with an ordinary dipper, it can be dipped out from one jar and poured into another, like water. It is poisonous when breathed, even though mixed with a large quantity of air.

43. Carbonic Oxide. Carbonic oxide, CO, is a gas formed when carbon burns with a limited supply of oxygen. It is often produced in an ordinary coal-fire, and is seen to burn with a pale blue flame. It is far more poisonous than carbonic acid.

CO may be obtained pure from several compounds of carbon. Thus, if crystallized oxalic acid, C2H2O1, be heated with strong sulphuric acid, the latter withdraws from it one molecule of water, H2O, leaving C2O, which at once splits up into CO and CO2. The CO2 may be removed by passing the mixed gases through a solution of caustic soda.

2

2

SULPHUR.

44. Sources of Sulphur.- Sulphur occurs in nature both free and combined: it is found free in certain volcanic countries, especially in Sicily and Iceland; it exists in combination with many metals, forming sulphides. These sulphides are the ores from which several of the metals are commonly obtained. Thus PbS, plumbic sulphide (galena), ZnS, zincic sulphide (zinc blende), and CuS, cupric sulphide (sulphide of copper), are the most productive ores of those metals.

In order to obtain pure sulphur, the mineral, containing