The affinity of nitrogen for other substances is not great, but it gives rise to five compounds, which are as below, in the order they are combined with oxygen:—
| Nitrous oxide (“laughing gas”) | (Monoxide) | N2O. |
| Nitric oxide | Dioxide | N2O2. |
| Nitrous acid | Trioxide | N2O3. |
| Nitric peroxide | Tetroxide | N2O4. |
| Nitric acid | Pentoxide | N2O5. |
These compounds are usually taken as representative examples of combining weight, and as explanatory of the symbolic nomenclature of chemistry, as they advance in such regular proportions of oxygen with nitrogen. The combining weight of nitrogen is 14, and when two parts combine with five of oxygen it makes nitric acid, and we put it down as N2O5; on adding water, HNO3, as we can see by eliminating the constituents and putting in the proportions. Actually it is H2N2O6, or, by division, HNO3.
Nitrogen plays a very important part in nature, particularly in the vegetable kingdom. Nitric acid has been known for centuries. Geber, the alchemist, was acquainted with a substance called “nitric,” which he found would yield a dissolvent under certain circumstances. He called it “dissolving fluid.” At the end of the twelfth century Albert Magnus investigated the properties of this acid, and in 1235 Raymond Lully prepared nitre with clay, and gave the liquid the name of “aqua-fortis.” But till 1849 nitric acid was only known as a hydrate,—that is, in combination with water,—but now we have the anhydrous acid.
Fig. 351.—Apparatus for obtaining nitrogen by using metal to absorb the oxygen of the air.
Oxygen and nitrogen combine under the influence of electricity, as shown by Cavendish, who passed a current through an atmospheric mixture of oxygen and nitrogen, in a tube terminating in a solution of potash, lime, and soda. Every time the spark passed, the volume of gas diminished, and nitric acid was formed, as it is in thunderstorms, when it does not remain free, but unites with ammonia, and forms a highly useful salt, which promotes vegetable growth. Here is another instance of the usefulness of thunderstorms, and of the grand provisions of nature for our benefit. Nitric acid is obtained by distilling nitre with sulphuric acid. The liquid is, when pure, colourless, and is a powerful oxidizer. It dissolves most metals, and destroys vegetable and animal substances. By an addition of a little sulphuric acid the water is taken from the nitric acid, and a very powerful form of it is the result. The acid is of great use in medicine, and as an application to bites of rabid animals or serpents. It converts cotton waste into “gun-cotton” by a very simple process of steeping, washing, and pressing. From the hydraulic press it comes in discs like “quoits,” which will burn harmlessly and smoulder away, but if detonated they explode with great violence. As a rule, when damp, it is not dangerous, but it can be fired even when wet. It will explode at a less temperature than gunpowder, and, moreover, yields no smoke, nor does it foul a gun. Gun-cotton, when dissolved in ether, gives us collodion for photographic purposes.
Fig. 352.—Nitric acid obtained from nitre and sulphuric acid.