Nitric acid is procurable in a more concentrated state, and with much less loss, by mixing very dry clay with saltpetre. This mixture is put into an earthern retort, and distilled with a strong fire. The clay combines with the potash, for which it has great affinity, and the nitric acid passes over, slightly impregnated with nitrous gas. This is easily disengaged by heating the acid gently in a retort, a small quantity of nitrous gas passes over into the recipient, and very pure concentrated nitric acid remains in the retort.
We have already seen that azote is the nitric radical. If to 20-1/2 parts, by weight, of azote 43-1/2 parts of oxygen be added, 64 parts of nitrous gas are formed; and, if to this we join 36 additional parts of oxygen, 100 parts of nitric acid result from the combination. Intermediate quantities of oxygen between these two extremes of oxygenation produce different species of nitrous acid, or, in other words, nitric acid less or more impregnated with nitrous gas. I ascertained the above proportions by means of decomposition; and, though I cannot answer for their absolute accuracy, they cannot be far removed from truth. Mr Cavendish, who first showed by synthetic experiments that azote is the base of nitric acid, gives the proportions of azote a little larger than I have done; but, as it is not improbable that he produced the nitrous acid and not the nitric, that circumstance explains in some degree the difference in the results of our experiments.
As, in all experiments of a philosophical nature, the utmost possible degree of accuracy is required, we must procure the nitric acid for experimental purposes, from nitre which has been previously purified from all foreign matter. If, after distillation, any sulphuric acid is suspected in the nitric acid, it is easily separated by dropping in a little nitrat of barytes, so long as any precipitation takes place; the sulphuric acid, from its greater affinity, attracts the barytes, and forms with it an insoluble neutral salt, which falls to the bottom. It may be purified in the same manner from muriatic acid, by dropping in a little nitrat of silver so long as any precipitation of muriat of silver is produced. When these two precipitations are finished, distill off about seven-eighths of the acid by a gentle heat, and what comes over is in the most perfect degree of purity.
The nitric acid is one of the most prone to combination, and is at the same time very easily decomposed. Almost all the simple substances, with the exception of gold, silver, and platina, rob it less or more of its oxygen; some of them even decompose it altogether. It was very anciently known, and its combinations have been more studied by chemists than those of any other acid. These combinations were named nitres by Messrs Macquer and Beaumé; but we have changed their names to nitrats and nitrites, according as they are formed by nitric or by nitrous acid, and have added the specific name of each particular base, to distinguish the several combinations from each other.
Table of the Combinations of Sulphuric Acid with the Salifiable Bases, in the order of affinity.
| Names of the bases. | Resulting compounds. | |||
| New nomenclature. | Old nomenclature. | |||
| Barytes | Sulphat of | barytes | Heavy spar. Vitriol of heavy earth. | |
| Potash | potash | Vitriolated tartar. Sal de duobus. Arcanum duplicatam. | ||
| Soda | soda | Glauber's salt. | ||
| Lime | lime | Selenite, gypsum, calcareous vitriol. | ||
| Magnesia | magnesia | Epsom salt, sedlitz salt, magnesian vitriol. | ||
| Ammoniac | ammoniac | Glauber's secret sal ammoniac. | ||
| Argill | argill | Alum. | ||
| Oxyd of | zinc | zinc | White vitriol, goslar vitriol, white coperas, vitriol of zinc. | |
| iron | iron | Green coperas, green vitriol, martial vitriol, vitriol of iron. | ||
| manganese | manganese | Vitriol of manganese. | ||
| cobalt | cobalt | Vitriol of cobalt. | ||
| nickel | nickel | Vitriol of nickel. | ||
| lead | lead | Vitriol of lead. | ||
| tin | tin | Vitriol of tin. | ||
| copper | copper | Blue coperas, blue vitriol, Roman vitriol, vitriol of copper. | ||
| bismuth | bismuth | Vitriol of bismuth. | ||
| antimony | antimony | Vitriol of antimony. | ||
| arsenic | arsenic | Vitriol of arsenic. | ||
| mercury | mercury | Vitriol of mercury. | ||
| silver | silver | Vitriol of silver. | ||
| gold | gold | Vitriol of gold. | ||
| platina | platina | Vitriol of platina. | ||
Sect. XIV.—Observations upon Sulphuric Acid and its Combinations.
For a long time this acid was procured by distillation from sulphat of iron, in which sulphuric acid and oxyd of iron are combined, according to the process described by Basil Valentine in the fifteenth century; but, in modern times, it is procured more oeconomically by the combustion of sulphur in proper vessels. Both to facilitate the combustion, and to assist the oxygenation of the sulphur, a little powdered saltpetre, nitrat of potash, is mixed with it; the nitre is decomposed, and gives out its oxygen to the sulphur, which contributes to its conversion into acid. Notwithstanding this addition, the sulphur will only continue to burn in close vessels for a limited time; the combination ceases, because the oxygen is exhausted, and the air of the vessels reduced almost to pure azotic gas, and because the acid itself remains long in the state of vapour, and hinders the progress of combustion.
In the manufactories for making sulphuric acid in the large way, the mixture of nitre and sulphur is burnt in large close built chambers lined with lead, having a little water at the bottom for facilitating the condensation of the vapours. Afterwards, by distillation in large retorts with a gentle heat, the water passes over, slightly impregnated with acid, and the sulphuric acid remains behind in a concentrated state. It is then pellucid, without any flavour, and nearly double the weight of an equal bulk of water. This process would be greatly facilitated, and the combustion much prolonged, by introducing fresh air into the chambers, by means of several pairs of bellows directed towards the flame of the sulphur, and by allowing the nitrous gas to escape through long serpentine canals, in contact with water, to absorb any sulphuric or sulphurous acid gas it might contain.