Chap. I. History. How obtain. સ. 1. Nitric Acid. This acid, which is one of the most important instruments of investigation of which the chemist is possessed, was certainly known to Geber, an Arabian chemist of the 7th century, and the author of the first purely chemical work with which we are acquainted. It is always procured from saltpetre or nitre, a salt which forms spontaneously on the surface of the earth in most parts of the world. The saltpetre, made use of in this country, is imported from India, where it is collected in great quantities from the soil. It is afterwards refined in Great Britain, and made fit for the manufacture of gunpowder, in which by far the greatest part of the saltpetre imported is expended. Saltpetre is a compound of nitric acid and potash. If sulphuric acid be mixed with it, the nitric acid is disengaged, and may be distilled over from a glass retort into a receiver. Nitric acid cannot be obtained free from water, but the sulphuric acid of commerce always contains a sufficient quantity of water to supply the nitric acid with that essential element. The To prepare nitric acid, the best way is to put 12-75 parts of saltpetre into a retort, and to pour over them 12:25 parts of the sulphuric acid of commerce (of the specific gravity 1·847). Lute an adopter to the beak of the retort, and fit the extremity of the adopter into the mouth of a glass receiver amply large enough to hold all the nitric acid which will come over. retort being placed in a sand bath and a fire kindled below it, the mixture of nitric and sulphuric acid speedily becomes liquid, and the whole nitric acid may be distilled over at a moderate temperature, and without any loss. The anhydrous nitric acid contained in 12·75 parts of nitre is 6·75, and the water contained in 12-25 of sulphuric acid is 2-25. But if the heat applied be not too great, one-half of this water remains in the retort united to the salt of potash formed by the sulphuric acid, so that the quantity of nitric acid obtained should weigh 7875 parts, and its specific gravity should be 1:55. But in general a little more than one-half of the water comes over, so that the specific gravity of the nitric acid obtained is rarely so high as 15. If the heat applied be a little too high, all the water in the sulphuric acid comes over with the nitric acid. The quantity of nitric acid obtained in that case is 9 parts, and its specific gravity is 1-4855. This is by far the most common strength of nitric acid obtained by this process. If instead of 12-25 of sulphuric acid we mix only 6·125 parts with 12-75 of nitre, only two-thirds of the nitre is decomposed The acid obtained by this process has a yellow colour; but it may be rendered colourless by simply raising it to the boiling temperature in a retort, taking care to remove the receiver while the acid is boiling hot, to prevent the deutoxide of azote thus driven off, from being again absorbed as the acid cools. If common nitre has been employed, the nitric acid obtained is always mixed with some muriatic. Nor can nitre be quite freed from all traces of common salt, though repeatedly crystallized. But if we put the impure nitric acid into a retort, and distil off about one-third of it by rather a slow fire, the remaining two-thirds in the retort will be quite pure, all the muriatic acid having passed along with the acid distilled over. This method, first proposed by Lassone and Cornette,* I have been in the habit of following, and easily obtain by means of it pure nitric acid; while the portion distilled over answers all the numerous purposes to which a mixture of nitric and muriatic acids is usually applied. Class I Div. I. Nitric acid thus prepared is a liquid colourless as water; but Properties. if we expose it to the direct rays of the sun, or even to the light of day, it gives out oxygen gas, and soon assumes a yellow colour. It absorbs deutoxide of azote with avidity, and becomes first yellow, then red, and at last brown, and so volatile that a very moderate heat converts it wholly into vapour. It has a peculiar smell, and gives out a white smoke when exposed to the air. Its taste is intensely sour, it reddens vegetable blues, and corrodes and destroys animal and vegetable substances with great energy. Hence it is often applied by * Mem. Par. 1781, P. 645. Chap. I. Strength. surgeons to ill conditioned ulcers as a caustic, and when so applied it answers better, and gives less pain when strong than when diluted with water. It has a strong affinity for water, and cannot be obtained free from that liquid. When the concentrated acid is exposed to the air, it absorbs moisture from the atmosphere. The following table shows the specific gravity of various atomic combinations of this acid with water.* Boiling point. The boiling temperature of this acid varies with its strength. The following table drawn up by Mr. Dalton, from his own experiments, shows the temperature at which acid of different densities begins to boil.† We see from this table that the boiling point of acid of the specific gravity 1.42, or acid composed of 1 atom acid + 4 atoms water is a maximum or 248°. The boiling point of acid either stronger or weaker than this is lower. The acid which occurs in commerce under the name of aquafortis or single nitric acid, is obtained by distilling a mixture of nitre and sulphate of iron in cast-iron retorts luted to large green glass receivers. It is colourless, or has only a very light shade of yellow. The specific gravity of what is usually made in this country is about 1-22, and it contains about 31 per cent. of real acid. It rarely or never contains any iron; but is never free from muriatic acid. other bodies. Most combustible substances decompose this acid; but it does Action on not act upon charcoal and sulphur unless the temperature be elevated. If a piece of charcoal be ignited and thrown into concentrated nitric acid it continues to burn with great violence, giving out red vapours. When boiled upon sulphur it gradually converts it into sulphuric acid. Phosphorus when thrown into concentrated nitric acid catches fire and burns with much splendour. On iron concentrated nitric acid does not act instantly, but if we elevate the temperature the action begins suddenly and becomes extremely violent; much heat being evolved, and even light it is said; though this has never happened in my trials. It acts also with great violence on zinc, tin, copper, and manganese. Mercury, silver, cadmium, bismuth, lead, cobalt, and nickel, are dissolved by it rapidly, and with effervescence. Upon gold, platinum, palladium, rhodium, iridium, and osmium, it either does not act at all or only very feebly. Concentrated nitric acid sets fire to various volatile oils. The combustion of oils by this acid was first taken notice of by Borrichius and Slare ;* but it is probable that Homberg communicated it to Slare. In order to set fire to the fixed oils, it must be mixed with some sulphuric acid; the reason of which seems to be, that these oils contain water, which must be previ * Phil. Trans. Abr. ii. 653, and iii. 663. Chap. I. ously removed. The sulphuric acid combines with this water, and allows the nitric acid, or rather the oil and nitric acid together, to act. The drying oils do not require any sulphurie acid: they have been boiled, and consequently deprived of all Composition. moisture. Dilute nitric acid has the property of giving a yellow colour to vegetable and animal substances. In this way it dyes the cuticle, and the yellow colour becomes orange when washed with soap, and remains indelible till the stained portion of the cuticle be removed. It is employed to give a yellow border to the blue or green woollen cloth used in this country for covering tables. Many vegetable and animal substances when digested in it are converted into oxalic acid, malic acid, carbonic acid, suberic acid, wax, or fatty matter like tallow, and sometimes also into hydrocyanic acid. It has the property of neutralizing bases, and forming a class of salts called nitrates. The first step to the knowledge of the constituents of this acid was made by Lavoisier, who proved by direct experiment, that it might be resolved into deutoxide of azote and oxygen gas; and that these two gases when placed in contact, immediately reunited and formed nitric acid. He concluded in consequence that nitric acid is a compound of deutoxide of azote and oxygen; and he even tried to determine the proportion of the two constituents, but the result obtained does not constitute a very near approximation to the truth. Mr. Cavendish showed that when azotic and oxygen gases are mixed, and electric sparks passed through the mixture in a glass tube standing over potash ley, the two gases unite together and form nitric acid, which gradually saturates the potash, and converts it into nitre. Hence it followed that the true constituents of this acid are azote and oxygen. From the proportions of the two gases which disappeared in this experiment, it appeared that nitric acid was a compound of 1 volume azotic gas, 2 volumes oxygen gas. and with this the analytical experiments of Berthollet agreed.† But it has been fully established by numerous experiments, among others by several of my own, that the true constituents of this acid are * Mem. Par. 1776, p. 673. First Principles, i. 100. Mem. D'Arcueil, iii. |