The deposits are found far from human habitations, and always upon porous rocks, or light earths, where the air can circulate freely, and where a considerable amount of moisture can be held suspended. Upon hard rocks no deposit is found, and rarely in sheltered places, unless there is considerable humidity. Nitrate of lime may often be observed upon old walls, forming a distinct efflorescence. Lavoisier found nitrate of potash, mixed with nitrate of lime, upon specimens of chalk from Roche Guyon and Mousseaux. These salts have been obtained in considerable quanti ties from the floors of old stables and cowsheds ; indeed, in some places, and at certain times, the collection has become quite a trade. It is, moreover, a noteworthy fact that the nitrate-bearing earths and rocks perpetually renew the formation when it is removed, so long as the necessary base is present. The deposits never penetrate far below the surface. Rain and dew dissolve the salts, the solutions rise by capillary attraction, and, evaporated by the action of the sun and air, form an efflorescence on the surface. The process of nitrification goes on most vigorously when animal or vegetable matter in a state of putrefaction is present. Oxidation of the ammonia, or nitro genized organic substances, then proceeds rapidly, especially when the temperature is higher than 20°. Nitrate of lime is artificially prepared on the Continent by mixing cinders, or any porous material, with decaying animal and vegetable matters. The masses are moistened from time to time with urine, turned over occasionally to expose every portion to the action of the air, and after the lapse of a couple of years, subjected to lixiviation to obtain the lime salt.
Nitric acid assumes a very important place in the arts and manufactures. Perhaps, with the exceptions of sulphuric and hydrochloric acids, it enters more largely into every-day commercial life than any other acid. It is chiefly esteemed for its solvent and oxidizing powers, and in these ways forms a most important agent in analysis, dissolving and oxidizing metals, and so separating them from the few which resist its action—e. g., silver from gold—peroxidizing antimony, tin, man ganese, iron, &c., and, generally, separating soluble and insoluble precipitates. Nitric acid forms a valuable test for organic bodies ; is employed in etching upon copper, steel, and stone ; is used as a solvent in the preparation of certain mordants, and imparts to others their potency by its oxidizing influence. In medicine it forms a tonic, and is also extensively used as a powerful caustic. It enters into the manufacture of nitro-benzine and many similar organic preparations, nitro-glycerine and gun-cottons. Finally, it forms a series of valuable salts termed nitrates, of which more will be said hereafter.
The methods of preparation are various, but, as a rule, exceedingly simple. The experiments of Cavendish, by which, passing a series of electric sparks through air, he first demonstrated the com position of the acid, have already been referred to. When nitrogen is mixed with twelve times its bulk of hydrogen, and burnt in oxygen, a small quantity of nitric acid, together with nitrate of ammonia, is found in the resulting water. By the decomposition of the oxides of nitrogen—as by electrolyzation, and by the action of water upon nitrous oxide, and nitric anhydride—consider able quantities of the acid may be produced. Other and more feasible methods that have been proposed, are the following ;—By heating chloride of manganese with nitre, 5 Mn CI + 5 Na 3 Mn 0 0 +5 Na Cl + 5 + O.
The mixture of nitric peroxide and free oxygen then brought together in presence of water, is converted into nitric acid. By the action of the sulphates of manganese, zinc, magnesium, calcium, &c., upon nitre, similar results arc obtained. When a strong solution of nitrate of barium is decomposed by equivalent quantity of oil of vitriol, sulphate of barium is precipitated, and a weak nitric acid of sp. gr. 1.03, may be decanted off and concentrated by boiling. All these processes, however,
belong, as yet at least, only to the region of the experimental chemist. The huge bulk of the nitric acid of commerce is obtained by heating nitrate of soda, or nitrate of potash, with sulphuric acid. Upon a small scale this operation may be performed with the apparatus shown in Fig. 147. Into a stoppered glass retort equal weights of nitre and sulphuric acid are placed, and a gradu-. ally increasing beat applied from a Bunsen's buruer. A bisulphate of soda, or potash, is formed in the retort, and nitric acid distils over and is collected in a flask or suitable receiver, kept cooled with water. When this operation is carefully con ducted, a very pure acid of 1.50 sp. gr. may be obtained, in weight equal to two thirds of the nitre employed. It is advisable to take equal quantities of acid and nitre, rather than the equivalent proportions, because in the latter case a much greater heat is required to set free all the nitric acid, and the neutral sulphate left behind sets into a hard mass, difficult to remove and in danger of cracking the vessel. By raising the temperature at which distillation is effected, a portion of the nitric acid undergoes decomposition.
Upon a large scale the decomposition of nitre by sulphuric acid is carried on in the various retorts shown in Figs. 148 to 154. Iron vessels for the distillation were fint employed by the French manufacturers, but their nee has gradually spread until only a comparatively small amount of nitric aoid is made in glass retorts. Perhaps the best form of retort is the cast-iron cylinder shown in Fig. 148. This is almost exactly similar to the retort used for the manufacture of hydrochlmic acid, and already described. Each retort may have its separate fireplace, or they may be set in pairs. The shape and substaneo aro alike in both cases, a good size being 6 ft. long, by 2 ft. 6 in. or 3 ft. diameter, with plates to 2 in. thick. The end over the fire is retnovahle—some times both ends, to facilitate cleaning out--and through it the charge of nitmte of soda, or potas sium, is introduced. The door is then securely luted on, and the necessary amount of sulphuric acid introduced through the funnel shown in the drawing. When the charging is completed this funnel is withdrawn, the hole stopped up with a plutcdlug, and the fire gently set away. Whoa 'the operation is completed, the cylinder is allowed to cool down, the door opened, and the mites of sulphate, or bisulphate, of soda raked out. The nitric acid disengaged passes off through earthen ware pipes, luted iuto the top or further end of the retort, into a row of Woulfe's bottles, or other suitable condensers. A cylinder of the description here given will work off 12 to 15 tons of nitre per week. It is better to take the acid off through the top of the retort to hinder any possible carrying over of the contents whon the disengagement of gas is active. When a cylinder is cracked it ncuty be repaired by bolting a piece of sheet iron, with red lead, on the inside, and countersinking the bolts: but it is very questionable if such attempts are advisable. The most profitable plan is to renow the cylinder as speedily as possible, and realize the old metal. In order to protect the metal from being eaten away by the nitric acid vapours, a retort of the description shown in Fig. 149 is occasionally used, the upper half being lined with fire-bricks. Or the cylinder may be turned round from time to time. If, however, the heat is carefully managed, and. no acid allowed to condense on the plate, a cylinder will rarely fail by the mere action of the acid.