Nitric anhydride, discovered by Devine in the year 1849, forms transparent, colourless crystals taking the shape of prisms with six faces. Mixed with water these crystals dissolve, with evolution of heat, and form the ordinary aqueous acid. The formula of the anhydride is boiling point 45° to 50°. This substance is of small value outside of the laboratog.
Nitric acid seems to have been known from very early times. As far back as the seventh century Geber, in his De Invention Veritatis,' says : " Sume libram Imam de vitriol! de cypro et libram salis petraz et unam quartam aluminis Iameni, extrahe aquam cum rubendine alembici." From this it appears that nitric acid was obtained by distillation from a mixture of saltpetre, alum, and sul phate of copper. According to Herapath, the Egyptians were acquainted with this substance, using a marking fluid containing nitrate of silver for its base. This opinion was founded upon a micro scopical examination of the hieroglyphics discovered upon the wrappings of a mummy, and seems to be a fair inference from all appearances. The name of aquafortis was bestowed by the alche mists, who made use of the acid in various ways, especially in the separation of gold and silver. The term "aqua fortis" was not, however, as is often supposed, confined to nitric acid. In the writings of the thirteenth century directions are given for preparing it from saltpetre and sul phate of iron. The present method of preparation—the distillation of saltpetre, or nitre, with sulphuric acid—was probably first employed by Glauber, and for a considerable period the product was known as " Spiritus nitri fumans Glauberi." In 1776 Lavoisier demonstrated that one consti tuent, at least, was oxygen, but little more was accomplished until Priestley and Cavendish experi mented upon the substance. The former, passing a series of electric sparks through air enclosed between two columns of litmus solution, observed that a red colour was produced, and that a contraction of the air volume took place. Cavendish used lime water and caustic potash in place of the litmus solution, and arrived at the belief that the reaction in question was caused by the production of an acid. He afterwards passed a series of electric sparks through a mixture of oxygen and nitrogen over caustic potash, and found that nitrate of potassium was produced. In the Phil. Trans.' for 1784, f. 119, Cavendish reports thus upon his experiment : "The phlogisticated air (nitrogen) was enabled by means of the electric spark to unite to, or form a chemical combination with, the dephlogistioated air (oxygen), and was thereby reduced to nitrous acid which united with the soap lees (caustic potash), and formed a solution of nitre ; for in these experiments these two airs actually disappeared, and nitric acid was formed in their room." In later times the constituents,
proportions, and properties of this body have been further investigated and determined by Davy, Gay-Lassac, and others.
Nitric acid dace not exist free in the mineral and vegetable kingdoms, but is found largely in both, combined with various bases—soda, lime, potash, and magnesia. If the old experiment of Cavendish, with oxygen and nitrogen, be performed with the addition of a little hydrogen gas, the action is much more marked, and a small quantity of nitrate of ammonia is formed. Hence, probably, the exist ence of this salt, in the rain water of thunderstorms. M. Bobierre in his report upon some re searches into the chemical composition of rain water collected at different altitudes, says, "I evaporated in an oiI bath 372 litres of rain water having carbonate of soda present, and determined month by month the amount of nitrogen in the fixed organic matter, the nitric acid, and the chlorine ; then by fractional distillation, by Boussingault's method, I determined the amount of ammoniacal nitrogen.
"Suspended neater was separated by filtration and examined by a microscope. I extract from my Memoir some of the principal figures, which show the nature of the results (at 47 metres height and dovrn below):— The action of the nitric acid present in the air of chemical works is of course to oxiclize the sulphurous acid. From what has been said respecting the other oxides of nitrogen it may he readily supposed that they have greater potency than the acid itself. This seems to be borne out by the following results of experiments—the sulphurous, acid in some cases appealing almost to be preserved bv the nitric acid :— The process of nitrification that has been referred to, the production first of nitric acid and then of a nitrate, is an important agency in nature, especially in warm climates, and where there is an unfailing supply of decaying organic matter. From this cause proceed the artificial nitre beds of the Continent, and the various deposits of nitrates of soda, potash, and lime occurring in South America, India, Persia, Ceylon, &cc. Concerning this more will be said in treating of the respective bases. The formation of the salt in all these cases probably proceeds from the same natural cause.