This substance usually occurs in cubic crystals, occasionally in octahedra. The crystals am often opaque ; they are anhydrous, melt at a low red heat, and volatilize completely at a higher temperature. Iodide while not deliquescent, is very soluble in water, and in dissolv ing produces a considerable fall of temperature. It dissolves in 0'7::5 parts of water at parts at 16°, 0.7 parts at 18°, and 0.5 parts at 120°. It is also soluble in alcohol ; iu 5.5 parts of specific gravity 0.85 at 12.5°, and in 40 parts of absolute alcohol at 13.5°. When heat, the alcohol dissolves a larger amount, the iodide separating again in needle-shaped crystals upon cooling. A saturated water solution boils at 120°. The deep-brown colour of an ordinary solution, owing to the presence of free iodine, is well known.
Nitrate of Potassium. (Fn., Nitrate de Potasse ; GER., Salpetersaiir•3 Ktli, or Formula, knowledge of this important salt has in all probability been a theory of gradual growth from very ancient times. The old alchemists named it " nitre" to distinguish it from " mtrum," the name by which soda was known before the term "natron" was introduced. Geber speaks of it as "sal pctrw," this designation evidently having its origin in the fact that the salt was obtained by grinding and lixiviating certain rocks. "Sal pctrosum" is mentioned in a Latin work of the seventh or eighth century. Agricola in his ' De Re Metallica' describes the refining of saltpetre by boiling the crude product of the washing of certain earths with quicklime and wood-ashes, lixiviating the liquors, concentrating, and crystallizing.
Nitrate of potassium occurs in considerable quantities in nature, in spring and river water, in the juices of certain plants—the sunflower, common borage, tobacco, &c.—but more widely as a con stituent of the soil, in many porous rocks, and as a product of the continual process known as "nitrification." Lemery first discovered the salt as a constituent of the juice of plants in 1717, and it has sinoe then been established that many species—especially the amaranthus—contain consider able proportions of it.
The prooess of nitrification is even yet but imperfectly understood. The simplest explanatiou seems to be that wherever organic substances containing nitrogen are exposed to the action of air ammonia is formed, and when an alkali—soda, potash, or lime—is present, a nitrate of the particular metal is produced by slow oxidation. Any circumstance that favours putrefaction assists nitrifica tion; hence a warm damp atmosphere-15° to 20°. Hence, too, the productiveness of tropical climates, where decaying organio matters yield a constant supply of ammonia. The best known natural depcsits, or "beds," of saltpetre are those of South America, India, Persia, Spain, and Hungary. Here the salts, formed in the natural manner described, carried down into the soil by
the agency of rain, dew, and rising again to the surface in the form of solution, are evaporated by the sun and air, and spread over the surface of the ground as a dirty white efflorescence. " Salt peire earth " of this description will test about as follows :— Nitrate of potassium .. 8.3 per cent. Carbonate of calcinm 35.0 per cent.
71 calcium „ Water .. „ Sulphate of lime .. „ Insoluble matter .. „ Chloride of sodium .. „ Other saltpetre deposits, with a somewhat different origin, are found in caverns and places where animals and birds congregate, and in the shape of excrement provide an unfailing supply of organic matter. The caves of Ceylon, Kentucky, Teneriffe, upon the coast of tho Adriatic and on the Missouri river are well known. A remarkable instance of rock deposit is found in the cave of Memoora, where the nitrate occurs in veins. The rock has been analysed as follows Nitrate of potash is dimorphous. It crystallizes in long hexagonal prisms and in rhombohedra. The crystals are anhydrous. They are white and inodorous, with a strong saline taste, and are neither deliquescent nor hygroscopic. Below a red heat, at 339°, nitrate of potassium melts to a colourless liquid with a specific gravity of 2.1. Upon cooling, the fused salt forms an opaque white mass, usually known as "sal prunelle." At a red heat, the salt is decomposed, yielding up oxygen, and finally nitrogen, and passing first into nitrite and then into protoxide and peroxide of potassium. Fused with carbon, sulphur, phosphorus, and other combustible snbstances, salt petre deflagrates, liberating oxygen. In this way gold, silver, and even platinum undergo oxidation, Its use as a constituent of gunpowder and other explosives is due to this potency as an oxidizing agent. It is very soluble in hot, but only slightly soluble in cold, water, as the following table shows:— Nitrate of potash may be obtained by adding nitric acid to a solution of pure carbonate or hydrate of potassium in very slight excess and crystallizing from the concentrated liquor. Or by crystallization from a concentrated solution of chloride or carbonate of potassium with nitrate of sodium. Upon a large scale, two processes are followed :—(1) Saltpetre earths are lixiviated, and the solutions concentrated and crystallized. These saltpetre earths are (a) of natural formation ; (b) artificially prepared. (2) By mixing chloride of potassium and nitrate of sodium, thus :— The nitrate crystallizes out from the concentrated solution, chloride of sodium being left. Very often carbonate of potassium is substituted for the chloride in this latter process.