Protoxide of nitrogen may be obtained in tho liquid state by subject ing it to a pressure of about forty atmospheres. Once compressed It may be preserved in a reservoir for two or three days, but soon begins to attack the brass or copper fittings. On opening the stopcock the first portions of the liquid freeze into a solid mass, resembling snow ; but it afterwards flows freely, and being received Into an open glass remains fluid for nearly an hour. During the whole time, however, it is slowly evolving gas.
Liquid 'protoxide of nitrogen is a colourless, mobile, and perfectly transparent fluid. A drop let fall upon the skin rapidly ovapomtes, producing a painful burn. Mixed with bisulphide of carbon and exposed to evaporation in vacuo, a reduction of temperature ensues to probably 252' below the freezing point of water, at all events to a much lower point than has hitherto been obtained by any other means. Metals dropped into liquid protoxido of nitrogen produoo a noise as though red-hot iron were immersed in water. Mercury causes the same noise, instantly freezes and affords a hard brittle mass, of the appearance of silver. Potassium floats upon the liquid, but experiences no change, and the same is the case with charcoal, sulphur, phospho rus, and iodine : strong sulphuric and nitric acids freeze immediately ; ether and alcohol merely mix with the liquid ; but water is instantly frozen with an almost explosively sudden elimination of gas.
2. Binoxide of nitrogen, dentoxidc of azote, or nitric oxide (NO.). Although this gaseous body had been obtained by Hales, yet it is to Dr. Priestley that wo owe the first distinct account of the method of procuring it and a description of its properties. When certain inetals such as mercury, or better copper in the state of borings, are put into dilute nitric acid, they are dissolved. During their solution, owing to the partial decomposition of a proportion of the nitric acid and the absorption of its oxygen by the metal, gaseous binoxide of nitrogen is plentifully formed mid evolved. It has the following properties :—it is gaseous, or permanently elastic, not having been hitherto condensed into a liquid by any degree of cold or pressure to which it has been subjected. It is colourless, and probably tasteless and inodorous; but when an attempt is made to inhale it, it excites violent spasm of the glottis by meeting and combining with the oxygen of the air, which converts it into nitrous acid gas, a very different compound. Its specific gravity is ; 100 cubic inches weighing rather more than 32 grains. Water at 60' dissolves about 11 per cent. It. has no action on litmus paper, nor does it in any way exhibit the properties of an acid ; but as already noticed, nitrous acid is formed as soon as it comes into contact with oxygen, and then it mildew, litmus pauper; this property distinguishes it from all other gases whatever, and renders it a test of the presence of uncombined oxygen : this was the gas first employed by Dr. Priestley in his eudiometrical researches. It supports combustion, if the temperature of the combustible be sufficient to decompose it into oxygen and protoxide of nitrogen, thus, charcoal and phosphorus immersed in it when they are in vivid com bustion, burn splendidly, but a burning candle is extinguished : sulphur also, if only sluggishly burning, is extinguished ; but when its temperature raised to near the boiling point it burns readily. It
is decomposed when passed through red-hot tubes; and the same effect is produced by a succession of electric sparks'. It is also decom posed when exposed to zinc or iron, which take away half the oxygen and convert it into protoxide of nitrogen. It is soluble in a solution of protosulphate of iron ; the solution has a deep green or brownish colour, and has been used in eudiomctry to ascertain the proportion of oxygen in gaseous mixtures; but it cannot be advantageously so employed.
3. Nitrous acid, hyponitrout arid was discovered by Gay Lusaac, who obtained it by adding excess of binoxide of nitrogen to oxygen gas, confined in a glass tube over mercury, containing a con centrated solution of potash ; under these circumstances 50 volumes of oxygen unite with 200 volumes of biuoxido .of nitrogen, forming nitrous acid which unites with the alkali. This acid may also be obtained by long exposure of binoxide of nitrogen gas to solution of potash, or by exposing to intense cold a mixture of 50 volumes of oxygen gas with 200 volumes of binoxide of nitrogen gas. By this operation the acid is obtained in a liquid state and possesses the follow ing properties : At common temperatures It is green or blue, but at V it is colour less; it is very volatile, eo that when expbsed to the air, it is rapidly converted into an orange vapour, the density of which is stated to ho 112. When water is added, the acid is decomposed and converted into nitric acid and nitric oxide, the latter escaping with effervescence. This acid does not readily combine with bases to form salts, being on their admixture converted into nitric acid and binoxido of nitrogen. Nitrites may however be formed by moderately heating certain nitrates, the acid of which then loses two equivalents of oxygen, nitrous acid remaining in combination with the base. Nitrite of potash is thus formed, the nitrate being heated until the oxygen evolved contains nitrogen. Solution of nitrite of ammonia, a convenient source of pure nitrogen, is most advantageously formed in the following way : one part of starch is gently heated with ten parts of nitric acid, and the nitrous acid gas produced passed into solution of potash, of sp. gr. 1.38, until the solution acquires an acid reaction ; a little caustic potash is then added, so as to render it distinctly alkaline, and the resulting liquid when required for the preparation of nitrogen is mixed with three times its bulk of a saturated solution of chloride of ammonium, and the whole gently heated; the nitrite of ammonia formed in this process is decomposed by the beat as described above. Nitrite of lead may be formed by heating metallic lead in a solution of the nitrate of that metal. Nitrate of silver is thrown down as a white precipitate on adding together solutions of nitrite of potash and nitrate of silver ; it is soluble in hot water and crystallises out on cooling.