512 CHEMISTRY [OXIDES AND ACIDS readiness may be deprived of its oxygen by the action of nascent hydrogen. In fact, hydrogen is never evolved by the action of metals on nitric acid, but instead oxides of nitrogen, nitrogen itself, ammonia, and other products of secondary reaction are obtained. The formation of these products is due to a somewhat complex series of reactions, which most probably are as follows. In the first place, by the removal of one of the atoms of oxygen the nitric acid is converted into nitrous acid, HN0 2 ; by further re duction this acid furnishes the so-called hyponitrous acid, HNO ; and by the continued action of the nascent hydro gen the hyponitrous acid is converted into hydroxylamine or oxyammonia, NH 2 (OH) , which finally is reduced to ammonia, NH 3 . The reactions which successively occur in the formation of these compounds are represented by the following equations : HNO, + Ho + H H = HN0. 2 + Nitrous acid. = HNO + Hyponitrous acid. = NH 2 (OH) Hydroxylamine. = NH, + H 2 Ammonia. H 2 H 2 Nitric acid. HNO 2 Nitrous acid. HNO Hyponitrous acid. NH 2 (OH) Hydroxylamine But the products of these reactions enter into reaction with each other, and are decomposed, and thus the gases are produced which are evolved when metals are dissolved in nitric acid. The nitric oxide doubtless results chiefly from the decomposition of the nitrous acid in the manner represented by the equation 3HN0 2 = 2 NO + HNO 3 + H 2 O . The nitrous oxide may be produced in two ways : from .hyponitrous acid, which immediately on formation is resolved into nitrous oxide and water: 2HNO = N 2 + H 2 O ; and by the action of nitrous acid on hydroxy lamine HN0 2 + NH 2 (OH) = N 2 + 2H 2 . The nitrogen produced is probably formed in a similar manner by the action of the nitrous acid on the am monia HN0 2 + H 3 N = N 2 + 2H 2 O ; and perhaps also, together with nitric oxide, by the action of nitric acid on hyponitrous acid. As the product of the action of nitric acid on a metal is always a mixture, it is evident that several of the reactions pointed out must occur simultaneously. The composition of the product varies, in a manner not yet understood, with the metal, the strength of the acid, and with the temperature. The approximate percentage composition of the gas obtained by the action of a mixture of nitric acid of the specific gravity 1 42 with twice its bulk of water on a number of metals is given in the following table : Name of Metal. N 2 O NO N Nickel 85 4 11 Cobalt 79 6 15 Tin ..- 68 23 9 Magnesium 61 17 22 Zinc 53 40 7 Lead 41 52 7 Cadmium 20 78 2 Thallium 19 70 11 Iron 6 89 5 Indium 4 91 5" Aluminium 1 97 2 Copper. . . . 1-5 97 1-5 Silver
97 3 It is difficult at present to explain the remarkable differ ence in the behaviour of the various metals with nitric acid which this table indicates. As we have stated above, the action of the metal probably consists simply in dis placing the hydrogen of the acid, and the gases evolved are
- he more or less direct products of the action of the hydrogen
so displaced at the moment of liberation on the acid which is always present in excess. The question, therefore, that we have to consider is, Why does the hydrogen displaced from nitric acid by different metals produce such very different effects 1 If the behaviour of closely related metals such as magnesium, zinc, and cadmium be compared, it is evident that magnesium is the most active, since it produces a relatively smaller quantity of nitric oxide, and relatively larger quantities of nitrous oxide and nitrogen, than either zinc or cadmium, cadmium being the least active ; in other words, the reduction of the nitric acid is most perfect when it is effected by the hydrogen displaced by the aid of magnesium, and least perfect when it is effected by the aid of cadmium. We know that when these metals act upon acids which are not attacked by nascent hydrogen, such as hydrochloric acid, for example, different amounts of heat are developed, most heat being developed by the action of magnesium, and least by the action of cadmium. But since the mere displacement of hydrogen in hydro chloric acid by different metals is attended with the develop ment of different amounts of heat, it appears probable that the same will be the case with nitric acid, and also that more heat would be developed in the reaction Mg + 2HN0 3 = Mg(N0 3 ) 2 + 2H than in the corresponding reactions with either zinc or cad mium ; and that on this account the reduction of nitric acid is carried furthest when magnesium is employed, and further with zinc than with cadmium. Whether this explanation is applicable to the action of metals generally there is not sufficient evidence at present to show. Nitrous Oxide, N 2 O ; Mol. wt., 43 96. This gas is obtained in a pure state by carefully heating ammonium nitrate, which at a temperature between 200 and 250 C. breaks up into water and nitrous oxide It is the chief constituent of the gas produced on dis solving magnesium, zinc, tin, nickel, or cobalt in nitric acid, and it is also furnished, but to a less extent, by other metals. Nitrous oxide is a transparent colourless gas, possessing a faint sweetish smell and taste ; it may be condensed by a pressure of about 50 atmospheres at 7 C. to a colour less liquid, and even frozen by the cold produced by its own evaporation. By exposing a mixture of liquid nitrous oxide and carbon disulphide to evaporation in vacuo, Natterer obtained a reduction of temperature which he estimated at - 1 40 C., which is lower than has been obtained by any other means. 100 volumes of water at C. dis solve 130 of nitrous oxide, but the solubility diminishes rapidly as the temperature rises. It supports the combus tion of bodies almost, if not quite, as well as oxygen, but is readily distinguished from that gas by its solubility in water, and by not forming red fumes when mixed with nitric oxide gas. The most remarkable property which nitrous oxide gas pos sesses is that of causing loss of sensibility in animals. When respired for a short time it produces a singular species of transient intoxication, attended in many instances with an irresistible tendency to muscular exertion, and often to uncontrollable laughter ; hence it has acquired the name of laughing gas. It is now largely used as an anaesthetic for producing insensibility to pain during surgical operations, and especially in the extraction of teeth.
Nitrous oxide is a perfectly neutral substance, and does