Davy had no sooner discovered that the gas might be respired, than he proceeded to attack the whole subject of the chemistry of the oxides of nitrogen, and of nitrous oxide in particular, and after ten months of incessant labour he put together the results of his observations in an octavo volume, entitled, “Researches, Chemical and Philosophical, chiefly concerning Nitrous Oxide, or Dephlogisticated Nitrous Air, and its Respiration. By Humphry Davy, Superintendent of the Medical Institution.” The book appeared in the summer of 1800, and immediately re-established its author’s character as an experimentalist. Thomson, in his “History of Chemistry,” says of it: “This work gave him at once a high reputation as a chemist, and was really a wonderful performance, when the circumstances under which it was produced are taken into consideration.” In spite, however, of the eulogies with which it was welcomed, its sale was never very extensive, and a second edition was not required. In fact, the work as a whole was hardly calculated to attract the general public, whose only concern with laughing-gas was in its powers as an exhilarant. Indeed, this aspect of the question is not wholly lost on Davy himself, who is careful to point out that “if the pleasurable effects or medical properties of the nitrous oxide should ever make it an article of general request, it may be procured with much less time, labour, and expense than most of the luxuries, or even necessaries, of life”; and in a footnote he adds. “A pound of nitrate of ammonia costs 5s. 10d. (its present price is 9d.!). This pound, properly decomposed, produces rather more than 34 moderate doses of the air, so that the expense of a dose is about 2d. What fluid stimulus can be procured at so cheap a rate?”
To the chemical student the book had, and still has, many features of interest. It contains a number of important facts, based on original and fairly accurate observation. In the arrangement of these facts “I have been guided as much as possible,” says their author, “by obvious and simple analogies only. Hence, I have seldom entered into theoretical discussions, particularly concerning light, heat, and other agents, which are known only by isolated effects. Early experience has taught me the folly of hasty generalisation.” The work is divided into four main sections. The first chiefly relates to the production of nitrous oxide, and the analysis of nitrous gas and nitrous acid. He minutely studies the mode of decomposition of ammonium nitrate (first observed by Berthollet), and shows that it is an endothermic phenomenon, varying in character with the temperature and manner of heating. He is thus led to offer the following Speculations on the Decompositions of Nitrate of Ammonia:—
“All the phenomena of chemistry concur in proving that the affinity of one body, A, for another, B, is not destroyed by its combination with a third, C, but only modified; either by condensation or expansion, or by the attraction of C for B. On this principle the attraction of compound bodies for each other must be resolved into the reciprocal attractions of their constituents, and consequently the changes produced in them by variations of temperature explained from the alterations produced in the attractions of those constituents.”
The singular property possessed by ammonium nitrate of decomposing in several distinct modes according to the rapidity of heating and the temperature to which the substance is raised, first indicated by Davy, has been minutely studied by M. Berthelot, who has shown that this comparatively simple salt may be decomposed in as many as six different ways. It may be (1) dissociated into gaseous nitric acid and ammonia; (2) decomposed into nitrous oxide and water; (3) resolved into nitrogen, oxygen, and water, (4) or into nitric oxide, nitrogen, and water, (5) or into nitrogen, nitrogen peroxide, and water; or lastly (6), under the influence of spongy platinum, it may be resolved into gaseous nitric acid, nitrogen, and aqueous vapour. These different modes of decomposition may be distinct or simultaneous; or, more exactly, the predominance of any one of them depends on relative rapidity and on the temperature at which decomposition is produced. This temperature is not fixed, but is itself subordinate to the rapidity of heating (cf. Berthelot’s “Explosives and Their Power,” translated by Hake and Macnab). The assertion of De la Metherie, that the gas produced by the solution of platinum in nitromuriatic acid was identical with the dephlogisticated nitrous air of Priestley (nitrous oxide), led Davy to examine the gaseous products of this reaction more particularly. He had no difficulty in disproving the statement of the French chemist; but his observations, although accurate, led him to no definite conclusion. “It remains doubtful,” he says, “whether the gas consists simply of highly oxigenated muriatic acid and nitrogen, produced by the decomposition of nitric acid from the coalescing affinities of platina and muriatic acid for oxygen; or whether it is composed of a peculiar gas, analogous to oxigenated muriatic acid and nitrogen, generated from some unknown affinities.” The real nature of the gas, which has also been considered by Lavoisier as a particular species, not hitherto described, was first established by Gay Lussac, when Davy had himself proved that “oxigenated muriatic acid” was a simple substance.
In the second section the combinations and composition of nitrous oxide are investigated, and an account is given of its decomposition by combustible bodies, and a series of experiments are described which are now among the stock illustrations of the chemical lecture-room. As to its composition, he says, “taking the mean estimations from the most accurate experiments, we may conclude that 100 grains of the known ponderable matter of nitrous oxide consist of about 36·7 oxygen and 63·3 nitrogen”—no very great disparity from modern numbers, viz. 36·4 oxygen and 63·6 nitrogen. He concludes this section with a short review of the characteristic properties of the combinations of oxygen and nitrogen, among which he is led to class atmospheric air.
“That the oxygen and nitrogen of atmospheric air exist in chemical union, appears almost demonstrable from the following evidences.
“1st. The equable diffusion of oxygen and nitrogen through every part of the atmosphere, which can hardly be supposed to depend on any other cause than an affinity between these principles.
“2dly. The difference between the specific gravity of atmospheric air, and a mixture of 27 parts oxygen and 73 nitrogen, as found by calculation; a difference apparently owing to expansion in consequence of combination.”
These “evidences” had already been adduced by others, as stated by Davy; the first was subsequently disproved by Dalton, the second was based on inaccurate analyses of air.
To these Davy added two other “proofs” which originated with him:—
“3dly. The conversion of nitrous oxide into nitrous acid, and a gas analogous to common air, by ignition.
“4thly. The solubility of atmospheric air undecompounded.”
Of these it may be stated that the first is invalid, and the second not true. Nitrous oxide may, under certain circumstances, give rise to a mixture of oxygen and nitrogen, but not necessarily in the same proportion as in common air; and the air boiled out from water has not the same composition as atmospheric air.