In his presidential address to Section A of the British Association at Southampton in 1882, Lord Rayleigh alluded to an investigation which he had begun on the densities of hydrogen and oxygen, relatively to each other. The object of the research was to discover whether the atomic weights of these gases, determinable from their densities and from the proportions by volume in which they combine, was actually as 1 to 16, or whether some fractional number was necessary to express the weight of an atom of oxygen relatively to that of hydrogen. In 1888 his first account of the determination was published in the Proceedings of the Royal Society. In 1889 he published a continuation of his first paper, and in 1892 he gave his final results; the number obtained was 15·882 for the atomic weight of oxygen, calculated from its density, hydrogen being taken as 1. In 1893 further experiments on densities were published,[25] those of oxygen and nitrogen being specially considered with reference to the density of air. He found the weights of one litre of oxygen, nitrogen, and air to be
| Oxygen | 1·42952 | grams |
| “Nitrogen” | 1·25718 | " |
| Air | 1·29327 | " |
A simple calculation leads to the composition of purified air. The percentage of oxygen must be 20·941, and that of “nitrogen” 79·059, in order to give a mixture of which the weight of a litre is 1·29327. Now, this corresponds with the results of the best analyses, quoted above. And the accuracy of these determinations of density is confirmed by this means, as well as by results of other experiments made by Leduc, von Jolly, and Morley.
But Lord Rayleigh was not content to prepare his gases by one process only. The oxygen, of which the mean value of the weight of a litre is given above, was prepared in three different ways: by the electrolysis of water, by heating chlorates, and by heating potassium permanganate. The results showed that the only difference which could be detected, and that an extremely minute one, must be attributed to experimental error. The actual weights of the contents of his globe were—
| Electrolysis, | May 1892 | 2·6272 | grams |
| " | " " | 2·6271 | " |
| Heating chlorates, | May 1892 | 2·6269 | " |
| " " | June " | 2·6269 | " |
| Heating permanganate, | January 1893 | 2·6271 | " |
These numbers are subject to a deduction of 0·00056, due to the fact that when the globe was empty of air, its capacity was somewhat reduced, owing to the external pressure of the atmosphere.
It was next deemed necessary to test whether nitrogen was homogeneous by preparing it too by several different methods. In the same paper Lord Rayleigh ([p. 146]) mentions that nitrogen, prepared from ammonia, its compound with hydrogen, is somewhat lighter than “atmospheric nitrogen,” the deficiency in weight amounting to about 1 part in 200. Now it is evident from inspection of the numbers quoted above, that the accuracy of the density determination may be trusted to within 1 part in 10,000, and that the balance would detect a discrepancy one-fiftieth of that observed in the densities of “atmospheric” and “chemical” nitrogen. In a letter to Nature, Lord Rayleigh asked for suggestions from chemists as to the reason of this curious anomaly, but his letter went without reply. He himself was inclined to believe that the difference was due to the decomposition of some of the ordinary molecules of nitrogen, usually believed to consist of two atoms in union with each other, into molecules consisting of one atom; and as it is held that equal numbers of molecules inhabit the same volume, temperature and pressure being equal, if the total number of molecules in his globe were increased by the splitting of some double-atom molecules into single-atom molecules, the effect would be that, owing to an admixture of some lighter molecules, the density would be somewhat reduced.
But two other suppositions were entertained as possible. The oxygen might have been imperfectly removed from the nitrogen derived from the atmosphere; or, on the other hand, the nitrogen from ammonia might conceivably have retained traces of hydrogen. In the former case, the nitrogen would have an increased weight owing to admixture of some heavier oxygen; in the latter, a diminished weight, due to the presence of the lighter hydrogen. The first of these suppositions is out of the question, inasmuch as it would have required that the nitrogen should contain one-thirtieth of its volume of oxygen, or one-sixth of that present in air, in order that its density should be raised by one two-hundredth; for the densities of oxygen and nitrogen are not so very different. The second supposition was negatived by introducing hydrogen purposely, and removing it by passing the gas over red-hot copper oxide, which oxidises the hydrogen to water. This yielded nitrogen of the same density as that which had not undergone that treatment.
One other possibility was considered: the atmospheric nitrogen might contain some molecules of greater complexity than two-atom molecules, say N3-molecules. Now it is known that when oxygen is electrified by the passage of a rain of small sparks through it, it acquires new properties: it possesses an odour, and attacks metallic mercury and silver. And this product, ozone, has been shown to consist of three-atom molecules of oxygen, by various experiments of which an account cannot be given here.
It was not inconceivable that if such a “silent electric discharge” were to be passed through “atmospheric” nitrogen, it might increase the number of such three-atom molecules, and might render the gas still denser; or if passed through “chemical” nitrogen, it might increase its density so as to make it equal to that of “atmospheric” nitrogen. Lord Rayleigh made such experiments, but without changing the density in the least: the nitrogen from ammonia or from oxides of nitrogen, which has been termed “chemical” nitrogen, still remained too light by about one two-hundredth, and the atmospheric nitrogen still remained too heavy by the same amount.