The Gases of the Atmosphere: The History of Their Discovery

by William Ramsay

At this stage Professor Ramsay asked and received permission to make some experiments on the nitrogen of the atmosphere, with the view of explaining its anomalous behaviour. He had several years before made experiments on the possibility of causing nitrogen and hydrogen to combine directly, by passing the mixture over heated metals; among these was magnesium, and although no direct combination to any great extent was observed, still it was noticed that magnesium was a good absorbent for nitrogen, when that gas was passed over the red-hot filings of the metal. This process was therefore applied to the absorption of “atmospheric” nitrogen, in order to find out whether any portion of it was different from the rest. The plan adopted was to heat turnings of magnesium, which can be made very thin and loose, to redness in a tube of hard glass, in contact with the nitrogen of the atmosphere, carefully purified from oxygen, which would otherwise have also combined with the metallic magnesium. As absorption proceeded, more nitrogen was admitted from a reservoir, and after a certain quantity had been absorbed, the residual gas was extracted from the tube by a mercury pump, and weighed.

The amount weighed was very small,—smaller perhaps than had up till then been thought possible, if accurate results were to be obtained. But here large differences were to be looked for. Only 40 cubic centimetres—the twenty-fifth part of a litre—was weighed; and its weight was only 0·050 gram. But with careful weighing the error should not exceed one five-hundredth of the amount weighed; and if there were to be any increase in density, that increase should be expected greatly to exceed this small fraction.

The first weighing—in May 1894—showed that the nitrogen had increased in density by reason of the operations, and instead of being fourteen times as heavy as hydrogen, it was nearly fifteen times as heavy.

Fig. 1.

The result was encouraging, and led to the probability of the nitrogen being altered in some way, or of the presence of some new component of the atmosphere. An experiment was therefore begun on a larger scale, the atmospheric nitrogen being passed backwards and forwards from one large glass gasholder A to another B, through a tube filled with magnesium heated to redness G, to absorb nitrogen; over red-hot copper oxide (a) (b), so that any carbonaceous matter such as dust should be oxidised to carbon dioxide and water; and these, if produced, were absorbed by placing in the train of tubes, one filled with a mixture of soda and lime F and I, to absorb any carbon dioxide which might possibly be formed, and two filled with pentoxide of phosphorus D and H, to dry the gas, so that water-vapour, carried along with the gas from the gasholders (which contained water) might be removed before the gas passed over the red-hot magnesium; for water acts on hot magnesium, forming oxide of magnesium and hydrogen, and the gas would have become contaminated with the latter had this precaution not been taken.

The process was continued for ten days, by which time most of the nitrogen had become absorbed. The apparatus was then somewhat altered, so as to make it possible to work with a smaller quantity of gas; but the tubes destined to absorb nitrogen, hydrogen, etc., were filled with the same materials as before. In a few days more the volume was reduced to one-seventh of what it had been when the transference to the smaller apparatus was made, or about one-eightieth of the original volume of the atmospheric nitrogen taken.

The gas was then weighed, this time in a larger bulb, the weight being 0·2190 gram; and such is the possibility of precision in weighing on a good balance, that a difference of one two-thousandth of the whole weight was detectable. The density of the gas was now found to be 16·1. At this stage it was still believed that the new gas was an ozone-like modification of nitrogen, difficult to attack by magnesium. It was supposed that just as oxygen, when exposed to an electric discharge, undergoes a cleavage of its molecules, two-atom molecules becoming one-atom molecules for an instant, which then unite to form three-atom molecules, so the action of the magnesium on the nitrogen might be to withdraw one atom of nitrogen from the two-atom molecule, leaving a single uncombined atom, which might not improbably find two partners, each of its own kind, to form with them a three-atom molecule—a sort of nitrogen-ozone, in fact. Hence it was resolved to continue the absorption with fresh magnesium for a still longer time, in the hope of its being possible to isolate the three-atom nitrogen molecules. But it became apparent that the bright metallic magnesium was now not much attacked; and on estimating the total amount of nitrogen absorbed, by treating the compound of nitrogen and magnesium with water, and liberating the nitrogen as ammonia, it appeared that only a small quantity of magnesium nitride had been formed. The density of this further purified gas was again determined, when it was found that a litre now weighed 1·7054 gram, corresponding to a density of 19·086.

A portion of this gas was mixed with oxygen and exposed to a rain of electric sparks in presence of caustic soda; in fact, Cavendish’s old plan of causing nitrogen to combine was now resorted to. Contraction occurred, and on removing the excess of oxygen, the diminution of volume was found to amount to 15·4 per cent of the original volume taken. Making the supposition that the gas of density 19 still contained nitrogen, and allowing for its influencing the density, it followed that the pure gas should be twenty times as heavy as hydrogen.

A tube such as is usually employed in examining the spectra of gases at low pressures was next filled with the gas of density 19. Such a tube, called a Pflücker’s tube, after its inventor, contains wires of platinum sealed through at each end, where it is about half an inch in width; the middle portion of the tube is about 3 inches long, and its bore is a fine capillary. When the platinum wires are connected with the secondary terminals of a Ruhmkorff’s coil, and the tube is partially exhausted, a brilliant glow appears in the capillary portion. If viewed through a glass prism, different gases show different sets of coloured lines crossing the usual gradation of colours of the spectrum. Thus hydrogen exhibits three striking lines, one bright red, one peacock blue, and one violet; nitrogen shows a large number of somewhat hazy bands, red, orange, yellow, and yellow-green in colour, besides a number of bands of a violet colour; but the new gas, while exhibiting the bands characteristic of nitrogen, showed in addition certain groups of red and green lines which did not appear to belong to the spectrum of any known gas.