gases were therefore submitted to the action of the electric discharge, but again both retained their original densities unaltered. The paper concludes without a suggestion as to the cause of the difference.

The exact sequence of events at this point is somewhat obscure. In the Gases of the Atmosphere (Edn. 1896, p. 159) Ramsay wrote: “At this stage Professor Ramsay asked and received permission to make some experiments on nitrogen of the atmosphere with a view to explaining its anomalous behaviour." In the Life of Lord Rayleigh the question of the terms of Ramsay's request, and of Lord Rayleigh's assent to it are discussed at some length. I do not think that the matter is one of very great interest or importance. The problem had been before the public for two and a half years, without a suggestion having been put forward in explanation of the apparent anomaly, and I feel sure that Lord Rayleigh would have been the last man to hinder anyone who wished to attempt to solve it. There is no indication in such of the contemporary correspondence between the discoverers as still exists of suspicion or sense of injustice on either side. Indeed the very idea would have been repugnant to either of them. One fact is perfectly clear. Lord Rayleigh and Ramsay stand out from amongst their contemporaries, chemists and physicists, as the two men who alone realised the significance of the apparent discrepancy in the densities of nitrogen. They were also the two men who alone were capable of developing the discovery. In genius, method and temperament each was in many respects the opposite and the complement of the other. No modern discovery ever awakened more interest than the discovery of argon; never did scientific men receive more gratuitous advice or criticism; but never was advice or criticism more completely sterile.

Ramsay had been engaged in a research on the surface tension of organic liquids. The experimental work had come to an end at the end of February. Some time was then spent in working out results which are summarized in his notebook. On April 11th there is a note headed, "Fractionation of formic acid." Then there is a break, and on the next page there is a sketch of the first apparatus (Fig. 1.) used for the absorption of nitrogen by magnesium. It is clear that he must have commenced experimental work immediately after the Royal Society meeting on April 19th, for on April 23rd he wrote to his wife:

"By the way curiously I am at work on nitrogen, but not from the commercial point of view, or rather Williams (Mr. Percy Williams, then his personal assistant) is. Nitrogen of the air is heavier than nitrogen from ammonia in the ratio 251 to 250. This would correspond to the addition of some light gas to the heavy one, or of some heavy gas to the light one. If the light gas was hydrogen it would need 7 parts in 2,000 to make it so much lighter. Now

FIG. 1.-APPARATUS USED IN FIRST EXPERIMENTS ON THE TREATMENT OF ATMOSPHERIC NITROGEN WITH MAGNESIUM.

no one has ever taken all the nitrogen out of air, or rather, after all the oxygen has been removed from air, no one has combined all the nitrogen. It is quite possible that there is some inert gas in nitrogen which has escaped notice. So Williams is at it now combining the nitrogen of the air with magnesium. and seeing if anything is over-anything not nitrogen. We may discover a new element."

Unfortunately no account of these early experiments is to be found in the notebook. The sketch referred to above, and the corresponding figure in the Philosophical Transactions of the Royal Society (Jan., 1895) paper are reproduced as Figure 1. The experiment to which the figure refers is described in the paper in the following words:

"A combustion tube, A, was filled with magnesium turnings, packed tightly by pushing them in with a rod. This tube was connected with a second piece of combustion tubing B by means of thick walled india-rubber tubing, carefully wired; B contained copper oxide, and in its turn was connected with a tube CD, one half of which contained soda lime, previously ignited to expel moisture, while the other half was filled with phosphoric anhydride. E is a measuring vessel, and F is a gas holder containing atmospheric nitrogen. In beginning an experiment the tubes were heated with long flame Bunsen burners and pumped empty; a little hydrogen was formed by the action of the moisture in the metallic magnesium, it was oxidised by the copper oxide, and absorbed by the phosphoric pentoxide. A gauge attached to the Sprengel pump connected with the apparatus showed when a vacuum had been reached. A quantity of nitrogen was then measured in E, and admitted into contact with the red-hot magnesium. Absorption took place rapidly at first, and then slowly, as shown by the gauge on the Sprengel pump. A fresh quantity was then measured and admitted, and this operation was repeated till no more could be absorbed. The system of tubes was then pumped empty by means of the Sprengel pump, and the gas was collected. The magnesium tube was then detached and replaced by another. The unabsorbed gas was returned to the measuring tube by a device shown in the figure (G), and the absorption recommenced. After 1,094 c.c.s of gas had been thus treated, there was left about 50 c.c.s of gas which resisted rapid absorption. It still contained nitrogen however, judging by the diminution of volume which it experienced when allowed to stand in contact with red-hot magnesium. Its density was, nevertheless, determined by weighing in a small bulb of 40 c.c.s capacity, filled first with air, and afterwards with the gas.

"The density of the residual gas was found to be 14.88, that of nitrogen obtained directly from air by removal of the oxygen being very close to 14.0.

The differences involved a difference in weight of 2.7 mg., much larger than any possible error."

Viewed from a distance of over thirty years this experiment seems simple enough; but from the standpoint of 1894 it was a very difficult one, requiring skill and much patience. How well this first experiment was conducted is shown by the fact that calculation, based upon knowledge gained later, shows that the density of the resulting gas should have been 14.75, which differs from the figure actually obtained by an amount of the order of the estimated experimental error.

There appears to be some doubt as to the exact form in which Ramsay communicated his result to Lord Rayleigh.

In the Life of Sir William Ramsay Sir William Tilden writes:

"After several previous letters to Lord Rayleigh he (Ramsay) writes on the 24th May, 1894:

"I intended to ask you to-day, what is probably quite unnecessary, not to say anything about the gas which I think I have got. It may turn out a mare's nest and it would be well that no one should know of its existence.

Another thing occurs to me. I have got a lot of magnesium nitride, which when treated with water yields ammonia. I can easily get nitrogen out of this ammonia, and I shall be glad to give it to you, if it can be conveyed to you in any way; or, what might be better, I could give you the ammonia as chloride of ammonium and you could liberate the ammonia and pass it, mixed with oxygen, over red-hot copper. I find on making a rough calculation. that on adding my 60 c.c.s of gas of sp. gr. 16 to the nitrogen from which it was obtained that it would amount to 3 p.c. of the total, and that such a mixture of N=4 (14?) and X-16 would give you the density you find. This is so far encouraging, but I must try to further purify the gas. I think that it still contains some nitrogen, and moreover it will be none the worse for another treatment with hot magnesium.

"Has it occurred to you that there is room for gaseous elements at the end of the first column in the periodic table? Thus,

Such elements should have the density 20 or thereabouts, and o.8 p.c. (1/120th

about) of the nitrogen of the air would raise the density of nitrogen that it would stand to pure nitrogen in the ratio 230 231."

Comment upon this letter is unnecessary.

Ramsay was a regular attendant at meetings of the Royal Society, and, no doubt, the progress of the experiments had been discussed in the course. of the usual tea interval before one or more meetings. However, the letter is of very great interest as the suggestions contained in it are quite prophetic. The method which has been described was used only in the first experiment as absorption took place much more slowly than when the gas was passed continuously through the apparatus. The free space in a tube packed with magnesium turnings may at the outset amount to 20 per cent. of the whole, but it rapidly diminishes as nitride is formed, and at a temperature of 800° the gas is only one-third as dense as at ordinary temperatures, so that the gas in the tube rapidly becomes poor in nitrogen. Also, a good deal of hydrogen is always formed by the action of water which is slowly given up by the glass, and apparently given off by the metal itself. A very little hydrogen would slow down the rate of absorption of nitrogen very materially if allowed to accumulate in the reaction tube.

The main investigation, which was actually concluded six years later, was commenced in July, 1894, and the first entry in the notebook is a drawing and description of the apparatus used. It is shown in facsimile in Fig. 2. The drawing is reproduced very faithfully on page 202 of the Philosophical Transactions of the Royal Society of January, 1895. The notes continue, “Four gasholders were passed through and absorbed, occupying from Tuesday, 10th July till Thursday, 19th. There was a residue of 1,500 c.cs. There may have been some leakage at first, for the absorptions were much more rapid later. But supposing that the total amount of nitrogen absorbed was 23,000 c.cs.The sentence is not finished, but later it is recorded that the gas was treated again with magnesium, and its volume reduced to 200 c.c., and as it is clear that this gas did not contain more than 50 per cent. of inactive gas, there must have been very considerable loss at some stage in the experiment. The apparatus used for the last experiment is shown in Fig. 3, which reproduces both Ramsay's original drawing, and the illustration in the Philosophical Transactions of January, 1895. The 1,500 c.cs. of gas in the holder over water was taken into the first of the two mercury-filled burettes, and passed thence through the tubes containing in order, phosphorus pentoxide, heated copper and copper oxide, and heated magnesium, into the second of the two mercury-filled burettes. It was passed backward, and forward, between these two burettes, the magnesium tube being renewed when necessary, and the gas in the tubes removed by means