Lord Rayleigh tried both ways, and he found that the nitrogen from the atmosphere was denser than that derived from ammonia. Sir William Ramsey then carried the matter a step further. He heated atmospheric nitrogen in the presence of magnesium, under which conditions some of the nitrogen combines with the latter element to form nitride of magnesium. That, it was found, made the remaining nitrogen denser still. The explanation then seemed obvious. Suppose we imagine a mixture of sawdust and iron filings: it will be heavier than an equal quantity of pure sawdust. And if we contrive to take away some of the sawdust from the mixture we shall find that what is left is heavier still, when compared with an equal bulk of pure sawdust. For it is clear that as we take away sawdust we thereby increase the proportion of the heavier iron filings and so we make the mixture heavier.
Applying a similar process of reasoning to these discoveries, the conviction grew that the nitrogen of the air was not pure, but that it had mixed with it a small proportion of some other gas of greater density. They soon succeeded in isolating this denser gas, to which they gave the name of argon. Its atomic weight was found, and, wonderful to relate, it was such that argon fell into a new column to the left of Group 1, as had been anticipated.
The discovery of argon was announced in 1894. The next year Sir William Ramsey, investigating a gas which had been discovered locked up in the interstices of a mineral called clevite, was able to state that it was helium, the element which had been previously noticed by the spectroscope in the sun. Like argon, it was found to be extremely inactive, and its atomic weight turned out to be such that it too fell into the "Zero Group."
In 1898 Professors Ramsey and Travers found two more gases in the air, krypton and neon, and a little later still, there was found mixed with the krypton a further new gas, xenon. All of these had their atomic weights found, and fell into that new column in the periodic table.
But what has all this got to do with liquid air? The two subjects are closely related, for it is by liquid-air machines that these rare gases are now obtained, and it was from liquid air that the last three were first discovered.
For air, as we well know, is a mixture of gases, and when extreme cold and pressure are applied these gases liquefy, each behaving according to its own nature. They do not all liquefy at the same time, nor on being relieved from the pressure and heated do all evaporate again at the same temperature. Although they emerge from the liquid-air machine in the form of a single liquid, it is really a mixture of liquids, each with its own boiling-point.
In an earlier chapter we saw how petroleum can be separated into its various constituents, such as petrol, by fractional distillation, advantage being taken of the difference in the "boiling-point" of the various "fractions." The boiling-point of a liquid is, of course, the temperature at which it turns freely into vapour, and just as petroleum when heated gives off first cymogene, next rhigolene, then petrol, benzine, kerosene and so on, in the order named, so liquid air, when it is evaporated, gives off its different constituents in order. Nitrogen, oxygen, argon, helium, krypton, neon and xenon can all be separated each from the others in this way, by "fractional distillation." The heat from the surrounding objects is allowed to get at the liquid, and the gases are then given off in the order of their boiling-points.
And thus we see how the mechanical production of cold has assisted in the pursuit of pure science. The newly-found gases are not of any great use at present. They are so inactive that possibly they never will be, with one exception, and that is neon. If an electric discharge be made to pass through a tube filled with this gas, a beautiful glow is the result, and it is just possible that neon tubes may become the electric light of the future. That is only a prediction, however, and a hesitating one at that.
The inactive elements may become of value in explosives. We have seen how important nitrogen is in these dangerous substances, the chief feature of which is their instability—their readiness, that is, to change into something else—which instability is due to the reluctance with which nitrogen enters into them. Now nitrogen, though inactive, is much less so than these others, and if a way should ever be found of inducing them to enter into a compound, that compound will probably be an extremely powerful explosive.