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.