Of course almost any gas may be found locally and occasionally in the atmosphere, but there are several that are always found wherever a refined analysis of the air is made, and others that are generally present. The following is a fairly complete list: Nitrogen, oxygen, water vapor, argon, carbon dioxide, hydrogen, helium, neon, krypton, xenon, niton (radium emanation), ozone, hydrogen dioxide, ammonia and other compounds of nitrogen. A number of these substances have only become known to science within the last quarter of a century. Argon, though it constitutes nearly 1 per cent of the atmosphere, escaped detection until the year 1894. The investigation of argon led to the discovery of some of the others. In 1895 it was found that the air, as well as certain minerals, contains helium. This substance was not new to science, but it had never before been found on earth. It was discovered in the atmosphere of the sun, by means of the spectroscope, as early as 1868. Terrestrial helium, neon, krypton, and xenon were all discovered by Sir William Ramsay, who also shared with Lord Rayleigh the distinction of discovering argon. Ramsay has published the following figures for the proportions in which some of the rare gases exist in the atmosphere: Helium 1 part in 245,320 by volume Neon 1 “ 80,800 “ “ Krypton 1 “ 2,000,000 “ “ Xenon 1 “ 17,000,000 “ “ Niton, or radium emanation, is one of the products of the disintegration of radium. Niton itself disintegrates very rapidly, one-half of any given quantity disappearing in about four days, and one of its products is helium. The amount of niton in the atmosphere is never more than an infinitesimal trace. Thus we are told that the total quantity of this substance present in the atmosphere of the whole earth up to an altitude of one kilometer (0.6 mile) weighs less than nine ounces, and that each cubic centimeter of air contains among its thirty million million million molecules only between one and two molecules of niton, on an average.