In order to understand the atmospheres of the planets, it is of great interest to ascertain the composition of the air that surrounds the Earth. Our knowledge in these matters has grown considerably of late. We shall in the main follow the presentation by Dr. Wegener of Marburg. We know at present with considerable accuracy which gases enter into the air. Besides the previously well-known nitrogen and oxygen which contribute the bulk, 78.1 and 20.9 per cent. respectively, of the total volume at the earth’s surface, we find water vapour in proportions changing with localities and times, and it is for this reason left out when fixing the various percentages; further, carbon dioxide 0.03 volume per cent. and the rare gases discovered by Rayleigh and Ramsay, argon, 0.932 per cent., neon 0.0012 per cent., helium 0.0004 per cent., krypton 0.000005 per cent., and xenon 0.0000006 per cent. Each one of these constituents diminishes in quantity with height in accordance with the so-called barometer-formula and the rate is the more rapid the heavier the gas. Krypton and xenon, therefore, which are two and one half and four times heavier than oxygen, occur mainly in the lower strata. The percentage of helium on the other hand, a gas eight times lighter than oxygen, should increase rapidly with height. If the air consisted of a mixture of oxygen and helium at 0° C. (32° F.) the former would decrease to one half at an elevation of 5 km. (3.1 miles) but the latter not before we had ascended 40 km. (25 miles) (eight times higher than for oxygen, as the weights are in the ratio of 1 to 8). At that altitude, oxygen would have decreased in the proportion 1:2^8 = 1:256. When, as actually is the case, there is 50,000 times more oxygen than helium at the surface this ratio should decrease in the proportion 128:1 at a height of 40 km. (25 miles). Ninety kilometers (56 miles) above the surface helium should overbalance oxygen and thereafter rapidly gain in preponderance. This holds true provided no agitation takes place in the form of vertical currents of air.