not appear at all in the spectrum of terrestrial "helium" derived from any source whatever.
Ramsay's acoustic experiments tend to show that helium, like argon, is monatomic, but can hardly be considered conclusive. If he is right, the atomic weight of helium regarded as a single element would be not far from four; but thus far all attempts to make it enter into chemical combination have failed, though it seems rather probable that in the uraninite minerals it is held by stronger bonds than those of mere occlusion.
Olszewski has tried his best to liquefy the gas, but thus far unsuccessfully; the methods that have conquered every other gas, hydrogen itself included, have failed with helium a circumstance very remarkable, since generally a.denser gas liquefies more easily than a lighter one, and hitherto hydrogen has stood pre-eminent in its refractoriness. The fact that the gas is probably a mixture may explain his failure: air is more difficult to liquefy than either oxygen or nitrogen.
Probably the question has suggested itself to every reader how it happens that helium, so conspicuous in the atmosphere of the sun and many stars, should be so nearly absent from our own atmosphere and so scantily present in any form upon the earth. The answer seems to depend upon two facts—the chemical inertness of the substance and its low density.
According to Johnstone Stoney's deductions from the accepted theory of gases, no free gas of low density can remain permanently upon a heavenly body of small mass and habitable temperature, but the molecules will fly off into space. A particle leaving the earth with a velocity of about seven miles a second would never return to it, this "limiting velocity" depending upon the mass of the earth and its diameter. Now, according to the dynamic theory of gases, the molecules of our atmosphere are flying swiftly about with velocities (at ordinary temperatures) of from fifteen hundred to fifteen thousand feet per second; the heavier molecules, like those of oxygen and nitrogen, move comparatively slowly, but if any lighter gas, like free hydrogen or helium, is present, its molecules take up velocities several times more swift, and any that may happen to be near the upper limits of the atmosphere would be likely to be thrown off into space. In the case of the moon even the oxygen and the nitrogen would go, since she is so small that a velocity not much exceeding a mile a second would carry them off. If this is correct, it is easy to see why we now have no appreciable quantity of free hydrogen or other light gas in our atmosphere.
But while we have no atmospheric hydrogen to speak of, hydrogen in combination is extremely abundant; one eighth part by weight of all the water in the sea is hydrogen; and hydrogen
