342 POPULAR SCIENCE MONTHLY 



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 ele- 

 ment 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 circum- 

 stance 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 proba- 

 bly a mixture may explain his failure : air is more difificult 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 inert- 

 ness 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 perma- 

 nently upon a heavenly body of small mass and habitable tempera- 

 ture, 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 mole- 

 cules, 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 at- 

 mosphere 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, hy- 

 drogen w combination is extremely abundant ; one eighth part by 

 weight of all the water in the sea is hydrogen ; and hydrogen 



