772 



SCIENCE. 



[N. S. Vol. VIII. No. 205, 



Such a process, prolonged over ages, will 

 ultimately remove from the atmosphere of 

 a planet all gases possessing less than a 

 certain minimum densitj'. 

 > The next question to which Dr. Stoney 

 addresses himself is : What rate of motion 

 must a molecule have in order that it may 

 escape from the attraction of the earth? 

 The least velocity which will enable such a 

 molecule to escape is about seven miles per 

 second. And it is assumed, from observa- 

 tions taken at high altitudes, that the tem- 

 perature of the upper regions of the atmos- 

 phere is about —66° Cent., or about —87° 

 Fahr. 



This velocity of seven miles a second 

 is, however, considerably greater than the 

 average velocity of a molecule of hydrogen, 

 which, at 32° Fahr., it will be remembered, 

 is only about a mile and a quarter. But it 

 is not greater than the velocity of some of 

 the molecules ; and these will, therefore, es- 

 cape. In fact. Dr. Stoney concludes that in 

 every gas a considerable proportion of the 

 molecules have a velocity at least ten times 

 as great as the mean. 



Now, ou this earth the important constitu- 

 ents of the atmosphere are nitrogen, oxj'gen. 

 argon, carbon dioxide, water-vapor and 

 ammonia ; and their densities are as follows, 

 that of hydrogen being taken as unity : 



Nitrogen 14 



Oxygen 16 



Argon 20 



Carbon dioxide 22 



Water-gas 9 



Ammonia 8.5 



We are here chiefly concerned with the 

 gases of the earth's atmosphere, but it may 

 be of interest to cast a glance at other con- 

 clusions which follow from Dr. Stoney's 

 speculations. 



The moon, the mass of which is much less 

 than that of the earth, would retain a gas 

 of density 40, or thereabouts ; but all less 

 dense gases would escape rapidly. From 



the planet Mercury water- vapor would at 

 once escape, and it is probable that both 

 nitrogen and oxygen would escape more 

 slowly. Argon and carbon dioxide might, 

 however, be permanent constituents of the 

 atmosphere of Mercury. Venus, on the 

 other hand, retains water-vapor ; but lighter 

 gases would escape. It must be remem- 

 bered that if the water were to escape from 

 a planet in the state of vapor its place 

 would be at once supplied by evaporation 

 of planetary seas, if there were any, and 

 that, in the long run, all the water would, 

 in the state of gas or water, leave the planet. 



Indeed, Dr. Stoney thinks it not unlikely 

 that we are slowly losing our stock of water. 

 This, however, need excite no alarm, and 

 our water will probably outlast our coal 

 many millions of years. For so few of the 

 molecules of water comply with the required 

 standard of velocity that the rate of loss is 

 almost infinitesimally small. 



Similarly, Dr. Stoney conjectures that 

 water cannot remain on Mars ; that all 

 known gases would be imprisoned by Jupi- 

 ter, and that Saturn, Uranus and Neptune 

 may probably be able to retain all gases 

 heavier than hydrogen. As for the sun, 

 its mass is so enormous relatively to that 

 of the planets that, even at the exceedingly 

 high temperature which its atmosphere pos- 

 sesses, it is impossible for any known gaB 

 to remove itself from the neighborhood of 

 the luminary. 



We must now take leave of Dr. Stoney's 

 fascinating hypotheses for a time, and con- 

 sider the recent discoveries of gaseous con- 

 stituents of our atmosphere. 



After the discovery of argon as a constit- 

 uent of air, in 189-4, one of the discoverers, 

 acting on advice given him by Professor 

 Miers, was so fortunate as to isolate helium, 

 a gas contained in certain rare minerals, 

 the best known of which bears the name of 

 cleveite. Helium had previously been de- 

 tected in the chromosphere, the colored at- 



