160 NOTES ON MARS. 



doubt the average speed at which the molecules of this gas are hurry- 

 ing' about is only 1 mile a second, and therefore onl}^ a seventh of the 

 critical velocity required to project a missile from the earth so as not 

 to return. But the molecules are continually changing their velocity, 

 and may sometimes attain a speed which is seven times as great as the 

 average. Suppose, therefore, that a certain quantit}^ of hydrogen 

 were diffused through our air. Every now and then a molecule of 

 hydrogen in its wandering would attain the upper limit of our atmos- 

 sphere, and then it would occasionally happen that with its proper 

 speed it would cross out into space beyond the region b}' which its 

 movements would be interfered with by the collisions between other 

 atmospheric molecules. If the attraction of the earth were sufficient to 

 recall it, then, of course, it would duly fall back, and in the case of 

 the more sluggishly moving atmospheric gases the ^'elocity seems 

 always small enough to permit the recall to ]je made. But it happens 

 in the case of hj^drogen that the velocity with which its molecules are 

 occasionall}^ animated rises beyond the speed which could l^e controlled 

 by terrestrial gravity. The consequence is that every now and then 

 a molecule of hydrogen would succeed in bolting away from the earth 

 altogether and escaping into open space. Thus it appears that every 

 molecule of free hydrogen which happened to be present in an atmos- 

 phere like ours would have an unstable connection with the earth, for 

 wherever in the vicissitudes of things it happened to reach the very 

 uppermost strata it would be liable to escape altogether. In the 

 course of uncounted ages it would thus come to pass that the particles 

 of hydrogen would all effect their departure, and thus the fact that 

 there is at present no fi?ee hj^drogen in the air over our heads may be 

 accounted for. 



If the mass of the earth were very nuich larger than it is, then the 

 velocities with which the molecules of hydrogen went their Avay would 

 never be sufficiently high to enable them to quit the earth altogether, 

 and consequently we might in such a case expect to find our atmos- 

 phere largely charged with hydrogen. Considering the vast abundance 

 of hydrogen in the universe, it seems highly probable that its absence 

 from our air is simply due to the circumstances wo have mentioned. 

 In the case of a globe so mighty as the sun, the attraction which it 

 exercises, even at the uppermost layers of its atmosphere is so intense 

 that the molecules of hydrogen never attain pace enough to enable 

 them to escape. Their velocity would have to be much greater than 

 it ever can be if they could dart away from the sun as they have done 

 from the earth. It is not, therefore, surprising to find hydrogen in 

 the solar atmosphere. In a similar manner we can explain the abun- 

 dance with which the atmospheres of other massive suns like Sirius or 

 Vega seem to be charged with hydrogen. The attraction of these vast 



