NOTES ON MAES. 159 



of which the directions and velocities of the individual molecules are 

 continually changing-. For each gas the molecules have, however, a 

 certain average pace, which is appropriate to that gas for that temper- 

 ature, and when two or more gases are blended, as in our atmosphere, 

 then each molecule of the constituent gases continues to move with its 

 own particular speed. Thus, in the case of the air, the molecules of 

 ogygen, as well as the molecules of nitrogen, tire each animated by 

 their characteristic velocity, and the same ma}^ be said of the molecules 

 of carbonic acid or of anj' other gas which in more or less abundance 

 may happen to be diffused through our air. For two of the chief 

 gases the average velocities of the molecules are as follows: Oxygen, 

 a quarter of a mile per second; hydrogen, 1 mile a second; in each 

 case the temperature is taken to be 64° C below zero, being presuma- 

 bh" that at the confines of the atmosphere. It will be noticed that 

 there is a remarkable difference Ijetween the speeds of the two mole- 

 cules here mentioned. That of hydrogen is by far the greatest of any 

 gas. 



We may now recall a fundamental fact in connection with an}^ celes- 

 tial body, large or small. It is well known that with the most power- 

 ful pieces of artillery that can be forged a projectile can be launched 

 with a speed of about half a mile per second. If the cannon were 

 pointed vertically upward, the projectile would soar to a great eleva- 

 tion, but its speed would gradually abate, and the summit of its jour- 

 ne}' would be duly reached, after which it would fall back again on the 

 earth. Such would undoubtedh' be the case if the experiment were 

 made on a globe resembling our own in size and mass. But on a globe 

 much smaller than the earth, not larger, for instance, than are some 

 of the minor planets, it is certain that a projectile shot aloft from a 

 great Armstrong gun would go up and up and would never return. 

 The lessening gravitation of the bod}- would fail to recall it. Of 

 course we are here reminded of Jules Verne's famous Columbiad. 

 According to that philosopher, if a cannon were pointed verticalh" and 

 the projectile were discharged with a speed of T miles a second, it 

 would soar aloft, and whether it went to the moon or not, it would at 

 all events not return to the earth except by such a marvelous series of 

 coincidences as those which he has described. But the story will, at 

 all events, serve to illustrate the fact that for each particular globe 

 there is a certain speed with which if a body leaves the globe it will 

 not return. 



It is a singular fact that hydrogen in its free state is absent from 

 our atmosphere. Doubtless many explanations of a chemical nature 

 might be offered, but the argument Dr. Stone}^ has brought forward 

 is most interesting, inasmuch as it shows that the continued existence 

 of hydrogen in our atmosphere would seem to be impossible. No 



