August 18, 189^. 



SCIENCE, 



87 



SCIENCE: 



Published by N. D. C. HODGES, 874 Broadway, New Yori 



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THE ATMOSPHEKE OF STELLAE SPACE. 



BT G. D. HVKTNG, CAMEBEDGE, ENGLAND. 



It was an interesting speculation that Sir E. Ball opened 

 up in this journal, a short time since, with regard to the 

 lunar atmosphere. His argument might easily be car- 

 ried further, and would take us, as I shall try to show, 

 into the realms of stellar space. It has been objected to 

 his theory that the velocity of the particles of air at ordi- 

 nary temjieratures, though on the average about five hun- 

 dred yards per second, is not enough to carry a particle 

 so quickly away from the moon that it would not be 

 drawn back again bj' its gravitation. This objection van- 

 ishes if we consider, not the average velocity, but the 

 velocities of individual j)articles, and the changes those 

 velocities rapidly undergo in consequence of frequent col- 

 lisions among the particles. It is not easy to grasp the 

 numbers involved in my argument, but I will state them 

 on the authority of Lord Kelvin's popular lecture on the 

 size of a^toms. He gives the number of particles in one cubic 

 centimetre, or one-sixteenth of a cubic inch, of atmospheric 

 air at ordinary barometric pressure and at ordinary tem- 

 perature, as not less than a million million of millions, or 

 10"^. Maxwell, in his article on "Atoms," in the Encyclo- 

 jJEedia Britannica, makes the number greater. These par- 

 ticles cannot move far, not more on the average than 

 about one hundredth of a thousandth of a centimetre, 

 without encountering one another, so that each particle 

 collides with one or another of its neighbors no less than 

 five thousand million times in every second. If we suji- 

 jpose the density of the moon's atmosphere to be only a 

 millionth of that of our atmosphere at the earth's surface, 

 there will stiU be at least a miLlion millions of particles in 

 one cubic centimetre of it, and the frequency of their en- 

 counters with each other will still be some thousands jaer 

 second for each of them. These encounters will cause 

 them to be perpetually changing their- velocities, and 

 while some will have, at any given instant, velocities many 

 times greater than the average, others will move at cor- 

 respondingly slower rates. The directions, also, of their 

 movements will be constantly changing from the same 

 cause. If we suppose two particles, moving with equal 

 velocities in directions at right angles to one another, to 

 come into direct collision, one of them will have its veloc- 

 ity increased in the ratio of the square root of two to one, 

 or rather more than seven to five, while the other wDI be 

 reduced to momentary rest. If, now, the former come 

 into a similar collision with a thii'd jsarticle, one of these two 



will acquire a still greater velocity. And considering the 

 prodigious number of the particles and the short distance 

 they can move without encountering others, it is evident 

 that there must be an immense variety of rates of motion 

 amongst them, and many of them must have velocities far 

 exceeding that necessary to carry them clear away from 

 the moon, or the earth, or even from the sun. In 

 fact, amongst so many millions of millions the chance that 

 some one will go on increasing its velocity at every one of 

 a large number of successive encounters is very great in- 

 deed, practically a certainty. If this be granted, some, if 

 it be but a small fraction of the whole, will be always 

 escaping from the outer surface of the lunar atmosjjhere 

 into the planetary space ; and the like must go on from the 

 atmosjjheres of other j)lanets, only the fraction of the 

 whole which get clear away from the bigger planets will 

 be so much less because of the greater attraction of the 

 bigger masses. 



One interesting consequence of this escape of only the 

 quicker moving particles, is that the temperature of inter- 

 planetary space must be thereby raised above that of the 

 outer regions of a planet's atmosjahere. For the temjaer- 

 ature is directly proportional to the average square of the 

 velocities of the particles, and as only the quickest fly off 

 for good, the average velocity of the remainder must be 

 less than that of those that break away. The process of dis- 

 sipating an atmosphere into space might be stojij^ed by its 

 own cooling effect. But it is obvious that there is an- 

 other cause which prevents anything like this. The 

 2)lanets are continually sweeping through the interjDlane- 

 tary sjaace where the escajDed particles are moving about, 

 and even if the density of this interj)lanetary atmosj)here 

 be only a millionth of a millionth of the density of that 

 at the earth's surface, still there will be at least a million 

 particles in each cubic centimetre, and some of them will 

 get swept up by the planets in their course and will not 

 get away again. Hence the process of dissipation will 

 cease when a planet picks up in its course through sj^ace 

 just as many as it loses by diffusion in the same time. It 

 follows from this that there must exist in planetary space 

 an atmosphere, greatly reduced in density, it is true, but 

 of the same chemical constitution as the earth's atmos- 

 phere. That is to say, the chemical constituents will be the 

 same, though not quite in the same proportions. For the 

 average velocity of the particles of nitrogen is a trifle 

 greater than that of the jjarticles of oxygen, and so the 

 former will escape into space rather more frequently in 

 proportion to their numbers than the latter. Besides, the 

 effect of gravity is to increase very slightly the propor- 

 tion of oxygen to nitrogen in the lower strata of the at- 

 mosj)here. Hence, for both reasons, the atmosi^here of 

 planetary space will be a trifle richer in nitrogen than the 

 air we breathe. There is so very little free hydrogen in 

 our- atmosphere that we cannot detect it, but for all that, 

 it is most 2)robable that there is a very little. And as 

 oxygen particles are sixteen times as heavy as those of 

 hydrogen, the projjortion of free hydrogen to the other 

 gases will be proportionally greater in the ujDper regions 

 of the air than in the lower; and since hydi-ogen ijarticles 

 move four times as quickly as oxygen particles, it follows 

 that the former will escape from the earth's attraction 

 about four times as fast, and so the proportion of hydi-o- 

 gen in jilanetary sf)ace may be sensibly greater than in 

 air we are able to test. A similar argument will api^ly to 

 -particles of water vapor, which are little more than half as 

 massive as particles of oxygen. If all the planets are thus 

 losing continually some of their atmospheres and picking 

 uj) an equal amount from the space they move in, it fol- 

 lows that all the planets must have atmospheres of simi- 

 lar constitution to our own. For each planet has for'ages 

 been losing some of its own and acquiring some of the air 



