August 10, 1900.] 



SCIENCE. 



203 



this : The molecules of gases beat upon each 

 other and rebound with great frequency and 

 high velocities. Both the frequency and the 

 velocities rise with temperature. The mo- 

 lecular velocity of a gas is inversely propor- 

 tional to the square root of its molecular 

 weight and hence for the lighter gases it is 

 very high. The velocity of a particular 

 molecule at any instant depends on the 

 nature of the last previous collision, being 

 sometimes much higher than the average 

 velocity and sometimes much lower. Now 

 it is obvious that if a molecule on the 

 outer border of the atmosphere collides with 

 another and rebounds outwards with a ve- 

 locity so great that the attraction of the 

 earth cannot overcome it, the molecule 

 will fly away into space and be lost. Dr. 

 Stoney has attempted to show that on ac- 

 count of the high velocities thus frequently 

 attained, hydrogen and even helium are 

 not permanently retained by the earth under 

 present conditions. He has also endeavored 

 to prove that the moon and other small 

 bodies cannot retain any of the atmospheric 

 gases, and that this is the reason they are 

 without atmospheres, and this latter view is 

 now quite generally accepted. 



Mr. S. E. Cook, however, has attempted 

 to show by mathematical analysis that the 

 rate of loss of hydrogen from the earth is at 

 present too small to be effective,* but he has 

 based his computations on the theoretical 

 parabolic velocity of the earth and not on 

 its available power of control in competition 

 with the sun and other bodies of the solar 

 system, and he also neglected the ionization 

 of the gases and the peculiar conditions 



the Moon, ' by Dr. G. Johnstone Stoney, Royal Dub- 

 lin Society, 1892, and ' Of the Atmospheres upon 

 Planets and Satellites,' by G. Johnstone Stoney. 

 Trans. Royal Dublin Society, Vol. IV., Part B, Oct., 

 1897. See also Paper I. above cited, pp. 653-683. 



■* ' On the Escape of Gases from Planetary Atmos- 

 pheres according to the Kinetic Theory,' by S. R. 

 Cook. Aslrophysical Journal, Vol. XI., No. 1, Janu- 

 ary, 1900, pp. 36-43. 



of the upper atmosphere.* We have en- 

 deavored to show that whatever doubt there 

 may be about the precise competency of 

 molecular velocities to cause loss of the 

 lighter gases at present, their retention 

 would be put in jeopardy if the temperature 

 of the earth were raised to 3500° or 4000° 

 C. as would be necessary to restore the earth 

 to the original gaseous condition postulated 

 by the Laplacian hypothesis. At such tem- 

 peratures water would be dissociated into 

 hydrogen and oxygen, if not ionized to a 

 higher degree ; the molecular velocities of 

 these gases would be exalted by the intense 

 heatf and, in such a state of gaseous ex- 

 tension, the outer part would be far from the 

 center of gravity where the control would 

 be enfeebled. J Under such conditions it 

 seems highly improbable that hydrogen 

 could be retained, and hence, the inference 

 that if the earth had passed through such a 

 history it would be deficient in hydrogen 

 compounds. Not only the atmosphere but 

 the ocean would seem to be put in jeopardy. 

 But this is by no means the crucial ap- 

 plication of the test. The Laplacian hy- 

 pothesis assumes that the material of the 

 earth and moon was detached from the solar 

 mass as an equatorial ring whose diameter 

 was essentially that of the earth's orbit. 

 Now the gravitation at any point on the 

 surface of such a ring would be very feeble 

 — very much feebler indeed than that on 

 the present surface of the moon where no 

 atmosphere is retained. At the same time, 

 by hypothesis, the temperature of the ring 

 was very high, and this high temperature 

 would only prevail if there were frequent 

 and intense collisions. But the rebound 

 from such intense collisions would carry 



* See reply of Dr. Stoney ' On the Escape of Gases 

 from Planetary Atmospheres according to the Kinetic 

 Theory.' J. G. Johnstone Stoney, Asirophysical Jour- 

 nal, p. 251, May, 1900. Ibid., II., June, 1900, p. 357. 



\ See table. Paper I., above cited, p. 661. 



t Paper I , pp. 659-661. 



