658 Th, (O (CLELAN OSI BISIL ION 
nature and the extremely high temperature necessary to main- 
tain its refractory substances in a volatile condition combine to 
suggest its speedy passage from the vaporous to the Saturnian 
or discrete solid condition from loss of heat. It seems a severe 
tax upon probabilities to suppose that such a ting would remain 
in the gaseous condition during the long period of its aggrega- 
tion into a spheroidal form. 
But a graver source of doubt is found in the high molecular 
velocities of the gases under these conditions. Dr. Johnstone 
Stoney * and others have attempted to show that the attractive 
power of small planets is insufficient to control gases of the 
higher molecular velocities, especially aqueous vapor. To this 
is attributed the measurable absence of atmospheres on the 
satellites and small planets. An endeavor to apply a similar 
line of reasoning to the conditions of the early earth leads to 
such disquieting results that I may be justified in briefly sketch- 
ing it. 
Each celestial body has an attractive power sufficient to con- 
trol molecules shot away from it at velocities below a certain 
limit. At these velocities the discharged molecules pursue ellip- 
tical paths and return to the starting point. At the limit of 
these velocities they pursue parabolic courses and never return. 
Hence arises the expression “‘ parabolic velocity ’’ to indicate the 
limital speed at which particles shot away from the body will 
not return. The parabolic velocity of the earth at its surface is 
about 6.9 miles (1118127) per second. A molecule discharged 
from it at that speed or a greater one will not return to it. The 
parabolic velocity is but an expression of effective gravity and 
t™“ On the Cause of the Absence of Hydrogen from the Earth’s Atmosphere and 
of Air and Water from the Moon.”” Royal Dublin Society, 1892. Since this paper 
was put in type I have been permitted to see an advanced copy of Dr. Stoney’s later 
paper, “Of Atmospheres upon Planets and Satellites,” Trans. Royal Dublin Society, 
Vol. VI, Part 13, Oct. 25, 1897, in which the author’s investigations are much more 
fully set forth and his conclusions greatly strengthened. He takes account of the 
rotary speed of the outer equatorial zone and of westerly winds as projectile aids, 
factors which are neglected in this discussion. He also bases a very strong argument 
on the absence of helium from the present atmosphere, which on account of its chem- 
ical inertness would accumulate if it were not discharged. 
