SOME FUNDAMENTAL CONCEITS OF EARTH HISTORY 167 
within their spheres of attraction and as they were competent 
to hold. In the case of the larger planets these doubtless included 
even the lightest gases, such as hydrogen and helium, but the 
smaller planets such as the * earth could hold only the heavier 
atmospheric gases, and these only after the temperatures had 
fallen to those of their present surfaces. The smallest planets, 
Mercury and Mars, and the planetoids and satellites never were 
, able to hold atmospheric gases or water vapor. Some smaller 
knots in the vicinity of the larger ones were within their spheres 
of control and so became satellite knots. From their smaller 
gathering power they would always remain relatively small. As 
a result of the nature of their origin the different knots would 
nave irregular spacings and masses. Hence their growth would 
be unequal and in ultimate character they would be different. 
It seems probable that the largest of the planets, Jupiter, has 
always been very hot. Indeed he is held by some astronomers to 
be self-luminous, a miniature sun. In the case of the earth knot 
the smaller size permitted rapid and probably complete cooling 
so that the juvenile earth was not very hot, either inside or out- 
side. Probably the core was never liquified, either from its 
original condensation or from later accretions of planetesimal 
matter. Whatever tendency there was in this direction because 
of friction or compression would be antagonized by the increasing 
pressure of overlying rock. 
The atmosphere of the earth is thought to have been derived, 
first from gases entrapped in the planetesimal matter and later 
released; second from gaseous matter which had been revolving 
about the growing earth — “the irreducible gaseous residium of 
the knot”; and third from matter which came in with iplanetesi- 
inals or as planetesimals. Its evolution began early and in a 
minor way is continuing at the present day. 
The hydrosphere, the water of the earth, was somewhat later 
in forming. Molecules of water-vapor have a greater velocity 
than do those of the atmospheric gases and hence would not con- 
dense into water until after an atmosphere had been well de- 
veloped. If, as computation shows to be probable, the earth- 
knot had 30 or 40 per cent of the present mass of the earth, it no 
doubt held water-vapor from the first, and so the hydrosphere 
would begin its development early in the planet’s evolution. In 
