HVPOTHESES BEARING ON CLIMATIC CHANGES 667 
per second. Now the temperatures of the supposed molten 
earth reached and probably much exceeded the temperatures of 
effective dissociation of water vapor. The dissociation is prob- 
ably due to violent impact of molecules of high velocities. It 
probably takes place in some degree even at moderate temper- 
atures." The proportion of dissociated molecules greatly 
increases with temperature until the dissociation so far exceeds 
the recombination that it may be said to be nearly or quite com- 
plete. Authorities differ as to the temperature of effective dis- 
sociation. The estimates commonly given lie in the lower half 
of the range of temperatures above assigned to the molten stage 
of the earth. If, therefore, the temperatures of the molten globe 
ranged as high as the current hypothesis seems to require, the 
dissociation of the aqueous vapor would seem to be inevitable 
and the loss of hydrogen would be endangered notwithstanding 
its disposition to recombine. 
If the retention of the atmosphere be put in jeopardy by the 
earth’s temperatures ina supposed liquid state much more would 
it be endangered if the temperatures were those of volatilization 
of the refractory material of the earth, as assumed by the 
Laplacean hypothesis, for not only would the molecular veloci- 
ties be enormously increased, but the extension of the mass 
would push its exterior portions out into the regions of low 
parabolic velocity. 
If the mass be still further dispersed into the vast gaseous 
ring of the Laplacean hypothesis the argument from molecular 
velocities is immeasurably strengthened, for not only must the 
temperatures requisite to the retention of the refractory sub- 
stances of the earth in the attenuated condition of such a gaseous 
ring be exceedingly high, but the parabolic velocity of the body 
with the free oxygen of the air is in a large measure covered by resting the argument 
chiefly on the absence of helium, which is chemically very inert. As helium is given 
off slowly by hot springs, it is urged that in the vast lapse of the geological ages it 
should have accumulated to an appreciable quantity if it had not escaped. As it has 
twice the molecular mass of hydrogen it is held that the minimum speed of control” 
at existing temperatures lies below the molecular velocities of gases which are twice 
as heavy atomically as hydrogen. 
* RISTEEN, Molecules and Molecular Theory, pp. 50-51. 
