JULY 7, 1899. ] 
central heat of the more concentrated bodies 
is conserved at the present time. Under 
this conception the history of the stellar uni- 
verse may be characterized as a progress- 
ive clearing-up of nebulosities and meteor- 
oidal dispersions and the concentration of 
its matter about certain points, leaving be- 
tween vast open spaces through which heat 
is now radiated away with a facility un- 
realized in the earlier stages. The quanti- 
tative value of such a suggestion must be 
left to the determination of astronomers 
who have the best data for forming a conjec- 
ture as to the ratio of matter to space in the 
stellar universe and as to the possibilities 
of its dispersion at a period coincident with 
the earlier stages of the earth’s history. 
4, A modification of the conditions as- 
sumed in the foregoing paragraphs may be 
postulated in which the earth is regarded 
as having made its early growth within the 
primordial meteoric aggregate, perhaps a 
great flattened meteoric spheroid, which in- 
itially extended beyond Neptune in nebular 
fashion and whose present attenuated rep- 
resentative may, perhaps, be found in the 
zodiacal light. In this case the thermal 
environment of the early earth was that 
furnished by the interior of the spheroid, 
though far out from the center. The con- 
ditions only became external gradually as 
the growth of the planets exhausted the 
peripheral portion of the meteoric spheroid. 
5. The foregoing hypotheses, which do 
not seem to be so completely out of accord 
with the possibilities of the case as to be 
inadmissible tentatively in the absence of 
a positive solution of the early terrestrial 
environment, are concerned with the ex- 
ternal relations of the earth. If we turn 
to the earth itself it may be remarked that 
the nature of its atmosphere very radically 
conditions the amount of heat requisite for 
the support of life. Dr. Arrhenius has re- 
cently made an elaborate computation rela- 
tive to the thermal influence of certain fac- 
’ SCIENCE. 
15 
tors of the atmosphere and has arrived at 
the conclusion that an increase of the at- 
mospheric carbon dioxide to the amount of 
three or four times the present content 
would induce such a mild climate in the 
polar regions that magnolias might again 
flourish there as they did in Tertiary times. 
On the other hand, he concluded that a re- 
duction of less than 50% would induce con- 
ditions analogous to those of the glacial 
period of Pleistocene times. The vast quan- 
tities of carbon dioxide represented in the 
carbonates and carbonaceous deposits of the 
earth’s crust imply great possibilities of 
change in the constitution of the atmos- 
phere of the earth in respect to this most 
critical element. 
6. But there are more radical consid- 
erations that relate to the early thermal 
history of the earth. To be sure, if we are 
forced to adopt the hypothesis of a white- 
hot liquid earth, with all its extravagant 
expenditures of energy in the early youth 
of the earth, we can take no advantage of 
these possible resources, but under the sup- 
position that the meteorites gathered in 
with measurable deliberation, it is theoret- 
ically possible to find conditions for a long 
maintenance of life on the earth, with little 
or no regard to the amount of heat which 
the early sun sent to it. In the earliest 
stages of the aggregation of the earth under 
this hypothesis, while it was yet small, it 
can scarcely be supposed to have been 
habitable, because its mass was not suffi- 
cient to control the requisite atmospheric 
gases, but when it had grown to the size of 
Mars, that is to a size representing about 
zy of its present aggregation, or, to be safe, 
when it had grown to twice the size of 
Mars, or about one-fifth of its present mass, 
it would have been able to control the at- 
mospheric gases and water, and, so far as 
these essential items are concerned, it 
would have presented conditions fitted for 
the presence of life. At this stage the 
