292 D. Kirkwood—Solar and Sidereal Heat. 
2. That not less than five hundred millions of years have 
been required for the stratification of the earth’s crust at the 
present rate of subaérial denudation ; and hence that the grav- 
itation theory of the origin of the sun’s heat is incompatible 
with geological facts. 
we suppose two solid opaque bodies, each equal to half 
the sun’s mass, to fall together in consequence simply of their 
mutual attraction, the collision would instantly generate suffi- 
cient heat to reduce the entire mass to a state of vapor. If, in 
addition to the motion resulting from their mutual attraction, 
we suppose the bodies to have had an original or independent 
motion towards each other of 202 miles per second, the con- 
cussion would produce 50,000,000 years’ heat; a motion of 678 
miles per second, together with that due to their mutual at- 
traction, would generate 200,000,000 years’ heat ; and a velocity 
of 1,700 miles per second would generate an amount of beat 
which would keep up the supply at the present rate for 800,- 
000,000 years. 
4. The sun and all visible stars may have derived their heat 
from the collision of cold, opaque masses thus moving in space. 
The nebulz are the products of the more recent impacts, and 
the stars have been formed by the condensation of ancient 
nebulee. 
5. This theory, while accepting the doctrine of the conser- 
vation of energy, indicates at the same time a possible supply 
of heat for several hundred millions of years; thus satisfying 
all moderate demands for geological time. 
The mathematical correctness of the theory here stated will not 
be called in question. We shall consider merely the probability 
of the facts assumed as its basis. To the present writer the 
hypothesis seems unsatisfactory for the following reasons: 
_ L The existence of such sidereal bodies as the theory assumes 
is purely conjectural, unless it be claimed that lost or missing 
stars have become non-luminous, of which we have no conclu- 
sive evidence. 
and in no case exceeding 200 miles per secon 
3. If the two iia whose collision the sun is supposed 
to have been formed were very unequal, as would be most 
Siete the amount of heat generated would be correspond- 
ingly less. 
4. Such collisions as the theory assumes are wholly hypo- 
thetical. It is infinitely improbable that two cosmical bodies 
should move in the same straight line; and of two moving in 
different lines, it is improbable that either should impinge 
ee ee i st -_ ei 
