460 Transactions.—G eology. 
and which revolved round its centre of gravity during the process of conden- 
sation. That, from time to time, extensive rings of this nebulous matter 
were left by the central condensing mass at points in the circumference 
where the centrifugal and gravitating forces became exactly balanced, and 
that these rings, still circulating round the central nucleus, broke up into 
masses which became endued with a motion of rotation and assumed a 
spheroidal form. These masses, acting in the same manner as the parent 
mass, and abandoning similar rings of outlying matter, led to the formation 
of the satellites of the various principal planets. During this process of con- 
densation, which, of course, took place in obedience to the all-pervading law 
of gravitation, the motion of the condensing particles of which each planet 
was composed, was converted into heat, and that to such a degree as would 
result in the fusion of the whole into one mass. Meyer, indeed, remarks, in 
a paper on Celestial Dynamics (as quoted by Nasmyth and Carpenter in 
their great work on the moon), that ‘‘ the Newtonian theory of gravitation, 
whilst it enables us to determine, from its present form, the earth's state of 
aggregation in ages past, at the same time points out to us a source of heat 
powerful enough to produce such a state of aggregation,—powerful enough 
to melt worlds; it teaches us to consider the molten state of a planct as the 
result of the mechanical union of cosmical masses, and to derive the 
radiation of the sun and the heat in the bowels of the earth from a common 
origin.” 
Sir Charles Lyell, however, though he did not dispute the opinions 
of Laplace and others as to the effect of the condensation of cosmical 
matter, appeared unwilling to admit the continued existence of internal 
heat to the extent contended for by leading physicists, and inclined 
rather to the opinion advanced by Poisson, ‘that in cooling by radiation to 
the medium which surrounded the earth, the parts which were first solidi- 
fied sunk, and that by a double descending and ascending current the great 
inequality was lessened, which would have taken place in a solid body 
cooling from the surface.” Iam here quoting directly from Poisson, and 
not from Sir Charles’ work, as I wish to show how completely at variance 
Poisson’s opinions are with the laws which govern heated matter passing 
into the solid condition. Nasmyth and Carpenter point out, with special 
reference to the opinions of Poisson and of those who held similar views, 
* that fusible substances are (with a few exceptions) specifically heavier 
whilst in their molten condition than in the solidified state, or, in other 
words, that molten matter occupies less space, weight for weight, than the 
same matter after it has passed from the melted to the solid condition,” 
. and they point to the remarkable facts, amongst others, that cold iron floats 
22N molten iron, cold silver upon molten silver, cold slag upon molten 
