Concrete Matter from Atomic Origins. -±01 
mean density somewhat greater than that of water, on the 
supposition of cubes randomly oriented at rest, as defined 
in § 5. 
§ 10. When the density becomes so great that the atoms 
begin to seriously jostle one another, we have the first step 
to the formation of concrete matter from atomic origins. 
Our present knowledge of the properties of matter does not 
suffice to allow us to follow, with definite and complete 
understanding, the progress after this step. We can see a 
prodigious cloud of atoms crowding turbulently around the 
centre. Atoms coming from all directions meet and collide. 
The energy of the relative motions of the atoms is still, let 
us suppose, sufficient to prevent them from ever remaining 
in contact except during very short times when any two of 
them are in collision. And let us suppose the central mean 
density to be still considerably less than "1 of the density of 
water. The whole assemblage now constitutes the gaseous 
fluid mass which forms the subject of Homer Lane's cele- 
brated problem*. As long as the whole mass remains thus 
in gaseous condition, loss of heat to the surrounding ether 
allows mutual gravitation to condense the whole assemblage, 
doing its work by increasing the kinetic energy of the relative 
motions. A result, as found by the mathematical solution of 
Homer Lane's problem, is that all of the assemblage outside 
a certain distance from the centre sinks in temperature, 
while all within that distance rises in temperature. I may 
here explain that the temperature of a " perfect gas ' } means 
the kinetic energy per unit mass of all the relative trans- 
lator v motions of its molecules in free paths from collision to 
collision. 
§ 11. During the whole time of the gaseous stage which 
we have been considering, the crowding becomes denser and 
denser in the central regions, until there every atom comes 
to be always in collision with all its nearest neighbours. 
Throughout all the space around the centre in which this 
condition has been reached the crowd constitutes a fluid of 
the species called liquid. In the outlying parts the crowd 
is much less dense ; each atom, instead of being always in 
collision, is in collision only during comparatively short 
intervals of time ; and, for the rest of its time, is moving in 
approximately straight lines, not perceptibly disturbed by 
* J. Homer Lane, American Journal of Science, July 1870 : A. Putter, 
Wiedemann's A nnalen, 1878 ... 1882 : Prof. A. Schuster, Brit, Assoc. 
Report, 1883 : Sir William Thomson. Phil. Mag. vol xxiii.p.287, 1887 • 
Prof. J. Perrv. 'Nature,' vol. lx. 1899 : T. J. J. See, Astr. Nachr. No. 4053, 
Bd. 169, 1905 : T. J. J. See, Astr. Nachr. No. 4104, Bd. 171, 1906 ; 
Lord Kelvin, Royal Society of Edinburgh, Jan. 21, 1907 : Lord Kelvin, 
< Nature,' Feb. 14, 1907. 
