400 Lord Kelvin on the Formation of 
continue to fall to reach the centre, after it has attained the 
densities indicated : — 
Time before reaching the M Dengi 
centre . J 
47-7 minutes 0*1 
21-3 „ 0-5 
15-1 „ 1-0 
6-3 „ 5-67 
If the beginning is an assemblage of cubes oriented at 
random, as in § 5, and with their centres in cubic order, as in 
§ 2, contacts would commence at a time about 8 minutes 
earlier than the time of coming to fit exactly on the supposi- 
tion of uniform orientation. 
§ 7. In the case of cubes initially oriented at random, 
collisions will not begin simultaneously as in § 4; but will begin 
with a crushing to powder at colliding edges or corners. 
The stupendous system of collisions which follows the com- 
mencement, would, if the material of the cubes is of any 
known substance, metallic or rocky, cause, in the course of a 
few minutes, melting of the whole mass : unless the pro- 
digious pressure in the central parts should have the effect of 
preventing fluidity in those parts, which does not seem probable. 
§ 8. The same general description is applicable to the 
ideal case of a vast number of large and small fragments of 
any shapes, instead of our equal cubic metres of homogeneous 
matter ; provided only that the initial distribution through the 
great spherical space is of uniform average density all through. 
§ 9. Let us now, instead of masses large or small of 
concrete matter, begin with a vast number of atoms ; or of 
atoms, and doublets such as 2 , N 2 , H 2 , given at rest dis- 
tributed uniformly in respect to average density through a 
sphere of a thousand million times the Earth's bulk : and 
having the sum of their masses equal to the Earth's mass. 
Every particle (atom or doublet) will have the same centre- 
ward velocity at the same time, as that found for the ideal 
cubes in § 6 ; until some of the atoms or doublets get into 
touch with neighbours, that is, come so near one another that 
mutual molecular forces become effective. This must be the 
case when the mean density is considerably less than one- 
tenth of the density of water, as we see by considering the 
known properties of gases and vapours, and of liquids of 
small density. Hence the time during which the atoms 
will continue to fall freely without jostling one another must 
be a few minutes less than the time of getting into touch at 
* Calculated by means of a formula given on page 538 of Lord 
Kelvin's u Baltimore Lectures," Appendix J). 
