406 Lord Kelvin on the Formation of 
Law (3). There is a certain definite line, fixed in direction, 
through the centre of inertia of the system, such that the 
sum of moments of momentum round it is greater than that 
round any other axis through the centre of inertia ; and the 
moment of momentum round every axis perpendicular to it, 
through the centre of inertia, is zero. 
That maximum moment of momentum is called the resultant 
moment of momentum of the system. Its axis may be called 
the rotational axis of the system. 
§ 20. Consider a vast assemblage of atoms, or of small 
bodies, given at rest at any time, distributed in any manner, 
uniformly or non-uniformly, through any finite volume of 
space. According to § 19, Law (1), the centre of inertia 
of the whole assemblage would remain at rest, and the total 
moment of momentum round every axis through it would 
remain zero, whatever motions the atoms receive in virtue of 
mutual gravitational attractions, and mutual repulsions in 
collision. 
§ 21. Consider now separately some part of the whole 
assemblage, which to avoid circumlocution I shall call part S, 
including all the primitive atoms or particles which at present 
form the Solar System, but not including any other great 
quantity of matter. Part S has, at each instant, a definite 
resultant moment of momentum round a definite axis through 
its centre of inertia ; and its centre of inertia is, at each instant, 
moving with a definite velocity in a definite direction. In a 
vast assemblage such as we were considering in § 20, which 
may be the whole matter of the universe (finite * in quantity 
.as it may with all probability be supposed to be), let there 
be denser parts and less dense parts. In the denser parts, 
there will be gravitational coalition ; in the less dense parts, 
there will consequently be rarefaction. The present existence 
of the Sun is undoubtedly due to gravitational coalition in 
some of the denser parts. The velocity of the centre of 
inertia of the Solar System is due to the gravitational attrac- 
tions of matter outside S, so also is the moment of momentum 
of the Solar System round any axis through its centre of 
inertia. The rarefaction of the distribution of particles, 
large or small, around S, leaves the matter belonging to S 
more and more nearly free from force acting on it from 
without ; and it becomes more and more nearly subject to 
Laws (10 and (2') of § 19. 
§ 22. The approximately constant momentum of the Solar 
System in its motion through space is chiefly the momentum 
of the Sun's motion, because his mass is much greater than 
* See Lord Kelvin's "Baltimore Lectures." Lee. XVI. § 15. 
