12 Lord Kelvin on the 



its constituents in any state of relative motion, no atom will 

 ily away from it, provided the total kinetic energy of the 

 given initial motion does not exceed some definite limit. A 

 gas is a vast assemblage of molecules thus defined, each 

 moving freely through space, except when in collision with 

 another cluster, and each retaining all its own constituents 

 unaltered, or only altered by interchange of similar atoms 

 -between two clusters in collision. 



§ 20. For simplicity we may suppose that each atom, A, 

 has a definite radius of activity, a, and that atoms of different 

 kinds, A, A', have different radii of activity, a, a! '; such that 

 A exercises no force on any other atom, A', A", when the 

 distance between their centres is greater than a + a! or a -f a!' . 

 We need not perplex our minds with the inconceivable idea 

 of " virtue," whether for force or for inertia, residing in a 

 mathematical point* the centre of the atom; and without 

 mental strain we can distinctly believe that the substance 

 (the 6i substratum " of qualities) resides, not in a point, nor 

 vaguely through all space, but definitely in the spherical 

 volume of space bounded by the spherical surface whose 

 radius is the radius of activity of the atom, and whose centre 

 is the centre of the atom. In our intermolecular forces thus 

 ■defined, we have no violation of the old scholastic law, 

 "Matter cannot act where it is not," but we explicitly violate 

 ihe other scholastic law, " Two portions of matter cannot 

 simultaneously occupy the same space. " We leave to gravi- 

 tation, and possibly to electricity (probably not to magnetism) , 

 the at present very unpopular idea of action at a distance. 



• § 21. We need not now (as in § 16, when we wished to 

 keep as near as we could to the old idea of colliding elastic 

 globes) suppose the mutual force to become infinite repulsion 

 before the centres of two atoms, approaching one another, 

 meet. Following Boscovich, we may assume the force to 

 vary according to any law of alternate attraction and repul- 

 sion, but without supposing any infinitely great force, whether 

 ■of repulsion or attraction, at any particular distance; but we 

 must assume the force to be zero when the centres are coin- 

 cident. We may even admit the idea of the centres being 

 absolutely coincident, in at all events some cases of a chemical 

 combination of two or more atoms; although we might con- 

 sider it more probable that in most cases the chemical com- 

 bination is a cluster, in which the volumes of the constituent 

 atoms overlap without any two centres absolutely coinciding. 

 § 22. The word " collision" used without definition in § 19 

 may now, in virtue of §§ 20, 21, be unambiguously defined 



* See Math, and Phys. Papers, vol. iii. art. xcvii. "Molecular Consti- 

 tution of Matter," § 14. 



