But the suggestion is perhaps more plausible if we look at the inverse relation, viz.\ $M$~as a function of~$\lambda$. If we can imagine the gradual destruction of matter in the world (e.g.\ by coalescence of positive and negative electrons), we see by~\Eq{(71.1)} that the radius of space gradually contracts; but it is not clear what is the fixed standard of length by which $R$~is supposed to be measured. The natural standard of length in a theoretical discussion is the radius $R$ itself. Choosing it as unit, we have $M = \frac{1}{2}\pi$, whatever the number of elementary particles in the world. Thus with this unit the mass of a particle must be inversely proportional to the number of particles. Now the gravitational mass is the radius of a sphere which has some intimate relation to the structure of the particle; and we must conclude that as the destruction of particles proceeds, this sphere must swell up as though some pressure were being relaxed. We might try to represent this pressure by the gravitational flux (\SecRef{63}) which proceeds from every particle; but I doubt whether that leads to a satisfactory solution. However that may be, the idea that the particles each endeavour to monopolise all space, and restrain one another by a mutual pressure, seems to be the simplest interpretation of~\Eq{(71.1)} if it is to be accepted. We do not know whether the actual (or electrical) radius of the particle would swell in the same proportion---by a rough guess I should anticipate that it would depend on the square root of the above ratio. But this radius, on which the scale of ordinary material standards depends, has nothing to do with equation~\Eq{(71.1)}; and if we suppose that it remains constant, the argument of \SecRef{66} need not be affected.