When we speak of the density of _matter_, we are really though not consciously expressing the group-density of the modified ether which constitutes matter,--not estimated per unit, but per aggregate; just as we might estimate the group or average density of a cloud or mist. Reckoned per unit, a cloud has the density of water; reckoned per aggregate, it is an impalpable filmy structure of hardly any density at all. So it is with a cobweb, so perhaps it is with a comet's tail, so also with the Milky Way, with the cosmos,--and, as it now turns out, with ordinary matter itself. For consider the average density of the material cosmos. It comes out almost incredibly small. In other words, the amount of matter in space, compared with the volume of space it occupies, is almost infinitesimal. Lord Kelvin argues that ultimately it must be really infinitesimal (_Philosophical Magazine_, Aug., 1901, and Jan., 1902), that is to say that the volume of space is infinitely greater than the total bulk of matter which it contains. Otherwise the combined force of gravity--or at least the aggregate gravitational potential--on which the velocity generated in material bodies ultimately depends, would be far greater than observation shows it to be. The whole visible universe, within a parallax of 1/1000 second of arc, is estimated by Lord Kelvin as the equivalent of a thousand million of our suns; and this amount of matter, distributed as it is, would have an average density of 1·6 × 10⁻²³ grammes per c.c. It is noteworthy how exceedingly small is this average or aggregate density of matter in the visible region of space. The estimated density of 10⁻²³ c.g.s. means that the visible cosmos is as much rarer than a "vacuum" of a hundred millionths of an atmosphere, as that vacuum is itself rarer than lead. It is because we have reason to assert that any ordinary mass of matter consists, like the cosmos, of separated particles, with great intervening distances in proportion to their size, that we are able to maintain that the aggregate density of ordinary stuff, such as water or lead, is very small compared with the continuous medium in which they exist, and of which all particles are supposed to be really composed. So that lead is to the ether, as regards density, very much as the "vacuum" above spoken of is to lead. The fundamental medium itself must be of uniform density everywhere, whether materialised or free. CHAPTER VII FURTHER EXPLANATIONS CONCERNING THE DENSITY AND ENERGY OF THE ETHER