The arguments on which an estimate may be made of the density or massiveness of the ether as compared with that of matter depend on the following considerations, the validity of which again is dependent upon an electrical theory of matter. In this theory, or working hypothesis, an assumption has to be made: but it is one for which there is a large amount of justification, and the reasons for it are given in many books,--among others in my book on _Electrons_, and likewise at the end of the new edition of _Modern Views of Electricity_, also in my _Romanes Lecture_, published by the Clarendon Press in 1903. Put briefly, the assumption is that matter is composed, in some way or other, of electrons; which again must be considered to be essentially peculiarities, or singularities, or definite structures, in the ether itself. Indeed, a consideration of electrons alone is sufficient for the argument, provided it be admitted that they have the mass which experiment shows them to possess, and the size which electrical theory deduces for them: the basis of the idea--which, indeed, is now experimentally proved--being that their inertia is due to their self-induction,--i.e. to the magnetic field with which they must be surrounded as long as they are in motion. The mass, or inertia, of an electron is comparable to the thousandth part of that of the atom of hydrogen. Its linear dimension, let us say its diameter, is comparable to the one-hundred-thousandth part of what is commonly known as molecular or atomic dimension; which itself is the ten-millionth part of a millimetre. Hence, the mass and the bulk of an electron being known, its density is determined, provided we can assume that its mass is all dependent on what is contained within its periphery. But that last assumption is one that quite definitely cannot be made: its mass is for the most part outside itself, and has to be calculated by magnetic considerations. (See Appendix 2.) These details are gone into in my paper in the _Philosophical Magazine_ for April, 1907, and in Chapter XVII of _Modern Views of Electricity_. But without repeating arguments here, it will suffice to say that although the estimates may be made in various ways, differing entirely from each other, yet the resulting differences are only slight; the calculated densities come out all of the same order of magnitude, namely, something comparable to 10¹² C.G.S. units,--that is to say, a million million grammes per cubic centimetre, or, in other words, a thousand tons to the cubic millimetre. But, throughout, we have seen reason to assert that the ether is incompressible; arguments for this are given in _Modern Views of Electricity_, Chapter I. And, indeed, the fundamental medium filling all space, if there be such, _must_, in my judgment, be ultimately incompressible; otherwise it would be composed of parts, and we should have to seek for something still more fundamental to fill the interstices.