CHAPTER XXI.

MATTER AND ETHER THEORIES.
Matter, like space and time, cannot be defined, but either the statement that matter is whatever occupies space or the statement that it is anything which can be seen, touched, or weighed, suggests its more important characteristics to anyone already familiar with it.
The means of measuring matter and some of its properties are treated in most text-books on mechanics, and I do not propose to discuss them. I confine the chapter to an account of some of the hypotheses formerly held by physicists as to the ultimate constitution of matter, but I exclude metaphysical conjectures which, from their nature, are incapable of proof and are not subject to mathematical analysis. The question is intimately associated with the explanation of the phenomena of attraction, light, chemistry, electricity, and other branches of physics.
I commence with a list of some of the more plausible of the hypotheses formerly proposed which accounted for the obvious properties of matter, and shall then discuss how far they explain or are consistent with other facts*. The interest of the list is
* For the earlier investigations I have based my account mainly on Itecent Advances in Physical Science, by P. G. Tait, Edinburgh, 1876 (chaps, xii, xiii); and on the article Atom by J. Clerk Maxwell in the Encyclopaedia Britannica or his Collected Works, vol. ii, pp. 445 — 484. For the more recent speculations see J. J. Thomson, Electricity and Matter, Westminster, 1904 ; J. Larmor, Aether and Matter, Cambridge, 1900 ; and E, T. Whittaker, IlistM'y of the 'Theories of Aether and Electricity, Dublin, 1910.
460 MATTER AND ETHER THEORIES [CH. XXI j
largely historical, for within the last few years new views as to the constitution of matter have been propounded, which cannot be discussed satisfactorily in a book like this.