for a moment from this point of view a theory of the atom which, though it is not in very close touch with physical phenomena, has yet the advantage of being so precisely defined that the properties of its atoms can be deduced by purely mathematical principles. The theory to which I allude is that known as the 'Vortex Atom Theory of Matter', which supposes that the Universe consists of an ideal substance known to mathematicians as a perfect fluid. Some portions of this are supposed to be rotating, the rest not: the rotating parts of the fluid on this theory are the atoms. It can be shown that any portion of this fluid which once possesses rotatory motion will never lose it, while if it does not at any instant possess it, it can never acquire it; the atoms on this theory possess at any rate some of the characteristics of real atoms, as they can neither be created nor destroyed. The atoms of one substance on this theory are differentiated from those of another, not merely by the quantity of the rotating liquid, but also by the speed with which it is rotating. The product of the angular velocity of rotation and the area of the cross section of the rotating fluid is called the 'strength' of the atom; it does not change, whatever vicissitudes the atom may experience, and, along with the volume of the rotating fluid, determines the property of the atom. Now let us consider some of the properties of the individual atoms in this theory, remembering that if we took a collection of a large number of them, the properties of the aggregate would be those of ordinary matter. The effective mass of one of these atoms would change when it came into collision with another atom; this is because the rotating portion of the atom has to drag along with itself a considerable volume of the liquid which is not rotating, so that the effective mass of the
