atom is the mass of the rotating portion, plus the mass of the liquid thus dragged along with it, and as some of this liquid may be detached from or added to the atom when it comes into collision with another atom, the effective mass of the atom will be changed by the collision. For the same reason, the effective mass of the atom changes with its velocity—the greater the velocity the smaller being the mass; so much is this the case that we have the paradoxical result that the momentum of the atom decreases as its velocity increases, and that the more slowly the atom moves the greater is the kinetic energy. Again, if all the atoms were made of vortices of the same 'strength', we should find that certain mechanical quantities would all be integral multiples of a definite unit, i.e. these dynamical quantities, though not matter, would yet resemble matter in having an atomic constitution, being built up of separate indivisible units. The quantity known as 'circulation' would have this property; it would always be an integral multiple of a definite unit, and would thus change by abrupt steps, and not continuously. When a particle describes a circular orbit the 'circulation' is proportional to its moment of momentum, and we see, that in a theory of this kind the moment of momentum of particles describing circular orbits would always be an integral multiple of a definite unit. We see from this example that when we have a structure as fine as that associated with atoms, we may find dynamical quantities such as moment of momentum, or it may be kinetic energy, assuming the atomic quality and increasing or decreasing discontinuously by finite jumps. In one form of a theory which has rendered great service to physical science—I mean Planck's theory of the 'quantum'—the changes from radiant to kinetic energy are supposed to occur
