Lord Kelvin's incipient kinetic theory of elasticity is a complicated matter, and I will only briefly enter upon it. But before doing so, I want to remove an objection which is sometimes felt, as to the fluid and easily permeable character of a medium of this great density,--that is to say, as to the absence of friction or viscosity--the absence of resistance to bodies moving through it. As a matter of fact there is no necessary connexion whatever between density and viscosity. 'Density' and 'Viscosity' are entirely different things; and, if there is no fluid friction, a fluid may have any density you please without interposing any obstacle to constant velocity. To _acceleration_ it does indeed oppose an obstacle, but that appears as essentially a part of the inertia or massiveness of the moving body. It contributes to its momentum; and, if the fluid is everywhere present, it is impossible to discriminate between, or to treat separately, that part of the inertia which belongs to the fluid displaced, and that part which belongs to the body moving through it,--except by theory. As for the elasticity of the ether, that is ascertainable at once from the speed at which it transmits waves. That speed--the velocity of light--is accurately known, 3 × 10¹⁰ centimetres per second. And the ratio of the elasticity or rigidity to the density is equal to the square of this speed;--that is to say, the elasticity must be 9 × 10²⁰ times the density; or, in other words, 10³³ C.G.S. units. That is an immediate consequence of the estimate of density and the fact of the velocity of light; and if the density is admitted, the other cannot be contested. But we must go on to ask, To what is this rigidity due? If the ether does not consist of parts, and if it is fluid, how can it possess the rigidity appropriate to a solid, so as to transmit transverse waves? To answer this we must fall back upon Lord Kelvin's kinetic theory of elasticity:--that it must be due to rotational motion--intimate fine-grained motion throughout the whole etherial region--motion not of the nature of locomotion, but circulation in closed curves, returning upon itself,--vortex motion of a kind far more finely grained than any waves of light or any atomic or even electronic structure. Now if the elasticity of any medium is to be thus explained kinetically, it follows, as a necessary consequence, that the speed of this internal motion must be comparable to the speed of wave propagation;--that is to say that the internal squirming circulation, to which every part of the ether is subject, must be carried on with a velocity of the same order of magnitude as the velocity of light.