Supposing, now, that we adopt provisionally the vortex theory,--the great power of which is well shown by the consideration just mentioned,--we must not forget that it is absolutely essential to the indestructibleness of the material atom that the universal fluid in which it has an existence as a vortex-ring should be entirely destitute of friction. Once admit even the most infinitesimal amount of friction, while retaining the conception of vortex-motion in a universal fluid, and the whole case is so far altered that the material atom can no longer be regarded as absolutely indestructible, but only as indefinitely enduring. It may have been generated, in bygone eternity, by a natural process of evolution, and in future eternity may come to an end. Relatively to our powers of comprehension the practical difference is perhaps not great. Scientifically speaking, Helmholtz and Thomson are as well entitled to reason upon the assumption of a perfectly frictionless fluid as geometers in general are entitled to assume perfect lines without breadth and perfect surfaces without thickness. Perfect lines and surfaces do not exist within the region of our experience; yet the conclusions of geometry are none the less true ideally, though in any particular concrete instance they are only approximately realized. Just so with the conception of a frictionless fluid. So far as experience goes, such a thing has no more real existence than a line without breadth; and hence an atomic theory based upon such an assumption may be as true ideally as any of the theorems of Euclid, but it can give only an approximatively true account of the actual universe. These considerations do not at all affect the scientific value of the theory; but they will modify the tenour of such transcendental inferences as may be drawn from it regarding, the probable origin and destiny of the universe.