The vortex theory appears to me to be beset by a difficulty of another kind, viz. that of reconciling it with the First Law of Motion. According to that law no body possessing inertia can deviate from the straight line unless forced to do so. A planet will not move round the sun unless it be constantly acted upon by a force deflecting it from the straight path. A grindstone will not rotate on its axis unless its particles are held together by a force preventing them from flying off at a tangent to the curve in which they are moving. Centrifugal force must always be balanced by centripetal force. The difficulty is to understand what force counterbalances the centrifugal force of the rotating material of the vortex-atom. It is not because the centrifugal tendency of the rotating material is controlled by the exterior incompressible fluid, for it offers no resistance whatever to the passage of the atom through it—in short, in so far as the motion of the atom is concerned, this fluid is a perfect void. Now, if this fluid can offer no resistance to the passage of the atom as a whole, how then does it manage to offer such enormous resistance to the materials composing the atom, so as to continually deflect them from the straight path and compel them to move in a curve? The centrifugal force of these vortex-atoms must be enormous, for on it is assumed to depend the hardness or resistance of matter to pressure. Now the centripetal force which balances this centrifugal force must be equally enormous. If, then, this perfect fluid outside the vortex-atom can exert this enormous force on the revolving material without being itself possessed of vortex-motion, there does not seem to be any necessity for vortex-motion in order to produce resistance. In short, how is the existence of the atom possible under the physical conditions assumed in the theory? How this may be, like the space of four dimensions, may be expressed in mathematical language, but like it, I fear, it is unthinkable as a physical conception. _Mr. William Crookes on the pre-nebular condition of matter._—In his opening address before the Chemical Section of the British Association in 1886, Mr. William Crookes entered at considerable length into the question of the genesis and evolution of the chemical elements. I shall here give a brief statement of his views as embodied in his important address, and this I shall endeavour to do as nearly as possible in Mr. Crookes’s own words.