changed from a liquid to a gaseous condition occupies a still larger volume in the latter than in its previous condition. The expansion in both cases is essentially the result of the increased repulsive motion that has been imparted to its atoms or molecules by the increased heat, and this increased repulsive power has overcome the attractive power of the atoms or molecules, with the result that they have been driven further and further apart, until, in the gaseous state, the atoms may be very far apart indeed. Wherever, therefore, we have heat of any kind, there we have a repulsive motion, such motion being proportionate to the heat radiated, that is, the aetherial waves propagated by the body. If, therefore, in the atomic world we find a repulsive motion, which is due to the vibratory motions of the Aether generated by heat, the question now confronts us, as to whether in the solar system, and indeed all through the universe, there is not the same repulsive motion from a central body due to the wave motions of the Aether termed Heat. May we not find in the repulsive power of heat in the atomic world, an indication of that very power for which we are seeking in the solar system--that is, a Centrifugal Force or motion which is the exact opposite of the Centripetal Force or attractive power of Gravitation? For if heat be a repulsive motion at all, then to be strictly logical it must be equally repulsive in relation to large masses, the sun and the planets for example, as it is in the atomic world, otherwise we have a phenomenon in Nature which contradicts itself, which assumption would be contrary to the simplicity which is to govern our philosophy, and also contradictory to experience, which is the primary factor of philosophical reasoning. Now what are the facts with reference to the sun, which is the central body of our solar system, and the source of all light and heat in that system? We will look at this aspect of the question under the heading of Radiant Heat. [Footnote 11: _Heat, a Mode of Motion._] ART. 64. _Radiant Heat._--The source of all light and heat, not only of our earth, but also of all the other planets, is to be found in the sun. We have therefore to deal, not with an atom which is generating heat waves on every side, but with a globe about 860,000 miles in diameter, and with a circumference of over 2,700,000 miles. This huge orb consists of a central body, molten or partly solid, with a temperature so hot that it is almost impossible to conceive its intensity. The quantity of heat emitted by the sun has been ascertained by Sir John Herschel from experiments made at the Cape of Good Hope, and by M. Pouillet in Paris.