upon, and also fills the space between those bodies. With this view of the centripetal force of Gravitation, our Philosophy is made to agree definitely with our experience, which teaches us beyond contradiction, that no body moves, unless it is either pushed or pulled by a physical medium. Unless this view of the centripetal force is accepted, we shall have to stumble on in darkness as to the physical cause of the centripetal force, and mentally accept the unphilosophical proposition, that a body can act on another in a way that we cannot understand, and by means which lie outside our experience and observation, and this hypothesis, as Newton and Herschel pointed out, is distinctly an unphilosophical proposition. [Footnote 40: _Modern Views of Electricity_, p. 221.] CHAPTER XII AETHER AND COMETS ART. 111. _Comets. What are Comets?_--In addition to the planets and asteroids which revolve around the sun, there are also other bodies termed Comets, which revolve round the solar orb. Unlike the planets, however, they do not all keep to the plane of the ecliptic, but approach to, and recede from the sun at all angles to that plane, as well as in that plane itself. Comets are supposed to be huge masses of gaseous matter, in a more or less condensed condition. That they are not composed of absolutely solid matter is proved by the fact that it is possible to see the stars through the gaseous matter of which they are composed. How the gaseous matter of which these comets are formed is originated, or how it is formed in solar or stellar space, has, I believe, up to the present never been explained, and indeed, with the idea of a frictionless Aether, I fail to see how any physical explanation of the origin and development of a comet can be satisfactorily given. With the conception of the Aether, however, that is put forward in this work, viz. that Aether is matter in its most rarefied and attenuated form, which can be condensed into a gaseous condition, with such a conception of the universal aetherial medium, the origin and development of gaseous matter from this Aether becomes a physical possibility. Lord Kelvin, in the _Philosophical Magazine_, July 1902, on the "Clustering of Gravitational Matter in any part of the Universe," has already suggested the possibility of the condensation of the Aether, but with the old idea of a frictionless Aether, that is, an Aether which does not possess mass, such a hypothesis is improbable. Because, if the Aether becomes condensed at all, it must be condensed into gaseous and solid matter, and all experiments and observation teach us that both these forms of matter possess mass and weight.