Scientific research has justified the conception of his first three hypotheses with respect to the universality, elasticity and vibrations of the aetherial medium, but up to the present I am not aware that science has accepted his fourth hypothesis. I propose to show how, from a strictly philosophical and logical standpoint, his fourth hypothesis is just as true as his first three hypotheses, and that it henceforth passes out of the realm of the hypothetical into the realms of fact and science, not only by philosophical reasoning, but by actual experiment made by some of the most advanced scientists of the present time. Let us consider the question first from the standpoint of the Rules of Philosophy. Our first Rule of Philosophy states, that any hypothesis must be simple in connection. Now I put it to any intelligent man, and ask him which is the simpler conception of Aether? To affirm that Aether is matter, and therefore subject to the properties of matter, as elasticity, density, inertia and Gravitation, or to affirm that Aether is matter, but while it is subject to some of the properties of matter, as elasticity, density and inertia, it is not subject to the very property which of all properties is the most fundamental, viz. Gravitation. There can, in my opinion, only be one answer to the question, so that, when we affirm that Aether is matter, we are compelled to affirm, in order to conform to the first Rule of Philosophy, that it is gravitative also. Faraday was also of the opinion that Aether was subject to the Law of Gravity, for, writing in _Experimental Researches_, he states: "The view now stated of the constitution of matter, would seem to involve the conclusion, that matter fills all space, or at least all space to which Gravitation extends, INCLUDING THE SUN AND ITS SYSTEM. For Gravitation is a property of matter, dependable on a certain force, and it is this force which constitutes matter." Let us also test the question by our second Rule of Philosophy, and we shall find greater evidence still for the statement that Aether is gravitative. What do experience and observation teach us with reference to matter? As we have already seen (Art. 37), if there is one truth that they teach us regarding matter, it is that it is gravitative. There is not the slightest evidence throughout the universe, as far as our observation can lead us to form an opinion, that there is any kind of matter which is not subject to the Law of Gravitation. Therefore to assume that Aether is matter, and yet not to assume that it is also subject to Gravitation, is to assume that which is directly opposed to the most fundamental principle of all philosophical teaching and scientific research. If Aether be matter, therefore, and yet is not gravitative, we shall have an anomaly in an otherwise universal law, as we shall have some kind of matter which fails to come within the scope of the universal Law of Gravitation.