THE UNIFORMITY OF MATTER AND ITS LAWS THROUGHOUT THE STELLAR UNIVERSE I have shown in the second chapter of this work that none of the previous writers on the question of the habitability of the other planets have really dealt with the subject in any adequate manner, since not only do they appear to be quite unaware of the delicate balance of conditions which alone renders organic life possible on any planet, but they have altogether omitted any reference to the fact that not only must the conditions be such as to render life possible _now_, but these conditions must have persisted during the long geological epochs needed for the slow development of life from its most rudimentary forms. It will therefore be necessary to enter into some details both as to the physical and chemical essentials for a continuous development of organic life, and also into the combination of mechanical and physical conditions which are required on any planet to render such life possible. THE UNIFORMITY OF MATTER One of the most important and far-reaching of the discoveries due to the spectroscope is that of the wonderful identity of the elements and material compounds in earth and sun, stars and nebulæ, and also of the identity of the physical and chemical laws that determine the states and forms assumed by matter. More than half the total number of the known elements have been already detected in the sun, including all those which compose the bulk of the earth's solid material, with the one exception of oxygen. This is a very large proportion when we consider the very peculiar conditions which enable us to detect them. For we can only recognise an element in the sun when it exists at its surface in an incandescent state, and also above its surface in the form of a somewhat cooler gas. Many of the elements may rarely or never be brought to the surface of so vast a body, or if they do sometimes appear there, it may not be in sufficient quantity or in sufficient purity to produce any bands in the spectroscope, while the cooler gas or vapour may either not be present, or be so dispersed as not to produce sufficient absorption to render its spectral lines visible. Again, it is believed that many elements are dissociated by the intense heat of the sun, and may not be recognisable by us, or they may only exist at its surface in a compound form unknown on the earth; and in some such way those lines of the solar spectrum which remain still unrecognised may have been produced. One of these unknown lines was that of Helium, a gas found soon afterwards in the rare mineral 'Cleveite,' and since detected frequently in many stars. Some of the stars have spectra very closely resembling that of the sun. The dark lines are almost as numerous, and most of them correspond accurately with solar lines, so that we cannot doubt their having almost exactly the same chemical constitution, and being also in the same condition as regards heat and stage of development.