CHAPTER XX. INORGANIC NATURE. Our world is a planet attendant on a star, and is a theatre of physical activities. It possesses a chemical composition which it shares with the surrounding universe, while its crust is composed of materials the arrangement of each part of which is intimately related to antecedent material conditions. The earth as a cosmical body-Physical activities-Chemical substances A CERTAIN general knowledge of the conditions, powers, and properties of the world about us has now become the common property of all moderately educated people, and it is not our intention to do more here than remind the reader of certain facts included in such general knowledge. So much, however, it seems requisite to do in order that there may be the needful data ready to hand for reference, when we come to consider certain scientific problems which will be spoken of in the last section. The earth, as every one now knows, is a sphere, revolving on its axis daily and accomplishing an annual revolution, and it is a planet, ie. one of those spheres revolving round the sun, which, together with their satellites, certain comets and clouds of meteoroids, constitute our planetary or solar system-itself one of the many systems of suns (with or without attendant planets) which make up the visible stellar universe. The various bodies of this universe, The earth which vary immensely as to size, are continually changing their relative positions according to the laws of mechanical as a cosmi motion together with the force of gravity, the result being that the members of such planetary systems as ours, revolve round their suns, or central bodies, in ellipses, variously attended by satellites revolving in turn, in ellipses, around their respective planets. In some distant systems. there may be more than one sun. Our solar system itself is rushing at the rate of ten thousand miles every halfhour in the direction of the star 7, in that apparent group of stellar bodies which is named the constellation Hercules. The known universe, or cosmos, is made up of bodies. variously composed of solid, fluid, and gaseous matter, which bodies differ greatly in density, some of them being much more dense than our earth, while others are composed of nothing but gases and vapours of great tenuity— those stellar aggregations of matter known as "nebulæ." The cosmical bodies shine by self-emitted light, as does our sun and the variously distant stars; or by reflected light, as do the planets and satellites of our solar system; and doubtless multitudes of planets of other systems, though some planetary bodies themselves are more or less self-luminous. Light travels at enormous speed (over 186,300 miles in a second) through whatever intervenes between, and connects together all the planetary and stellar bodies. A universally diffused highly elastic substance of extreme tenuity, termed "ether," is commonly said to be thus interposed, and we are taught by physicists. that minute waves of a certain kind traversing this ethereal medium constitute light-waves of different lengths giving rise to our perceptions of different colours.* According to this view, wherever light can travel there must be ether; and it can travel through every known interval of every other substance, even including the most perfect so-called vacuum which we can make. It follows, therefore, that we have no evidence of the existence of a real vacuum anywhere, but rather that there is and can be none. The distance from us of the stellar bodies, the so-called "fixed stars," being enormous-that of the nearest star being about 200,000 times more distant from us than the sun-the time which light must take in passing from * See above, p. 99. them to our eyes is to be measured by years-three years from the nearest star. Even the light of the sun takes more than eight minutes to cross the ninety-two thousand seven hundred millions of miles of ether which are interposed between it and us. The surface and atmosphere of the sun is a region of intense heat and activity. Amidst flames of hydrogen thousands of miles high, metallic vapours are continually ascending, to be condensed and then fall down in showers of red-hot metal. The sun is 852,900 miles in diameter, and is 1,252,700 times the volume of the earth; while the largest of the planets, Jupiter, is but 1233 times the earth's volume. The planets of our system, with their satellites, move round the sun in one direction; but there is an exception in the case of the satellites of Uranus, which move in a retrograde direction, and in planes nearly perpendicular to the ecliptic. The planets all revolve on their axes during their revolution round the sun, and generally their attendant satellites revolve round the planets they attend more slowly than such planets revolve on their own axes. An exception, however, occurs in the case of the planet Mars, one of the satellites of which circulates round it in less than onethird of the time that planet takes to revolve on its own axis. The descent of meteorites upon the surface of this Physical planet give us the plainest proof that the same chemical activities. substances exist in the solar system external to this earth as exist in the earth itself. But that careful dissection of light which is known as "spectrum analysis" shows us that a similar identity of materials exists between the substances which compose our earth, and those which enter into the composition of even the most distant stellar bodies yet discovered. Thus the action of gravity and the energies known as light, heat, mechanical motion, and chemical action, as also, doubtless, those activities spoken of as electric and magnetic, seem to be diffused throughout the visible. universe. These physical activities are spoken of as the "physical forces," and brilliant modern discoveries have shown that there is a quantitative equivalence between the different successive activities of the same or of different |