To begin at the beginning, we must go back to the time when the sun was only a great gaseous nebula filling all the space included in the orbit of Neptune. This nebula was not in itself hot, but as it rotated it contracted. Now, heat is really only a form of energy, and energy and heat can be interchanged easily. This is a very startling thing when heard for the first time, but it is known as surely as we know anything and has been proved again and again. When a savage wants to make a fire he turns a piece of hard wood very very quickly between his palms--twiddles it, we should say expressively--into a hole in another piece of wood, until a spark bursts out. What is the spark? It is the energy of the savage's work turned to heat. When a horse strikes his iron-shod hoofs hard on the pavement you see sparks fly; that is caused by the energy of the horse's leg. When you pump hard at your bicycle you feel your pump getting quite hot, for part of the energy you are putting into your work is transformed into heat; and so on in numberless instances. No energetic action of any kind in this world takes place without some of the energy being turned into heat, though in many instances the amount is so small as to be unnoticeable. Nothing falls to the ground without some heat being generated. Now, when this great nebula first began its remarkable career, by the action of gravity all the particles in it were drawn toward the centre; little by little they fell in, and the nebula became smaller. We are not now concerned with the origin of the planets--we leave that aside; we are only contemplating the part of the nebula which remained to become the sun. Now these particles being drawn inward each generated some heat, so as the nebula contracted its temperature rose. Throughout the ages, over the space of millions and millions of miles, it contracted and grew hotter. It still remained gaseous, but at last it got to an immense temperature, and is the sun as we know it. What then keeps it shining? It is still contracting, but slowly, so slowly that it is quite imperceptible to our finest instruments. It has been calculated that if it contracts two hundred and fifty feet in diameter in a year, the energy thus gained and turned into heat is quite sufficient to account for its whole yearly output. This is indeed marvellous. In comparison with the sun's size two hundred and fifty feet is nothing. It would take nine thousand years at this rate before any diminution could be noticed by our finest instruments! Here is a source of heat which can continue for countless ages without exhaustion. Thus to all intents and purposes we may say the sun's shining is inexhaustible. Yet we must follow out the train of reasoning, and see what will happen in the end, in eras and eras of time, if nothing intervenes. Well, some gaseous bodies are far finer and more tenuous than others, and when a gaseous body contracts it