We know very little about this phenomenon. Even the position of the Zodiacal Light along the zodiac which has given rise to its name has been questioned, and it would appear from recent investigations that the glow is situated in the plane of the solar equator. However that may be, the view is very generally held that the glow is due to particles which come from the sun or enter into it. We have already adduced arguments to prove that the mass of solar dust cannot be unimportant; this dust may therefore be the cause of the phenomenon which we have just been discussing. VI END OF THE SUN—ORIGIN OF NEBULÆ We have seen that the sun is dissipating and wasting almost inconceivable amounts of heat every year: 3.8. 10^{33} gramme-calories, corresponding to 2 gramme-calories for each gramme of its mass. We have also obtained an idea as to how the enormous storage of heat energy in the sun may endure this loss for ages. Finally, however, the time must come when the sun will cool down and when it will cover itself with a solid crust, as the earth and the other planets—so far, probably, in a gaseous state—have done long since or will do before long. No living being will be able to watch this extinction of the sun despairingly from one of the wandering planets; for, in spite of all our inventions, all life will long before have ceased on the satellites of the sun for want of heat and light. The further development of the cold sun will recall the actual progress of our earth, except in so far as the sun will have no life-spending, central source of light and heat near it. In the beginning the thin, solid crust will again and again be burst by gases, and streams of lava will rush out from the interior of the sun. After a while these powerful discharges will stop, the lava will freeze, and the fragments will close up more firmly than before. Only on some of the old fissures volcanoes will rise and allow the gases to escape from the interior—water vapor and, to a less extent, carbonic acid, liberated by the cooling.