Our astronomers tell us that the solar heat is gradually diminishing. In time the sun will cease to shine, as the earth did long since. Newcomb says that in 5,000,000 years, at the present rate, the sun will have shrunk to half its present size, and that it cannot sustain life on the earth more than 10,000,000 years longer. Of this we may be assured, there is enough heat to support life on our globe for millions of years to come. The sun consists of a central orb, liquid or solid, of exceeding brightness, which of itself would give a continuous spectrum, or in other words which emits all kinds of light. The sunlight is decomposed by means of the spectroscope, already alluded to, in order to discover the kind of elements it is composed of. Therefore Tyndall says: "I think we now possess knowledge sufficient to raise us to the level of one of the most remarkable generalizations of our age. It has long been supposed that the sun and planets have had a common origin and that hence the same substances are more or less common to them all. Can we detect the presence of any of our terrestrial substances in the sun?... I have said that the bright bands of a metal are characteristic of the metal; that we can without seeing the metal declare its name from the inspection of the bands. The bands are, so to speak, the voice of the metal declaring its presence. "Professor Kirchhoff finds iron, calcium, magnesium, sodium, chromium, etc., in the sunlight spectrum. We know also the total amount of solar heat received by the earth in a year, and we can calculate the entire quantity of heat emitted by the sun in a year. Conceive a hollow sphere to surround the sun, its center the sun's center, and its surface at the distance of the earth from the sun. The section of the earth cut by this surface is to the whole area of the hollow sphere as 1 to 2,300,000,000; hence the quantity of solar heat intercepted by the earth is only 1/2300000000 of the total radiation.