"This, like your former question, is altogether beyond our knowledge. All that we certainly know is that God made it thus. The process of creation, if indeed God did not create the earth by a word, without a process, is a matter of sublimest and most venturesome conjecture. According to the opinion of some, the elements of which the earth is composed were created separate and uncombined, and were suffered afterward to unite by their chemical affinities. This chemical combination would be nothing else than a tremendous conflagration, and the result would be the most intense heat of which we can form a conception. Others have dreamed of a 'fire-mist' created of God and by some means condensed into worlds. The temperature of the earth is maintained, so far as we know, only by the poor conducting quality of the enveloping crust preventing its cooling. At the present rate of radiation, millions of years would be required to render the change of temperature perceptible. "What is the third great natural source of heat? I will ask Mr. Hume." "Mechanical action, or force transmuted to heat." "Will you please explain this?" "Strictly speaking," said Mr. Hume, "this is not to be counted an original source of heat. But heat is used in the production of winds and waves, the flow of rivers, and all the ceaseless activities of the world, and this force reappears from time to time transmuted again to heat. Whenever in the friction of air and of water, in the dashing of matter against matter and force against force, motion and force seem to be lost, heat is produced. The water of the sea after long storms is said to be sensibly warmed. We can appreciate the amount of heat generated in this manner only by considering in how many thousand ways force is meeting force and motion is destroyed. All this lost motion--lost as sensible motion--reappears as atomic motion, that is, as heat. Such heat has been applied to artificial uses. Heat generated by the friction of iron plates ground together has been used for heating buildings." "And this transmutation of living force and heat," added Mr. Wilton, "is but one of many illustrations of God's economy in the management of heat. Nothing is wasted. The voices of Nature all echo the words of Jesus: 'Gather up the fragments, that nothing be lost.' "The fourth source of heat is chemical action. What is the chief form of this which is used for the production of heat? Samuel may tell us." "Combustion, I think, sir."