"Our idea of the structure of the solar system was at first only a theory. The astronomer does not see the planets revolving in regular circles through the heavens and moving around the sun. He only sees the shining points moving back and forth upon the concave vault, doubling and crossing their tracks apparently in the greatest disorder. How shall their motions be explained? Astronomers have found that the motions of planets revolving around a central sun, when seen from one of the planets, must present just these apparent irregularities. This explanation is so full and complete that it is now counted not a theory, but an established fact. The same may be said of the shape of the earth. "The dynamic theory of heat explains the phenomena of heat better than any other explanation that has been proposed. It explains the radiation of heat from the sun or from any other hot body: vibrations or impulses are propagated through that ether which is supposed to fill all space. It explains the conduction of heat through solid bodies in the same manner. It explains the expansion of bodies: the atomic motion forces the atoms of bodies farther apart. It explains the production of heat by friction or collision, which no other theory is able to do: the shock of the collision generates this atomic vibration. It explains the production of heat by combustion: the atoms of oxygen and carbon or hydrogen dash against each other and generate heat by the collision. This theory explains the transmutation of motion, or living force, and electricity, into heat, and the transmutation of heat into electric or mechanical force. These points will come up again, and I now only refer to them in answering Samuel's question. The dynamic theory explains the phenomena of heat and its relations to force, light, and electricity exceedingly well, and for this reason men look upon it with favor and count it as probably true. If in the progress of scientific investigation it shall be found to explain all the new facts discovered and meet well all the demands made upon it, it will at length be received as an admitted principle in physical science. The _wave_ theory of light and the _vibratory_ theory of sound may be looked upon as thus established. "At our next lesson we shall take a rapid review of the effects and laws of heat." CHAPTER IV. HEAT A GIFT OF GOD. The class is again promptly in place and ready for work. "As I announced a week ago," said Mr. Wilton, "we will to-day take a rapid review of the effects and laws of heat. Will you tell us, Peter, the first and chief of these effects?" "Yes, sir: combustion." "What is combustion?" "Commonly the rapid union of oxygen with some combustible substance, attended with the evolution of heat." "Was your answer correct, then?" "No, sir," said Peter, blushing; "I spoke before I thought." "Will you correct your answer?" "The first and chief effect of heat is expansion."