"My introduction has become much longer than I designed, but now let us turn our attention to the subject of the lesson. "To aid us in understanding God's wise arrangements in the management of heat, we need, first, to consider what heat is and to review the laws of its action. Without this, we could look on and wonder at God's working in nature, but could not explain that which we saw. "Ansel, will you state the theories which have been held touching the nature of heat?" "I will do it as well as I can. The ancient philosophers supposed fire to be one of the four elements of which all bodies were composed. The three other elements were earth, air, and water. These four elements were mingled in various proportions. Of these, fire was esteemed the purest and most ethereal; this constantly tended upward to the empyrean, the highest heaven, where the element of fire and light was supposed to exist unmingled and pure. In the seventeenth century, Beccher and Stahl, two German chemists, brought forward what is known as the _phlogistic hypothesis_. They supposed that every combustible body held in composition a pure, ethereal substance which they called _phlogiston_, a Greek word which signifies _burned_, and that in combustion this phlogiston escaped. Flame was supposed to be this escaping phlogiston. These were the notions held about fire and combustion, but they are hardly worthy to be called theories of heat. The discovery of oxygen by Dr. Priestley of England, in 1774, and the introduction of the balance by Lavoisier of France, joined with the ever-enlarging circle of facts to be explained, rendered the phlogistic hypothesis untenable, and it was thrown aside. "Until a few years since the _caloric_ theory was generally received. According to this theory, heat is a _substance_, a subtle ether, diffused through all bodies and surrounding their atoms. This ether has been supposed to have a strong attraction for the atoms of every other substance, while between its own atoms a strong repulsion exists. In solid bodies each atom of matter, or in compound bodies each cluster of atoms, has been supposed to be surrounded by a little atmosphere, so to speak, of caloric, which prevented the atoms from coming into absolute contact. According to this theory, heat expands bodies by increasing and deepening these minute atmospheres, thus pressing the atoms farther from each other." "You need not explain this theory farther," said Mr. Wilton; "we have hardly time to go into the history of theories. Tell us the latest received theory." "The theory now commonly believed is called the _mechanical_ or _dynamic_ theory. According to this theory, the essence of heat is _motion_. A hot body is one whose atoms are in a state of rapid and intense motion or vibration; and the sensation of heat on touching a hot body arises from the impact, or rapid blows, of the agitated atoms, communicating the same atomic vibration to the flesh and nerves of the hand."