Heat, a form of molecular motion. According to the theory of heat, every molecule in a substance is moving to and fro. The hotter a substance, the more violently the molecules bound back and forth. If they begin to subside and swing more quietly in their places, we say that the substance is cooling. The change from a solid to a liquid or to a gaseous condition is thought to be due to the violent jostling of molecules, whereby they get beyond each other's attractive influence. An increase of heat is accompanied by a closely corresponding increase of volume. This increase of volume varies for different substances. Lead and zinc, for instance, increase and shrink more rapidly than iron. The amount of heat required to make, let us say, a pound of a substance too hot to handle varies greatly. Marble and aluminum require the application of seven times as much heat as lead and gold to bring them to the same temperature. Tin heats up ten times as fast as ice, and alcohol nearly twice as fast as water. A proper degree of heat is one of the necessary conditions of all life. So far as the earth is concerned, all heat is derived from the sun. Even the heat obtained from fuel and chemical action comes originally from the sun. Winter clothing is designed to prevent heat from leaving the body. Some substances conduct heat more readily than others. Wool is a poor conductor. Cotton is better. Wool feels warm to the body because it does not carry away heat--really because it remains cold. The heat of the body requires to be maintained by eating food containing fat or some form of carbon. In attempts to investigate heat, scientists have ascertained with reasonable certainty that in freezing air the distance to which a molecule can dart before bumping into another is so short that a quarter of a million such trips, end to end, would make only an inch in length; but that a molecule darts backward and forward often enough to travel over a quarter of a mile per second. The further statement is made that each molecule of air, that is at freezing, is run into at a rate of 5,000,000,000 collisions per second. In the case of heated air, these figures must be increased. According to this theory the amount of heat in a body is the amount of molecular motion. The more a body cools, the less its molecular motion, until at 273 Centigrade degrees below freezing, all molecular motion ceases. This temperature is written-273 deg. C., and is called the absolute zero. This theoretical temperature has never been reached, but scientists claim to have come within fourteen centigrade degrees of it. Prof. Dewar has succeeded in reaching a temperature of-443 deg.F. The highest temperature yet attained is thought to be that of the electric arc, or 3,500 deg. C. Heat may be changed into light and electricity. It is in turn produced by them. Our great source of heat, light, and electricity is the sun. Whether they travel to us separately or whether they are three manifestations of the same energy is not yet understood, but is under investigation. It is believed that solar heat will one day do much of the work now done by coal, wind, and water. Solar mills, constructed somewhat on the windmill plan, but consisting of huge tin mirrors to catch heat, are already in use to drive machinery. Ericsson states that one-half square mile of Arizona sun has energy enough to drive 64,800 steam engines of 100 horse power each. When a substance burns--combines with oxygen--heat is evolved; when iron rusts, heat is evolved; but the amount is too slight for measurement. When a combustible substance burns, it generates a definite amount of heat. Two pounds of petroleum, conditions remaining the same, produce twice as much heat as one pound. A pound of burning petroleum, however, produces more heat than a pound of coal. The unit of measure now in common use is the British Thermal Unit or B. T. U., which may be defined as the quantity of heat required to raise the temperature of one pound of pure water one degree Fahrenheit, at or near its point of greatest density. This point of greatest density is approximately 3.91 deg. F. We can measure the amount of heat produced by means of an instrument called a calorimeter. To secure such a measurement, a definite amount of material is burned in the calorimeter in such a way as to raise the temperature of a surrounding quantity of water. The change in temperature of the water can be accurately determined by means of a very delicate thermometer. By repeated trial with the calorimeter, the amount of heat yielded per pound of fuel has been approximated. The heat for several kinds of fuel is per pound: Fuel. B. T. U. Carbon . . . . . . . . 14,600 Hard coal . . . . . 14,900 Soft coal . . . . . 14,000 Lignite . . . . . . . 12,000 Dry peat . . . . . . 10,000 Oak wood . . . . . . 5,000 Long leaf pine. 9,000 Dry tan bark . . 6,000 Dry straw . . . . . 6,000 Petroleum . . . . . 20,000 Natural gas . . . 30,000 Hydrogen . . . . . . 62,000 See CLOTHING; FOOD; THERMOMETER; EXPANSION.