HEAT. Until the early part of the 19th century, it was generally believed that heat is a substance devoid of weight (imponderable), and diffused through the mass of bodies. This hypothetical substance was called caloric. Many phenomena seemed to be explained by the as sumption of the existence of caloric, but finally, through the experiments of Davy and Rumford, in which heat was actually created from me chanical energy, the old caloric theory was aban doned. In its place we now have the molecular motion theory. According to this theory heat is nothing but a violent agitation of the mole cules of matter. These molecules are extremely minute, but have a definite size and weight for each definite substance. It has been estimated that a molecule of water has a diameter of about one forty-millionth of an inch. Though mole cules are small in size, their velocity, even at ordinary temperatures, is very great. In air, in which the molecules dart about in straight lines until they encounter other molecules, they at tain a speed of 1,470 feet a second at the freezing temperature. The average length of their path between two encounters — the vaean free path about 1-277,000 inch, and the number of molecules in a cubic inch of air is 443 million million million. Each molecule experiences about 5,030,000,000 collisions a second.
Expansion of Solids, Liquids and Gases. — The molecules of any substance attract one another with a force called cohesion. It is co
hesion that prevents a wire from breaking when it supports a heavy weight. The pressure of the atmosphere also helps to hold 'the molecules of a body together. Opposed to both of these forces is heat. The effect of the agitation of the molecules is to make them jostle one an other apart. Thus it is that in general an in crease of temperature results in expansion. In solids, in which the cohesion is enormous, the expansion for a given increase of temperature is very slight, especially when the test is made at low temperatures. At higher temperatures, when the molecules have somewhat weakened their mutual hold through having moved further apart, an increase of temperature equal to the previous increase generally results in a somewhat greater expansion. To express such ideas technically we employ the expression efficient of linear expansion, which means the fraction of its length that a bar expands when heated 1° Centigrade. As the ' length varies with the temperature, the length at the freezing point, 0° C., is taken as the standard length. Using then this expression, we may say that the coefficient of expansion of a solid gen erally increases with the temperature. The co efficient of linear expansion of a number of sub stances will be found in the following table: