CONDUCTION OF HEAT

is the propagation of heat from one body to another, with which it is in contact, or from molecule to molecule in a homogeneous substance. Heat is considered to increase the vibration of the molecules of a substance, but no visible motion of the matter takes place. The increased vibration of the warmer molecules must be by impact handed on te the neighboring colder molecules, so that heat flows from the part of higher temperature to that of lower temperature. Gases, liquids, and solids conduct heat. Liquids po e poor conductivity, and this is important for animal and plant life in lakes. Water is cooled from the top. The surface water cools to 4°, and sinks till the whole of the water is at 4°. As it cools still further, it grows lighter and floats, finally freezing, but owing to the poor conductivity the water at the bottom is rarely colder than 4°. It is difficult to estimate the conductivity of liquids, because the proce is interfered with by connection. As a rule, good conductors of electricity are good conductors of heat. The conduction in gases is almost negligible.