HEAT, in natural philosophy, the term used chiefly to mean, not the sen sation which our bodies feel when we say that they are hot, but the particular state or condition of matter which causes this sensation. The accepted hypothesis is that heat is caused by an oscillatory or vibratory motion of the particles of a body. It is thus a condi tion of matter and not a substance. The hottest bodies are those in which the vibrations move quickest through the widest space. It is called also the Mechanical or Dynamical Theory of Heat.
Heat makes bodies, whether solid, liquid, or gaseous, expand, while cold contracts them. Water is a partial ex ception to the rule. In the case of a solid, heat can produce fusion at a cer tain definite temperature; in that of liquids vaporization. It is transmitted by radiation or by conduction. Radiant heat is that produced by radiation. Latent heat is that which is absorbed by solid bodies when they are subjected to calorific influence far more than suffi cient to make them melt, and when at the very time they are in process of fusion.
The heat does not raise the temperature of the solid until it is completely lique fied. There is also a latent heat of vapor ization, being heat absorbed by liquids when being converted into vapor. Latent is opposed to sensible heat. Heat may be reflected or refracted, or, by being regd., arly reflected in all directions, i may become scattered or diffused heat reflection and refraction may also polar ize its rays, as happens to those of sole' light. The heat which falls on a bod3 is called incident heat. Specific heal is the quantity of heat required to rais( the temperature of a body of a giver weight 1' ; the unit of measure being the quantity required to raise the sam( weight of water to the same tempera ture. Heat may be produced by solar radiation, chemical action, friction, Ares sure, percussion, absorption, and imbibi tion; by the conduction of powerfu magnets and bodies in motion, etc.