3. _Heat._ When a body which occasions the sensation of heat on our organs is brought into contact with another body which has no such effect, the hot body contracts and loses to a certain extent its power of communicating heat; and the other body expands. Different solids and fluids expand very differently when heated, and the expansive power of liquids, in general, is greater than that of solids. It is evident that the density of bodies must be diminished by expansion; and in the case of fluids and gases, the parts of which are mobile, many important phenomena depend upon this circumstance. For instance, if heat be applied to fluids and gases, the heated parts change their places and rise, and the currents in the ocean and atmosphere are due principally to this movement. There are very few exceptions to the law of the expansion of bodies at the time they become capable of communicating the sensation of heat, and these exceptions seem to depend upon some chemical change in the constitution of bodies, or on their crystalline arrangements. The power which bodies possess of communicating or receiving heat is known as _temperature_, and the temparature of a body is said to be high or low with respect to another in proportion as it occasions an expansion or contraction of its parts. When equal volumes of different bodies of different temperatures are suffered to remain in contact till they acquire the same temperature, it is found that this temperature is not a mean one, as it would be in the case of equal volumes of the same body. Thus if a pint of quicksilver at 100 deg. be mixed with a pint of water at 50 deg., the resulting temperature is not 75 deg., but 70 deg.; the mercury has lost thirty degrees, whereas the water has only gained twenty degrees. This difference is said to depend on the different _capacities_ of bodies for heat. Not only do different bodies vary in their capacity for heat, but they likewise acquire heat with very different degrees of celerity. This last difference depends on the different power of bodies for _conducting_ heat, and it will be found that as a rule the densest bodies, with the least capacity for heat, are the best conductors. Heat, or the power of repulsion, may be considered as the _antagonist_ power to the attraction of cohesion. Thus solids by a certain increase of temperature become fluids, and fluids gases; and, _vice versa_, by a diminution of temperature, gases become fluids, and fluids solids.