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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° be mixed with a pint of water at 50°, the resulting temperature is not 75°, but 70°; 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 versâ, by a diminution of temperature, gases become fluids, and fluids solids.

Proofs of the conversion of solids, fluids, or gases into ethereal substances are not distinct. Heated bodies become luminous and give off radiant heat, which affects the bodies at a distance, and it may therefore be held that particles are thrown off from heated bodies with great velocity, which, by acting on our organs, produce the sensations of heat or light, and that their motion, communicated to the particles of other bodies, has the power of expanding them. It may, however, be said that the radiant matters emitted by bodies in ignition are specific substances, and that common matter is not susceptible of assuming this form; or it may be contended that the phenomena of radiation do in fact, depend upon motions communicated to subtile matter everywhere existing in space.

The temperatures at which bodies change their states from fluids to solids, though in general definite, are influenced by a few circumstances such as motion and pressure.

When solids are converted into fluids, or fluids into gases, there is always a loss of heat of temperature; and, vice versâ, when gases are converted into fluids, or fluids into solids, there is an increase of heat of temperature, and in this case it is said that latent heat is absorbed or given out.

The expansion due to heat has been accounted for by supposing a subtile fluid, or caloric, capable of combining with bodies and of separating their parts from each other, and the absorption and liberation of latent heat can be explained on this principle. But many other facts are incompatible with the theory. For instance, metal may be kept hot for any length of time by friction, so that if caloric be pressed out it must exist in an inexhaustible quantity. Delicate experiments have shown that bodies, when heated, do not increase in weight.

It seems possible to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity and through the greatest space; that in fluids and gases the particles have not only vibratory motion, but also a motion round their own axes with different velocities, and that in ethereal substances the particles move round their own axes and separate from each other, penetrating in right lines through space. Temperature may be conceived to depend upon the velocity of the vibrations, increase of capacity on the motion being performed in greater space; and the diminution of temperature during the conversion of solids into fluids or gases may be explained on the idea of the loss of vibratory motion in consequence of the revolution of particles round their axes at the moment when the body becomes fluid or aeriform, or from the loss of rapidity of vibration in consequence of the motion of particles through greater space.