The forms and appearances of the beings and substances of the external world are almost infinitely various, and they are in a state of continued alteration. In general, matter is found in four forms, as (1) solids, (2) fluids, (3) gases, (4) ethereal substances. 1. _Solids._ Solids retain whatever mechanical form is given to them; their parts are separated with difficulty, and cannot readily be made to unite after separation. They may be either elastic or non-elastic, and differ in hardness, in colour, in opacity, in density, in weight, and, if crystalline, in crystalline form. 2. _Fluids._ Fluids, when in small masses, assume the spherical form; their parts possess freedom of motion; they differ in density and tenacity, in colour, and in opacity. They are usually regarded as incompressible; at least, a very great mechanical force is required to compress them. 3. _Gases._ Gases exist free in the atmosphere, but may be confined. Their parts are highly movable; they are compressible and expansible, and their volumes are inversely as the weight compressing them. All known gases are transparent, and present only two or three varieties of colour; they differ materially in density. 4. _Ethereal Substances._ Ethereal substances are known to us only in their states of motion when acting upon our organs of sense, or upon other matter, and are not susceptible of being confined. It cannot be doubted that there is such matter in motion in space. Ethereal matter differs either in its nature, or in its affections by motion, for it produces different effects; for instance, radiant heat, and different kinds of light. All these forms of matter are under the influence of active forces, such as gravitation, cohesion, heat, chemical and electrical attraction, and these we must now consider. 1. _Gravitation._ When a stone is thrown into the atmosphere, it rapidly descends towards the earth. This is owing to gravitation. All the great bodies in the universe are urged towards each other by a similar force. Bodies mutually gravitate towards each other, but the smaller body proportionately more than the larger one; hence the power of gravity is said to vary directly as the mass. Gravitation also varies with distance, and acts inversely as the square of the distance. 2. _Cohesion._ Cohesion is the force which preserves the forms of solids, and gives globularity to fluids. It is usually said to act only at the surface of bodies or by their immediate contact; but this does not seem to be the case. It certainly acts with much greater energy at small distances, but the spherical form of minute portions of fluid matter can be produced only by the attractions of all the parts of which they are composed, for each other; and most of these attractions must be exerted at sensible distances, so that gravitation and cohesion may be mere modifications of the same general power of attraction.