The earth, so far as we are acquainted with it, is composed of about sixty principles, which we call elementary. These are the most simple states to which we can reduce matter, and from them all the forms of creation yet examined by the chemist are produced. These elementary principles are, some of them, permanently gaseous under the ordinary temperature, and others exist as solid masses; the difference between the two conditions being regulated, as it appears, by the opposing forces of heat and cohesive attraction. Matter has been regarded by some as infinitely divisible; but the known conditions of chemical combinations lead to the conclusion that there are limits beyond which matter cannot be divided.[25] The theory of atoms having determinate characters, and possessing symmetric forms, certainly has the advantage of presenting to the human mind a starting point--a sort of standing ground,--from which it can direct the survey of cosmical phenomena. The metaphysical hypothesis, which resolves all matter into properties, and refers all things to ideas, leaves the mind in a state of uncertainty and bewilderment. Adapting the views of Dumas, with some modifications,[26] it will be found more satisfactory to regard the _ultimate atoms_ of matter as points beyond the reach of our examination; which, according to a law, determined by the influences of the so-called imponderable forces, unite to form _molecules_. Again, these molecules combine to form the _particles_ of the mass which we may regard as the limit of mechanical division. The particles of solid bodies are solid, those of fluids fluid, and those of gaseous bodies are themselves aëriform; but it does not follow that the molecules of any body should be necessarily solid, fluid, or aëriform, from the circumstance of their having formed the particles of a body in one of these states. As this planet--a molecule in space--is formed of aggregated atoms, and enveloped by its own physical agencies--and as it is involved in the infinitely extending influences of other planetary molecules, and thus forms part of a system--so the molecules of any mass are grouped into a system or particle, which possesses the great characteristic features of the whole. In an aëriform body the particles are in a state of extreme tenuity, the molecules being themselves, by the influence of some repulsive force, just on the verge where cohesion exerts its decaying power. In fluid bodies the attenuation of the particles is less--the particles and also the molecules are nearer together,--whereas, in the solid body, the forces of cohesion are most strongly exerted, and all the molecular conditions brought more powerfully into action.