Chemistry is the science which teaches the composition, properties, and uses, of all material bodies. It is the business of Mineralogy to investigate the properties and uses of inorganic substances as they naturally occur, but it is the office of chemistry to reduce, not only them, but all organic bodies, to their primitive elements, and having discovered the component parts and their proportions, to aim at their reproduction, when such reproduction would be beneficial to man. Under this head we shall give but a short sketch of this extensive science, as it will be necessary to explain the nature and mode of operation of those potent agents, Light, Heat, and Electricity; and as each substance which is the object of chemical research will be described under the initial letter of its name. For the history of this science we must refer our readers to Brande's Manual, or some other systematic work on the science, as its details would be far too multifarious to be compatible with the limits of this work. The science of chemistry will be found to depend chiefly on the operation of the different degrees of attraction which the various elementary bodies have for each other. Every mass of matter on the earth, or diffused through the universe, is found to tend towards every other mass.
This tendency is expressed by the term gravitation, and it is found to increase in intensity as the masses are nearer in the proportion of the squares of the distances between them. If, however, we examine this power in individual masses, we shall find it act with much greater intensity, and to vary its force in different bodies; it is then called affinity. The former species of attraction, or that acting on masses at sensible distances, forms the particular object of study in mechanical philosophy; while the action of the particles on each other at insensible distances is the object of chemical philosophy. When this affinity acts between particles of the same kind, it is called cohesion; but when it operates between particles of a different kind, it is called chemical attraction, the attraction of composition, or sometimes simply affinity. One particle of copper is attached to another particle of copper by cohesion; also a particle of sulphate of copper to another particle of the same substance; but when a particle of copper unites with a particle of sulphuric or nitric acid, it is an instance of chemical affinity. The attraction of cohesion, as well as the power of repulsion, which is opposed to it, are both opposed to chemical attraction.
A lump of sugar or salt is much more readily dissolved if previously broken into pieces or pulverized, than if left in the solid form which cohesion imparts to it. Chemical attraction has been distinguished into three degrees or states of energy: the result of the lowest kind is mixture; of the second, solution; and of the third and most energetic, composition. Chemical mixture takes place when the particles of two bodies are in a like state, and the power of cohesion, with regard to them, so far suspended, as to admit of that freedom of motion between themselves upon which fluidity depends; thus, two liquids or two aeriform fluids may admit of mixture, but two solids can be chemically mixed only by diminishing their cohesion by means of heat to such a degree as to bring one or both to the state of liquids. Between some fluids there appears to be no attraction, and hence they do not admit of mixture; thus, if water and oil, or water and mercury be agitated together, they will separate as soon as they are allowed to remain at rest, the denser body occupying the lower place; but sulphuric acid and water, or alcohol and water, have a strong attraction for each other, and when mixed, do not separate by repose, although the density of the ingredients materially differs.
The atmosphere in which we live and breathe is an example of the mixture of aeriform bodies: it is composed of two gases - oxygen and nitrogen, which are mingled with surprising uniformity. They may be separated from each other, and the properties of the latter exhibited, by burning a little sulphur under a bell glass over water. By this process the oxygen is removed, combining with the sulphur, and the nitrogen remains. The properties of these gases will be discussed hereafter under their respective names. Unlike liquids, all aeriform bodies have the property of mixing together. This difference is considered to arise from a modification of attraction between then* constituent particles. In the case of liquids not mixing, the attraction of cohesion between similar particles is probably greater than the chemical attraction between the dissimilar particles. In gaseous bodies the attraction of cohesion does not appear to exist, and chemical mixture operates unopposed. The expansive power of heat is opposed to cohesion, and hence this attraction may be so far counteracted, that solid bodies shall assume the fluid state, and may then be mixed together; thus, melted tin may be mixed with melted lead or copper, and their particles remain combined after they have resumed the solid state.
Chemical mixture may take place between two bodies in any proportion. Equal measures of sulphuric acid and water may be commingled, or a single drop of the former may be mixed with a pint or a gallon of the latter, and the mixture will be uniform and perfect. Chemical mixture is often attended by condensation or diminution of volume. If equal measures of sulphuric acid and water are mixed, the mixture will not fill two measures: this contraction is usually accompanied with an increase of temperature. If four parts of sulphuric acid be suddenly mixed with one of water, both at ordinary temperatures, the heat of the mixture will be higher than that of boiling water. The properties of bodies are not essentially changed by moisture; in general, however, those of the more active prevail. A few drops of sulphuric acid will communicate a sour taste to a quart of water. The separation of liquid mixtures may be effected either by the addition or subtraction of heat, arising from the unequal effect produced by this agent on different bodies. By carefully applying heat to a mixture of alcohol and water, the spirit will rise in vapour, and leave the water pure; this process is called evaporation.