If the more evaporable fluid is condensed and received into a separate vessel, it is termed distillation. This process may easily be performed in a small way by using a retort and receiver. In the annexed cut a is the retort, containing the mixed liquid, which may be spirit and water, or a little wine or ale; b is the receiver, into which the neck of the retort must pass through a. cork; c is the spirit lamp, which furnishes heat for the distillation. As soon as the heat is applied, the spirit being more easily vapourized, rises to the top of the retort, where, being condensed into the liquid state, it slowly trickles down the neck into the receiver. The neck of the retort and the receiver may be kepi cool, by applying cloths, constantly moistened with cold water, to their externa) surface. The spirit may also be separated from the water by intense cold, which freezes the water, and leaves the spirit in its liquid state. Mixtures of gaseous fluids are not separable by heat or cold, for they all expand equally by equal additions of heat; but steam, and other vapours, may be separated from gases by reduction of temperature, which destroys the elasticity of the formed, but not of the latter.

The second degree of chemical attraction is solution, which operates between solids and liquids, or gases and liquids: the liquids, in these cases, are called solvents. Between some liquids and solids attraction does not exist, or it is overbalanced by the cohesive power. Resin and sealing wax are insoluble in water, but readily dissolve in alcohol. Chemical attraction may be exerted in different degrees between one body and several others. There is an affinity between alcohol and water, whereby they are capable of mixing; there is also a mutual affinity between alcohol and resin, by which the former dissolves the latter; but there is no affinity between water and resin. Now, if water be added to a solution of resin in alcohol, the resin will resume the solid form; the attraction between the particles of alcohol and those of water is greater than that between the particles of resin and alcohol: the consequence is, that the alcohol quits the resin and combines with the water, and the resin falls to the bottom of the liquid. This is called elective attraction, because the alcohol may be considered as exercising a choice between the substances with which it is capable of combining. This resumption of the solid form by a body previously dissolved in a liquid, is termed precipitation.

The power of solution is limited, as liquids cannot combine with more than a certain definite quantity of any solid or aeriform body. The point at which the action between the two bodies ceases is called the point of saturation. Up to this point the two bodies may combine in any proportions. A solvent that has been saturated with one substance, is often capable of combining at the same time with ethers; thus water that is saturated with saltpetre will dissolve a considerable quantity of common salt. The influence of heat upon the power of solution corresponds with the difference between cohesion and elasticity. Upon solid bodies it generally increases the power of the solvent by diminishing their cohesion. Upon aeriform bodies it diminishes the power by increasing their elasticity. Solvents may be separated from the bodies with which they are united by alterations of temperature. If a solution of alum or common salt be boiled, the liquid will be dissipated in vapour, but the solid salt will remain. If a solution of carbonic acid in water be frozen, the gas will escape and the water remain in the solid state. The result of the highest degree of chemical attraction is composition, which may take place between bodies under every modification of cohesive attraction.

Bodies that unite in this intimate manner combine only in definite proportions, which are invariable in the same compound. Combination usually produces a total change in the sensible properties of the combining substances; and the process is generally accompanied by change of temperature, to such an extent sometimes that light and heat are emitted in abundance. If copper filings and sulphur be mixed togethei, and heat applied as soon as the sulphur melts, a violent action will take place, the copper will become red hot, and a black brittle body will be formed, with properties totally different from those of its two ingredients. This compound, which is the sulphuret of copper, is often found ready formed in mineral veins; but whether existing naturally, or formed by art, its composition is definite and invariable. It is found to consist of 64 parts by weight of copper, and 16 of sulphur. There is, however, another compound of copper and sulphur, in which the proportions are 64 copper to 32 of sulphur. In this it will be seen that the quantity of sulphur is exactly double of that which the first combination contains.

This is a fact of common occurrence; many bodies will unite in two or three different proportions, but they are no less definite, each being a multiple, or a submultiple, of the preceding; that is, it is double, triple, or half, and so on. This is well illustrated in the compounds of mercury. If this brilliant and fluid metal be agitated for a long time in contact with the common air, it will unite with the oxygen of that mixture, and will be converted into a black, insipid, insoluble powder, which consists of 200 parts of mercury to 8 of oxygen. If instead of being agitated at common temperatures with the air it be kept nearly boiling in it, it will be converted into a red shining mass, endued with an acrid metallic taste, and which is soluble in water, and poisonous. This consists of 200 parts of mercury, and 16 of oxygen. Again, if mercury be rubbed in a mortar with sulphur, it unites with a given proportion. If the mercury be poured into melted sulphur, and strongly heated, a compound will be formed which rises in vapour, and concretes on cooling into a brilliant red substance, known as cinnabar or vermilion. In this the quantity of sulphur is double of that in the preceding compound.