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 It frequently happens that the conpound of two principles cannot be destroyed either by a third or fourth separately applied; but if the third and fourth be combined, and placed in contact with the former compound, a decomposition, or a change of principles will ensue. Thus when lime is added to a solution of sulphate of soda, no decomposition happens, because the sulphuric acid attracts the soda more strongly than it attracts lime. If muriatic acid be applied to the same compound, still its principles remain undisturbed, because the sulphuric acid attracts soda more strongly than the muriatic acid does. But if the lime and muriatic acid previously combined be mixed with the sulphate of soda a double decomposition is effected. The lime leaving the muriatic acid, unites with the sulphuric acid ; and the soda, being separated from the sulphuric acid combines with the muriatic. These decompositions are rendered more intelligible by the following diagram.
Muriate of Soda.
Sulphate
of
Soda.
Soda. Muriatic acid.
Sulphuric acid. Lime.
Muriate
of Lime.
Sulphate of Lime. On the outside of the vertical brackets are placed the original compounds, sulphate of soda and muriate of lime; and above and below the diagram, the new compounds. The upper line being strait, indicates that the muriate of soda remains involution, and the middle of the lower line being directed downwards, shows that the sulphate of lime is precipitated.
OA*. The turbid, or milky appearance which is seen on adding the clear solutions of muriate of lime and sulphate of soda together, is owing to the fact that one of the newly formed salts, sulphate of lime, is insoluble in water, and consequently is precipitated, or falls to the bottom of the vessel occasioning in its descent the turbidness. The same appearance is seen in every case where an insoluble precipitate is formed by chemical decomposition.
81. Experiments illustrating chemical affinity.
Exp. Mix together some olive oil and water in a vial. The oil will rise to the top of the water, and cannot be made to unite with it, there being no affinity between them. But if some pieces of potash be added, and the vial shaken, there takes place a chemical union between the three substances.
Oba. The potash having an affinity both for the oil and the water, it attracts each with a force sufficient to effect their union. In this manner soap is formed.
83. Bodies do not act on each othw unless either one I or both are in a state of solution, or at least contain water.
Illus. Spread thinly on a piece of tinfoil three or four inches square, some dry nitrate of copper and wrap it up. No effect will be produced. Unfold the tinfoil and having sprinkled the nitrate of copper with the smallest possible quantity of water, wrap the tinfoil up again as quickly as possible, pressing down the edges closely. Considerable heat attended with copious red fumes, will now be excited; and if the experiment has been dexterously made, sparks of fire or even name will be emitted.
Olts. This shows that nitrate of copper has no action on tin, except it is in a state of solution.
84. Experiments illustrating single elective affinity. Exji. 1. Add to a solution of soap in water a few drops of
sulphuric, or any other acid. The acid will combine with the A potash ; the oil will be set free, and will rise to the top. * Obs. The same effect is produced by those waters which naturally contain a small quantity of any acid, as the carbonic. These waters are called hard and will not wash, because the acid which they contain decomposes the soap.
2. To a solution of camphor in spirits or alcohol, add a quantity of water. The mixture will instantly become white and turbid, and the camphor will rise to the top in flakes.
Obs. The alcohol has a stronger attraction for the water than it has for the camphor, it therefore leaves the latter and unites to the former, while the camphor being insoluble in water takes the solid form, and rises to the top because it is of less specific gravity than the fluid.
85. The properties characterizing bodies when separate, are destroyed by chemical combination, and new properties appear in the compound.
Ilhis. Thus, muriate acid and lime, which in a separate state have a most corrosive taste, lose this entirely when mutually saturated ; the compound is extremely soluble, though lime itself is very difficult of solution: the acid no longer reddens syrup of violet, nor does the lime change it as before to green. The resulting compound, muriate of lime also exhibits new properties. It has an intensely bitter taste; is susceptible of crystalization, and the crystals when mixed with snow or ice, generate a degree of cold sufficient to freeze mercury.

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