Another variety is that of double elective affinity, iu which two compounds mutually decompose each other. Thus, sulphuric acid, carbonic acid, strontia, and potash, are so characterised with respect to mutual affinities, that if sulphate of strontia and carbonate of potash be made to act on each other, both are decomposed, and the components re-unite to form sulphate of potash and carbonate of s troll tia.
Chemical affinity works much more power fully when the bodies are in the liquid than the solid state, in most cases ; while others, again, require the gaseous state for its full development.
Proportion, heat, electricity, and light, all influence the strength and nature of affinitive combination. If sulphuric acid and alcohol be combined in the ratio of equality, they produce sulphuric tether ; but if the propor tion of acid be ten times as much as that of , alcohol, olefiant gas is produced. With re spect to heat, the difference is not less striking. If we mix oxygen and hydrogen gases, they will remain in a state of mixture for an inde finite period without combining ; but if flame he applied to them they combine with explo sion, and water is formed. Water dissolves certain salts, but to a limited extent only when cold ; boil it, and the solvent power is greatly increased. When mercury is moderately heated in atmospheric air it is converted into -peroxide, by combining with the oxygen of the air: heat the compound thus formed more strongly than was required for its pro duction, and the affinity is destroyed : oxygen gas is given out, and the mercury returns to its metallic state. And so on of other bodies.
Electricity possesses remarkable power over chemical affinity. Indeed so intimate is the connection between the two, that it is now known that no chemical action takes place
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without electric action being also manifested.
This part of the subject, developed as it- has been by professors Faraday and Daniell, is noticed wider ELECTROCHEMISTRY. In the mean time the following will serve as an ex ample of the reversal of chemical affinity by electricity. Immerse a piece of copper in a solution of nitrate of silver, the copper is solved and the silver precipitated ; if we re verse the experiment, and put a piece of sil ver into a solution of nitrate of copper, no change is effected ; if, however, the silver while immersed be touched by a piece of iron, the order of affinity is reversed, the copper is precipitated, and the silver is dis solved.
Light is capable of controlling chemical affinity, both with respect to decomposition and combination. If a mixture of hydrogen and chlorine gases be exposed to the sun's rays, they combine with explosion, and form muriatic acid : this effect does not appear to be produced by the heat which accompanies the light, for a considerably higher tempera ture is not capable of producing the combina tion. Or if pale nitric acid be subjeCted to the action of light, it suffers decompo sition to a certain extent, oxygen gas being evolved.
Chemical affinity has a most important influence on many branches of manufacture. Bleaching, dyeing, and calico-printing depend wholly on it ; as these processes are guided by relative powers of combination in different substances. All metallurgic processes in which mixed metals are concerned, such as steel, brass, pewter, bronze, Lte. ; and nearly all operations in which heat or solution are em ployed—are in like manner dependent on the affinities of different substances one for ano ther.