Conversations on Chemistry Part 59

The 3d law of chemical attraction is, that _it can take place between two, three, four, or even a greater number of bodies_.

CAROLINE.

Oxyds and acids are bodies composed of two const.i.tuents; but I recollect no instance of the combination of a greater number of principles.

MRS. B.

The compound salts, formed by the union of the metals with acids, are composed of three principles. And there are salts formed by the combination of the alkalies with the earths which are of a similar description.

CAROLINE.

Are they of the same kind as the metallic salts?

MRS. B.

Yes; they are very a.n.a.logous in their nature, although different in many of their properties.

A methodical nomenclature, similar to that of the acids, has been adopted for the compound salts. Each individual salt derives its name from its const.i.tuent parts, so that every name implies a knowledge of the composition of the salt.

The three alkalies, the alkaline earths, and the metals, are called _salifiable bases_ or _radicals_; and the acids, _salifying principles_.

The name of each salt is composed both of that of the acid and the salifiable base; and it terminates in _at_ or _it_, according to the degree of the oxygenation of the acid. Thus, for instance, all those salts which are formed by the combination of the sulphuric acid with any of the salifiable bases are called _sulphats_, and the name of the radical is added for the specific distinction of the salt; if it be potash, it will compose a _sulphat of potash_; if ammonia, _sulphat of ammonia_, &c.

EMILY.

The crystals which we obtained from the combination of iron and sulphuric acid were therefore _sulphat of iron_?

MRS. B.

Precisely; and those which we prepared by dissolving copper in nitric acid, _nitrat of copper_, and so on. --But this is not all; if the salt be formed by that cla.s.s of acids which ends in _ous_, (which you know indicates a less degree of oxygenation,) the termination of the name of the salt will be in _it_, as _sulphit of potash_, _sulphit of ammonia_, &c.

EMILY.

There must be an immense number of compound salts, since there is so great a variety of salifiable radicals, as well as of salifying principles.

MRS. B.

Their real number cannot be ascertained, since it increases every day.

But we must not proceed further in the investigation of the compound salts, until we have completed the examination of the nature of the ingredients of which they are composed.

The 4th law of chemical attraction is, that _a change of temperature always takes place at the moment of combination_. This arises from the extrication of the two electricities in the form of caloric, which takes place when bodies unite; and also sometimes in part from a change of capacity of the bodies for heat, which always takes place when the combination is attended with an increase of density, but more especially when the compound pa.s.ses from the liquid to the solid form. I shall now show you a striking instance of a change of temperature from chemical union, merely by pouring some nitrous acid on this small quant.i.ty of oil of turpentine--the oil will instantly combine with the oxygen of the acid, and produce a considerable change of temperature.

CAROLINE.

What a blaze! The temperature of the oil and the acid must be greatly raised, indeed, to produce such a violent combustion.

MRS. B.

There is, however, a peculiarity in this combustion, which is, that the oxygen, instead of being derived from the atmosphere alone, is princ.i.p.ally supplied by the acid itself.

EMILY.

And are not all combustions instances of the change of temperature produced by the chemical combination of two bodies?

MRS. B.

Undoubtedly; when oxygen loses its gaseous form, in order to combine with a solid body, it becomes condensed, and the caloric evolved produces the elevation of temperature. The specific gravity of bodies is at the same time altered by chemical combination; for in consequence of a change of capacity for heat, a change of density must be produced.

CAROLINE.

That was the case with the sulphuric acid and water, which, by being mixed together, gave out a great deal of heat, and increased in density.

MRS. B.

The 5th law of chemical attraction is, that _the properties which characterise bodies, when separate, are altered or destroyed by their combination_.

CAROLINE.

Certainly; what, for instance, can be so different from water as the hydrogen and oxygen gases?

EMILY.

Or what more unlike sulphat of iron than iron or sulphuric acid?

MRS. B.

Every chemical combination is an ill.u.s.tration of this rule. But let us proceed--

The 6th law is, that _the force of chemical affinity between the const.i.tuents of a body is estimated by that which is required for their separation_. This force is not always proportional to the facility with which bodies unite; for manganese, for instance, which, you know, is so much disposed to unite with oxygen that it is never found in a metallic state, yields it more easily than any other metal.

EMILY.

But, Mrs. B., you speak of estimating the force of attraction between bodies, by the force required to separate them; how can you measure these forces?

MRS. B.

They cannot be precisely measured, but they are comparatively ascertained by experiment, and can be represented by numbers which express the relative degrees of attraction.

The 7th law is, that _bodies have amongst themselves different degrees of attraction_. Upon this law, (which you may have discovered yourselves long since,) the whole science of chemistry depends; for it is by means of the various degrees of affinity which bodies have for each other, that all the chemical compositions and decompositions are effected.

Every chemical fact or experiment is an instance of the same kind; and whenever the decomposition of a body is performed by the addition of any single new substance, it is said to be effected by _simple elective attractions_. But it often happens that no simple substance will decompose a body, and that, in order to effect this, you must offer to the compound a body which is itself composed of two, or sometimes three principles, which would not, each separately, perform the decomposition.

In this case there are two new compounds formed in consequence of a reciprocal decomposition and recomposition. All instances of this kind are called _double elective attractions_.

CAROLINE.

I confess I do not understand this clearly.

MRS. B.

You will easily comprehend it by the a.s.sistance of this diagram, in which the reciprocal forces of attraction are represented by numbers: