MRS. B.
Every chemical combination is an illustration of this rule. But let us proceed—
The 6th law is, that the force of chemical affinity between the constituents 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.