endeavour to decompose this salt by nitric
acid, which tends to combine with soda, we shall be equally
unsuccessful, as nitric acid tends to unite with the alkali by a force
equal only to 7.

In neither of these cases of simple elective attraction, therefore, can
we accomplish our purpose. But let us previously combine together the
lime and nitric acid, so as to form a nitrat of lime, a compound salt,
the constituents of which are united by a power equal to 4. If then we
present this compound to the sulphat of soda, a decomposition will
ensue, because the sum of the forces which tend to preserve the two
salts in their actual state is not equal to that of the forces which
tend to decompose them, and to form new combinations. The nitric acid,
therefore, will combine with the soda, and the sulphuric acid with the
lime.

CAROLINE.

I understand you now very well. This double effect takes place because
the numbers 8 and 4, which represent the degrees of attraction of the
constituents of the two original salts, make a sum less than the numbers
7 and 6, which represent the degrees of attraction of the two new
compounds that will in consequence be formed.

MRS. B.

Precisely so.

CAROLINE.

But what is the meaning of _quiescent_ and _divellent_ forces, which are
written in the diagram?

MRS. B.

Quiescent forces are those which tend to preserve compounds in a state
of rest, or such as they actually are: divellent forces, those which
tend to destroy that state of combination, and to form new compounds.

These are the principal circumstances relative to the doctrine of
chemical attractions, which have been laid down as rules by modern
chemists; a few others might be mentioned respecting the same theory,
but of less importance, and such as would take us too far from our plan.
I should, however, not omit to mention that Mr. Berthollet, a celebrated
French chemist, has questioned the uniform operation of elective
attraction, and has advanced the opinion, that, in chemical
combinations, the changes which take place depend not only upon the
affinities, but also, in some degree, on the respective quantities of
the substances concerned, on the heat applied during the process, and
some other circumstances.

CAROLINE.

In that case, I suppose, there would hardly be two compounds exactly
similar, though composed of the same materials?

MRS. B.

On the contrary, it is found that a remarkable uniformity prevails, as
to