proportions, between the ingredients of bodies of similar
composition. Thus water, as you may recollect to have seen in a former
conversation, is composed of two volumes of hydrogen gas to one of
oxygen, and this is always found to be precisely the proportion of its
constituents, from whatever source the water be derived. The same
uniformity prevails with regard to the various salts; the acid and
alkali, in each kind of salt, being always found to combine in the same
proportions. Sometimes, it is true, the same acid, and the same alkali,
are capable of making two distinct kinds of salts; but in all these
cases it is found that one of the salts contains just twice, or in some
instances, thrice as much acid, or alkali, as the other.

EMILY.

If the proportions in which bodies combine are so constant and so well
defined, how can Mr. Berthollet's remark be reconciled with this uniform
system of combination?

MRS. B.

Great as that philosopher's authority is in chemistry, it is now
generally supposed that his doubts on this subject were in a great
degree groundless, and that the exceptions he has observed in the laws
of definite proportions, have been only apparent, and may be accounted
for consistently with those laws.

CAROLINE.

Pray, Mrs. B., can you decompose a salt by means of electricity, in the
same way as we decompose water?

MRS. B.

Undoubtedly; and I am glad this question occurred to you, because it
gives me an opportunity of showing you some very interesting experiments
on the subject.

If we dissolve a quantity, however small, of any salt in a glass of
water, and if we plunge into it the extremities of the wires which
proceed from the two ends of the Voltaic battery, the salt will be
gradually decomposed, the acid being attracted by the positive, and the
alkali by the negative wire.

EMILY.

But how can you render that decomposition perceptible?

MRS. B.

By placing in contact with the extremities of each wire, in the
solution, pieces of paper stained with certain vegetable colours, which
are altered by the contact of an acid or an alkali. Thus this blue
vegetable preparation called litmus becomes red when touched by an acid;
and the juice of violets becomes green by the contact of an alkali.

But the experiment can be made in a much more distinct manner, by
receiving the extremities of the wires into two different vessels, so
that the alkali shall appear in one vessel and the acid in the ot