ed from bones, in which it is contained in the state of phosphat
of lime; from this salt the phosphoric acid is separated by means of the
sulphuric, which combines with the lime. In its pure state, phosphoric
acid is either liquid or solid, according to its degree of
concentration.

Among the salts formed by this acid, _phosphat of lime_ is the only one
that affords much interest; and this, we have already observed,
constitutes the basis of all bones. It is also found in very small
quantities in some vegetables.

CONVERSATION XVIII.

OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATIONS OF OXYGEN WITH
NITROGEN AND CARBON; AND OF THE NITRATS AND CARBONATS.

MRS. B.

I am almost afraid of introducing the subject of the NITRIC ACID, as I
am sure that I shall be blamed by Caroline for not having made her
acquainted with it before.

CAROLINE.

Why so, Mrs. B.?

MRS. B.

Because you have long known its radical, which is nitrogen or azote; and
in treating of that element, I did not even hint that it was the basis
of an acid.

CAROLINE.

And what could be your reason for not mentioning this acid sooner?

MRS. B.

I do not know whether you will think the reason sufficiently good to
acquit me; but the omission, I assure you, did not proceed from
negligence. You may recollect that nitrogen was one of the first simple
bodies which we examined; you were then ignorant of the theory of
combustion, which I believe was, for the first time, mentioned in that
lesson; and therefore it would have been in vain, at that time, to have
attempted to explain the nature and formation of acids.

CAROLINE.

I wonder, however, that it never occurred to us to enquire whether
nitrogen could be acidified; for, as we knew it was classed among the
combustible bodies, it was natural to suppose that it might produce an
acid.

MRS. B.

That is not a necessary consequence; for it might combine with oxygen
only in the degree requisite to form an oxyd. But you will find that
nitrogen is susceptible of various degrees of oxygenation, some of which
convert it merely into an oxyd, and others give it all the acid
properties.

The acids, resulting from the combination of oxygen and nitrogen, are
called the NITROUS and NITRIC acids. We will begin with the NITRIC, in
which nitrogen is in the highest state of oxygenation. This acid
naturally exists in the form of gas; but is so very soluble in water,
and has so great an affinity for