per

does not crystallize so readily. Sinapate of lime dissolves very
readily, and when evaporated forms mamillary concretions.
Sinapic acid gives a purple red colour to the salts of the
oxide of iron, and by this action a very minute quantity of iron
may be detected. By this reaction Henry and Garot dis-
covered the presence of sinapic acid in pepper corns and in the
seeds of red cabbage, turnips, and radishes, and in water dis-
tilled from pepper. But they found no traces of it in the leaves
of cresses and cochlearia.

This acid occasions a precipitate in diacetate of lead, nitrate of mercury, and nitrate of silver; the two latter in curdy concretions, which are separated by boiling, and become black. The sinapate of silver is not soluble in nitric acid. Sinapic acid gives a green colour to a solution of sulphate of copper, and a white precipitate gradually falls.*

SUCH are the properties of the compound oxygen acids, so far as the present state of our knowledge enables us to go. It is not unlikely that some of them will hereafter require to have their position altered. For it is more than probable that mellitic acid and croconic acid contains nothing but oxygen and carbon, and that therefore they should be placed along with oxalic acid in the first division of this class. The atomic composition of these three acids is probably as follows:

Class I.

Div. II.

But as the absence of hydrogen in the two last of these acids has not been demonstrated by sufficiently accurate experiments, I think it better for the present to let them retain their place among the compound oxygen acids.

If the analysis of hydro-carbo-sulphuric acid be accurate, it contains no oxygen. Zeise has endeavoured to show that there exists a principle to which he has given the name of xanthine, composed of

Not having Henry and Garot's paper at hand, I have extracted this Section from Berzelius' Lärbok i Kemien, 4 delen, p. 103, which is a pretty faithful abridgment of the paper in question, so far as my recollection goes.

Chap. I.

Carbo-hydro. gen acids,

which is capable like cyanogen of combining with the different acidifiable bases, and converting them into acids. Hydro-carbo-sulphuric acid he considers as a compound of 1 atom of xanthine and 1 atom of hydrogen. Should this opinion be confirmed by future experiment, we must add another class of acids under the name of xanthine acids. But as Mr. Zeise's experiments are obviously insufficient to establish his opinions upon a sufficiently firm base, I thought it better in the mean time to place this acid along with the oxygen acids with a compound base, to which it obviously bears a striking analogy.

We have no idea at present in what way the numerous atoms composing these compound acids are arranged. Hence a knowledge of their mere atomic constituents is of little importance, excepting so far as it gives us the atomic weight of each acid, and enables us to form some judgment respecting the comparative simplicity or complex nature of each. The following table exhibits the constitution of the first subdivision of these acids so far as it has been determined:

We see from this table that the smallest number of atoms in any of these acids is in the formic which contains only 6 atoms, while the most complex is the ulmic, which contains no fewer than 64 atoms. In the gallic and ulmic acids the atoms of oxygen and hydrogen are just in the proportion to constitute water. In the formic, acetic, succinie, citric, croconic, mellitic, tartaric, vinic, and mucic, the hydrogen is in too small a ratio to saturate the oxygen. Formic acid might be resolved into an atom of water and an atom of olefiant gas, and citrie into two atoms

of water and two atoms of dicarbonic oxide.

In benzoic, cam

Class I.

Div. 11.

phoric, suberic, and kinic, it is in too great a proportion.
The following table exhibits the constitution of such of the
fatty acids as have been analyzed.

The striking difference between these acids and the last set consists in the great number of atoms of carbon and hydrogen, compared to the small number of atoms of oxygen. In no case do we find the number of atoms of carbon and hydrogen equal. But in general the approach to equality is so evident that I am disposed to consider these acids as compounds of oxygen with solid or liquid multiples of carbo-hydrogen, containing many more atoms of carbon and hydrogen than any yet met with by chemists. The atomic weights of the solid acids are in general higher than those of the liquid. Are we to infer from this that the tendency to solidity increases with the number of atoms of carbo-hydrogen that unite into one complex atom? The following table exhibits the constitution of the acids composed of oxygen, carbon, and azote:

In these the atoms of azote are always few, scarcely exceeding 2, while those of oxygen and carbon are numerous, with the exception of uric acid, which is much simpler in its composition than the other two.

azote acids,

The following table shows the constitution of those acids Hydro-carb. that are composed of oxygen, carbon, hydrogen, and azote.

The following table shows the atomic constitution of those acids that are composed of oxygen, carbon, hydrogen, and sulphur:

The sulpho-vinic is a compound of two atoms sulphuric acid and two atoms olefiant gas. It is not unlikely that sulphonaphthalic acid is a compound of two atoms sulphuric acid with one atom of naphthaline. The probability is that the constitution of the other two acids, included in this table, is not accurately made out.

It is very doubtful whether these compounds owe their acid qualities to the oxygen which they contain. Certainly the degree of acidity is not proportional to the proportion of this principle which they contain. There can be no doubt that each acid as a whole possesses electro-negative qualities; but these cannot be ascribed to the oxygen, which in the fatty acids must be completely overpowered by the great number of electropositive atoms with which it is combined.

CLASS II.

CHLORINE ACIDS.

I have no doubt that the acids formed by the combinations of chlorine with the acidifiable bases are numerous, and they will probably one day rival the oxygen acids themselves. But they have scarcely yet attracted the attention of chemists. The consequence is, that the facts respecting these acids hitherto ascertained, if we except muriatic acid, which had been known and investigated long before its constitution was suspected, are very few. I shall lay them before the readers of this work in the two following sections.

Class II.

SECTION I.—OF HYDRO-CHLORIC OR MURIATIC ACID.

There is little doubt that this acid was known in the seventh History. century, as it is distinctly alluded to in the writings of Geber. But Glauber seems to have been the chemist who contrived the present mode of obtaining it, by distilling a mixture of sulphuric acid and common salt. It was distinguished by the names of spirit of salt, marine acid, and muriatic acid, doubtless because it is obtained from common salt. Mr. Cavendish first obtained this acid in the gaseous state, and mentioned the circumstance in his paper on Factitious Airs, published in 1776;† but he does not seem to have been aware of the nature of the elastic fluid which he obtained. The subject was afterwards taken up by Dr. Priestley, who ascertained the nature and properties of muriatic acid gas, and must therefore be considered as the true discoverer of it. In 1774 Scheele discovered chlorine gas, and stated the composition of muriatic acid to be chlorine united to phlogiston. By phlogiston he meant hydrogen. Therefore, according to the opinion of Scheele, muriatic acid is a compound of chlorine and hydrogen. This opinion was neglected for many years; but was revived again in 1810 by Sir H. Davy, in consequence chiefly of the experiments of Gay-Lussac and Thenard. The numerous experiments of Davy, assisted by the subsequent discovery of iodine, and its striking analogy to chlorine, gradually established the theory of Davy. I am not aware of any chemist who at present supports the old hypothesis of Berthollet.

Nothing is easier than to procure gaseous muriatic acid in small quantities. We have only to put a little common salt into the small tubulated

retort A, capable of holding three or four cubic inches, and then to pour by the tubular a quantity of sulphuric acid, while the bent extremity of the beak is plunged into a mercurial

b

trough under the mouth of a small glass jar standing inverted over it and filled with mercury. The gas issues at first rapidly from the beak b. A portion must be allowed to escape before

* Muria is used by Cicero for brine or salt water. + Phil. Trans. vol. lvi. p. 157.

Priestley on Air, ii. 276.