Chap. L

Composition.

Its taste was bitter and acid, accompanied by an impression of something metallic like that of salts of copper. When heated in a glass tube it melted before it was heated to 212°, and on cooling crystallized from central points; but the whole ultimately became solid. When more highly heated water was first given off, and the acid assumed a slight red tint, but no sulphurous acid was yet driven off, or charring produced, and when tested by muriate of barytes it gave but very slight traces of sulphuric acid. In this state it seems to have been anhydrous. A higher temperature caused a little naphthaline to rise, the red liquid became deep brown, and then asudden action commenced at the bottom of the tube, which spread over the whole, and the acid became black and opaque. When the heat was continued naphthaline, sulphurous acid, and charcoal, were evolved; but a portion of sulpho-naphthalic acid remained undecomposed unless the temperature was raised to redness.

Sulpho-naphthalic acid combines with and neutralizes the different bases, and all the salts of it hitherto formed are soluble in water. When strongly heated they burn with a dense white flame, leaving a sulphate or bisulphate according to the nature of the base.

To determine the nature of this acid Mr. Faraday subjected sulpho-naphthalate of barytes to analysis, and obtained from 100 parts of the salt

Barytes
Sulphuric acid
Carbon

Hydrogen

27.57

30.17

41.90

2.877

102.517

The reason was, that the analysis being made upon only a few grains, any error committed was multiplied many times when it was reduced to 100 grains. But if we consider the salt as a compound of Barytes

So that there was an excess of 2.517 per cent.

Sulpho-naphthalic acid

27.57 or 9.5

72.43 or 24.54

it is obvious that the atomic weight of the acid must be 24.5.

Mr. Faraday considers the constituents of the acid to be

2 atoms sulphuric acid

20 atoms carbon

8 atoms hydrogen

10

15

1

26

are the proportions which correspond with the atomic weight of the acid as deduced from the analysis of sulpho-naphthalate of barytes. The exact results of the analysis are

2.13 atoms sulphuric acid,

19.75 atoms carbon,

8.13 atoms hydrogen.

Were we to consider sulpho-naphthalic acid as a compound of 2 atoms sulphuric acid and 1 atom of naphthaline, and were we to admit naphthaline to be a compound of

which accords with some of the analyses of it that have been made, the atomic weight of sulpho-naphthalic acid would be 24.625. I am inclined to the opinion that this is its true constitution. It resembles sulpho-vinic acid in containing two atoms of sulphuric acid, so that the saturating power of this acid is reduced to one half when it combines with naphthaline. A more detailed examination of this acid will probably throw new light upon the constitution and atomic weight of naphthaline. On this account it seems entitled to more attention than has been hitherto paid to it; for I am not aware that Mr. Faraday's experiments have been repeated. I indeed prepared one or two of the salts which he described, merely to judge of their appearance; but did not subject them to any additional examination.

SECTION LV.-OF VEGETO-SULPHURIC ACID.

This acid, which is obviously analogous to the two preceding, History. was discovered by Braconnot in 1819, during his researches on the action of concentrated sulphuric acid on vegetable substances.*

When pieces of linen or hemp cloth are left in contact with concentrated sulphuric acid they speedily lose their texture and

Ann. de Chim. et de Phys. xii. 172.

Chap. I.

Properties.

are converted into a pulp. When this pulp is diluted with water the greater portion of the linen dissolves, together with the sulphuric acid, leaving a quantity of black matter which is to be separated. The solution is now to be saturated with carbonate of lead in order to get rid of the sulphuric acid; and the sulphate of lead being separated a current of sulphuretted hydrogen gas is passed through the liquid to throw down the lead which was in solution. The liquid thus freed from lead being concentrated cautiously, leaves a transparent substance resembling very closely gum arabic in its properties.

If this gum be mixed with very dilute sulphuric acid, and boiled for some time, it is converted into two very remarkable substances. By far the greatest part of it assumes the form of a crystallizable sugar; but another portion in which the sulphuric acid that constituted a portion of the gun is wholly concentrated, possesses the characters of an acid, and constitutes the substance to which Braconnot has given the name of vegetosulphuric. This acid may be separated from the sugar by digestion in rectified spirits, which dissolve the acid together with a small portion of the sugar. Being evaporated to the consistence of a syrup, it was agitated in a close vessel with ether, which assumed a straw colour, and being evaporated left vegetosulphuric acid in a state of purity.

This acid is nearly colourless, its taste is very acid, almost caustic, setting the teeth on edge. It is deliquescent and incapable of being crystallized. When exposed to a temperature a little elevated it gradually assumes a brown colour. When exposed to the heat of a water-bath it is decomposed, and becomes black at a temperature rather lower than that of boiling water. When in that state, if we dilute it with water, flocks of vegetable matter, partly charred, are deposited. And when muriate of barytes is dropt into the liquid, a precipitate of sulphate of barytes falls. It is decomposed still more rapidly at a temperature above that of boiling water, and suffocating vapours of sulphurous acid are exhaled.

This acid occasions no precipitate when dropt into metallic solutions; it does not even throw down any thing when mixed with nitrate of barytes or diacetate of lead. It effervesces strongly with the carbonates, and dissolves metallic oxides, forming salts which are incrystallizable, deliquescent, and insoluble in rectified spirits. These vegeto-sulphates when heated are decomposed, sulphurous acid being evolved, and a sulphate mixed with charcoal remaining. It dissolves iron and zine with

a copious evolution of hydrogen gas. The vegeto-sulphates of barytes and lead are very soluble in water, and have the appearance of gum.

No attempt has been made to analyze any of the vegetosulphates, or to determine the constituents of vegeto-sulphuric acid. It obviously contains sulphuric acid as a constituent, and its analogy with the two preceding acids is so close that there can be little doubt of its being a compound of 2 atoms sulphuric acid and 1 atom of sugar of linen. Farther researches on it would be interesting, because they would be likely to throw light on the atomic weight of linen sugar; from which some conclusions might be drawn respecting the atomic weights of the different species of sugar, respecting which we at present know but little.

Class I. Div. II.

SECTION LVI.-OF SINAPIC ACID.

This acid was discovered by MM. Henry, junior, and History. Garót, in the expressed oil of the sinapis alba; and as they found that sulphur was one of the constituents of it, they gave it the name of acide sulphosinapique. It appears to exist in various cruciform plants. The method of obtaining this acid is as follows:

Digest the expressed oil of sinapis alba for a couple of weeks in 1 its weight of alcohol of the specific gravity at least of 0-827 (but the stronger the better), shaking it frequently during the digestion. Then draw off the alcohol, mix it with a little water, and putting it into a retort distil off the half of the alcohol. When the matter in the retort is allowed to cool a precipitate of fatty matter falls which had been held in solution by the alcohol. Free the liquid from this fatty matter, and leave it to spontaneous evaporation. The sinapic acid falls impure in the form of a granular reddish mass. It contains a fatty matter which is removed by means of ether, with which the impure acid is repeatedly macerated in a close vessel. The first portions of the ether acquire a carmine red colour, from fatty matter dissolved. As soon as ether ceases to act upon the acid it is dissolved in a little water, the solution is filtered and left to spontaneous evaporation, or it may be left in the vacuum of the air-pump over sulphuric acid. The acid shoots into small, glittering, half spherical groups of crystals, or into small stars.

Sinapic acid thus obtained has a light yellowish colour. Its Properties. taste is bitter, sharp, and sulphureous. It does not redden

Chap. I.

Composition.

litmus tincture, but blackens it like a solution of a hypo-sulphate. But it reddens the tincture made from the petals of the mallow. Whether these crystals contain water has not been ascertained. They are soluble in water and alcohol, and slightly soluble in ether. The aqueous solution may be evaporated to dryness, and the acid exposed to a heat of 230° without decomposition. When the dry acid is distilled in a retort it is decomposed. A little of the acid indeed sublimes unaltered, but by far the greatest part is resolved into carbonic acid, sulphuretted hydrogen, and ammonia, and a coal which remains in the retort. Even when the aqueous solution is long boiled the acid is partly decomposed, and sulphuretted hydrogen exhales, which blackens paper dipped in acetate of lead.

The composition of this acid is very remarkable, since it contains no fewer than 5 constituents, namely, oxygen, hydrogen, carbon, azote, and sulphur. The proportions of these constituents as determined by Henry and Garot are as follows:

None of the salts which this acid forms having been subjected to analysis, we do not know its atomic weight. But the smallest number of atoms which correspond with the preceding analy

Hence the atomic weight is either 13-125, or a multiple of

that number.

The alkaline and earthy sinapates have a bitter taste. Sinapate of potash and of soda shoot out into small crystals about the size of a millet seed. Sinapate of ammonia forms small translucent crystals, which appear to be octahedrons. Sinapate of barytes forms small needles which effloresce in the air and are very soluble in water. Sinapate of strontian is similar, but