dissolve it even when assisted by heat, and though it be weakened by the addition of water. It is not so easily melted nor so easily decomposed by heat as common sugar. When distilled it yields a white sublimate and an ammoniacal liquid, showing that azote exists in it as a constituent. When heated with nitric acid it forms no mucic acid. But another acid is formed to which Braconnot has given the name of nitrosaccharic.

When nitric acid is poured upon sugar of glue no solution takes place, but the sugar becomes perfectly white in consequence of the solution or decomposition of the colouring matter. When the mixture is heated solution takes place, but there is no effervescence takes place, nor any disengagement of red vapours. The evaporation being continued (taking care not to apply too strong heat) till the whole is sufficiently concentrated, it becomes solid on cooling, having the appearance of a crystalline mass. This mass was cleaned by pressure between the folds of blotting paper, and crystallized anew. In this state it constitutes nitro-saccharic acid, the weight of which exceeds considerably the whole weight of the sugar of glue employed.

Class I.

Div. II.

It is very soluble in water, and readily crystallizes in fine Properties. transparent, flat, striated prisms, similar in appearance to the crystals of glauber salt. Its taste is acid and slightly sweet, very similar to that of tartaric acid. When heated it swells very much, melts and gives out an acid vapour. It produces no change in metallic or earthy solutions. It combines in two proportions with potash, forming a neutral and a binitro-saccharate, both of which crystallize in fine needles. Its taste is nitrous with an impression of sweetness. When thrown upon burning coals it detonates like nitre. The nitro-saccharate of lime crystallizes in beautiful needles. It is not deliquescent and is scarcely soluble in alcohol. Thrown upon burning coals it melts in its water of crystallization, and then detonates like

nitre.

With oxide of copper this acid forms a crystallizable salt not altered by exposure to the air; with magnesia an uncrystallizable deliquescent salt; with oxide of lead it forms an uncrystallizable salt not altered by exposure, and resembling gum in appearance. This salt detonates feebly when heated. Nitrosaccharic acid dissolves iron and zinc, with the disengagement of hydrogen gas, and forms with them salts which do not crystallize.

Chap. I.

Formation.

Leucine.

These facts (which are all that have hitherto been determined) leave no doubt that nitro-saccharic acid is a combination of nitric acid and sugar of glue. It is a remarkable circumstance that bases have no disposition to separate the nitric acid from the sugar, but that they combine with both at once. This is a sufficient proof that the sugar of glue does not unite with nitric acid as a base, but so as to form a new compound acid.

It would have been interesting to have known the atomic weight of this acid, and likewise the proportion in which the nitric acid and sugar of glue unite. But no attempts seem to have been made by Braconnot to determine the composition of any of its salts.*

SECTION XLVIII.—OF NITROLEUCIC ACID.

This acid, which has hitherto been but very imperfectly examined, was discovered by Braconnot about the same time with the nitro-saccharic acid. It was obtained by the following process:

A piece of muscle of beef as lean as possible, was well divided and steeped in water till every thing soluble in that liquid was taken up. It was then subjected to pretty strong pressure in a linen cloth. 30 grammes (463 grains) of this fibrin being mixed with their own weight of concentrated sulphuric acid, softened and dissolved without the acid becoming coloured or the disengagement of any sulphurous acid. It was heated to render the solution more complete, and allowed to cool that a crust of tallow which had appeared might be separated. The solution was then diluted with about six cubie inches of water, and boiled for nine hours, renewing the water from time to time as it evaporated. The acid was then saturated with chalk, and the liquid being separated by the filter and evaporated, yielded an extract not in the least sweet, but having a very marked flavour of roasted meat. When boiled repeatedly in alcohol of the specific gravity 0-847, the alcoholic liquids on cooling deposited about 15 grains of a white matter, which Braconot distinguished by the name of leucine.

Leucine when dried is a white powder-but it still retains a little animal matter separable by tannin. To free it from this matter the leucine was dissolved in water, and a little tannin cautiously added. After some hours the liquid was filtered

* Ann. de Chim. et de Phys. xiii. 113.

and then evaporated till a pellicle formed on its surface. This pellicle being removed after 24 hours, small white grains were found at the bottom of the vessel. This was pure leucine. When leucine dissolved in hot water is left for spontaneous evaporation the leucine forms on the surface small round circles like buttons, having a depression in the centre. Leucine has an agreeable taste of boiled meat or soup. It is lighter than water. When heated it melts at a temperature much higher than that of boiling water, emitting the smell of roasted meat, and partly subliming in small white opaque crystals. The liquid which comes over contains empyreumatic oil and ammonia.

Leucine dissolves readily in nitric acid. If we heat the solution to drive off the excess of nitric acid, we observe scarcely any effervescence and no emission of red vapour. The residue evaporated in the sand-bath in a gentle heat assumes the form of a solid crystalline mass, which being dried between folds of bloating paper and redissolved in water, crystallizes in fine divergent needles. These needles constitute nitroleucic acid.

Class L

Div. II.

acid.

This acid forms with the salifiable bases salts quite differ- Nitro-leucic ent from the nitro-saccharates. Nitroleucate of lime is a salt not altered by exposure to the air, and which crystallizes in small groups. When thrown on burning coals it melts in its water of crystallization, but not so readily as nitro-saccharate of lime. Nitroleucate of magnesia crystallizes and is not altered by exposure to the air. The remaining properties of this acid are still unknown. It is obviously a combination of nitric acid with leucine, and therefore analogous to nitrosaccharic acid in its nature.*

SECTION XLIX. -OF PURPURIC ACID.

This acid was discovered by Dr. Prout, in the year 1818, History. while engaged in examining the pink-coloured substance formed by heating a solution of uric acid in nitric acid. His mode of obtaining the acid was as follows:

He digested pure uric acid in dilute nitric acid. An effervescence took place, and the acid was dissolved. The excess of nitric acid was neutralized by ammonia, and the evaporation continued. The colour of the liquid became gradually of a deeper purple and dark red granular crystals (sometimes of a greenish hue externally), soon began to separate in abundance.

* Ann. de Chim. et de Phys. xiii. 118.

Chap. I.

Properties.

These crystals were composed of purpuric acid and ammonia; they were dissolved in a solution of caustic potash, and heat applied to the solution till the red colour entirely disappeared. The alkaline solution was then gradually dropped into dilute sulphuric acid, which uniting with the potash, left the acid principle in a state of purity.*

Purpuric acid thus obtained is a cream-coloured powder, which, when examined with a magnifier, especially in water, appears to possess a pearly lustre. It has no smell nor taste. Its specific gravity is considerably above that of water, though from its minute state of division it usually takes a considerable time to subside in that fluid.

It is very little soluble in water; 10,000 parts of that liquid not being capable of dissolving one part of the acid. In alcohol and ether it is quite insoluble. It dissolves readily in the concentrated mineral acids and in the alkaline solutions; but it is insoluble, or nearly so, in dilute sulphuric, muriatic, and phosphoric acids, and also in solutions of oxalic, tartaric, and citrie acids. Concentrated nitric acid dissolves it readily with effervescence, and purpurate of ammonia is obtained. Chlorine likewise dissolves it, and produces a similar change on it. It dissolves also when assisted by heat in concentrated acetic acid.

It does not sensibly alter the colour of litmus paper. When exposed to the air it does not deliquesce. But it becomes gradually red, and some purpurate of ammonia seems to be formed in it.

When heated it neither melts nor sublimes, but acquires a purple hue from the formation of ammonia, and afterwards burns gradually without yielding any remarkable odour. When distilled it yields a good deal of carbonate of ammonia, some prussic acid, and a little fluid having an oily appearance, while a pulverulent charcoal remains.

It combines with bases, and forms salts to which the name of purpurates has been given. The greater number of these purpurates have a red colour, though some of them are green and some of them yellow. Purpuric acid is capable of decomposing the carbonates when assisted by heat, and it does not, as far is known, combine with any other acid.

Dr. Prout made some trials to determine the composition of this acid by heating it with peroxide of copper. The result of his experiments gave the constituents as follows:

Phil. Trans. 1818, p. 420.

According to this analysis the equivalent number for purpuric acid is 5.5 or a multiple of it.*

pur

It may be requisite to state that the attempts which I made to procure this acid by the process of Dr. Prout did not succeed, nor did the phenomena which I observed exactly agree with those described by Vauquelin, who considers the colouring matter and the acid as two different substances. Lassaigne conceives that he has verified this opinion of Vauquelin by decomposing the purate of ammonia by means of the galvanic battery. He got a colourless acid collected round the positive pole, while the alkaline substance round the negative pole was deeper coloured than ever. I suspect that these discordancies are connected with some diversity in the properties of the calculi from which the uric acid was obtained by the different experimenters.

Erythric Acid.

This acid was formed by Brugnatelli. His method was to dissolve uric acid in a slight excess of nitric acid, to evaporate slowly, and then put the solution by to crystallize in a cool place. These crystals became slowly purple when exposed to the air. The purple colour appears more rapidly if they be dissolved in water and a few drops of ammonia added. The nature of these crystals has not yet been satisfactorily explained. It would seem to be a salt, but what the constituents of this salt are I do not know. I have never had an opportunity of seeing Brugnatelli's paper, which puts it out of my power to enter into farther particulars respecting erythric acid. This name was given from the Greek word gulgos, red, in consequence of the red colour which it assumes when exposed to the air.

SECTION L.-OF ALLANTOIC ACID.

This is the name which M. Lassaigne has given to the acid History. discovered by Vauquelin and Buniva in the liquor amnii of the

* Prout, Phil. Trans. 1818, p. 420.

† Ann. de Chim. et de Phys. xxii. 334.