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SECTION XXVIII.

OF CITRIC ACID.

Experiment 1. Saturate any quantity of boiling les mon juice, by adding to it, gradually, chalk, in small quantities, until the effervescence ceases, on adding to it a new portion of chalk, During this process a white precipitate will be formed, and fall down to the bottom, consisting of citric acid and lime (citrate of lime.) Separate this precipitate by transferring the whole on a strainer, and pour water over it till this fluid passes tasteless. Transfer the washed precipitate into a matrass, or other convenient vessel, and pour over it as much sulphuric acid as will neutralize the chalk employed, having previously diluted the acid with six times its weight of water. Boil the whole about half an hour, agitating it with a wooden spatula during the whole time, and then pour it on a filter, taking care to return the fluid, which passes through, back upon the filter until it becomes perfectly clear. Having done this, evaporate it in a shallow vessel to the consistence of a thin syrup, and leave it to crystallize undisturbed. The crystals obtained are citric acid; in order to obtain them in a s:ate of purity they must be re-dissolved, the solution filtered, and re-crystallized repeatedly.* Four parts of chalk require in general, for saturation, 94 parts of lemon juice. The citrate of lime produced amounts to about 7} parts. To decompose this, nearly 20 parts of sulphuric acid are necessary.*

Rationale. The citric acid combines with the lime into a citrate of lime, which is decomposed by sulphuric acid; sulphate of lime is formed, and citric acid is disengaged.

Remark. Citric acid exists in the juice of lenions and oranges; in unripe grapes, cranberries, bilberries, and a variety of other sour fruits.

* Accum.

Citric acid crystallizes in the form of rhomboidal prisms, which suffer no alteration from exposure to air. They are easily dissolved by water, and excite a very sour taste, which, when diluted, is exceedingly pleasant. One part of boiling water dissolves two of citric acid. It acts upon-iron, zinc, tin, lead, and various other metals. Sulphuric acid chars it, and forms vinegar. Treated with nitric acid it becomes converted into oxalic and acetic acid. Exposed to a red heat it becomes decomposed into carbonic acid, and carbonated hydrogen gas, and a little charcoal remains behind.

This elegant acid is very useful to manufacturers on account of its solubility. One ounce of cold water will dissolve th of an ounce of it, and boiling water will dissolve double its weight.

Experiment 2. This acid may be kept ready mixed with either of the dry carbonates of potash or soda; and, as no effervescence ensues till the mixed powder is put into water, we have a ready mode of making a pleasant saline draught.

Experiment 3. Dip a piece of white calico in a cold solution of sulphate of iron, and suffer it to become entirely dry. Then imprint any figures upon it with a strong solution of colourless citric acid, and allow this to dry also. If the piece be then well washed in pure warm water, and afterwards boiled in a decoction of logwood, the ground will be dyed either of a slate or black colour, according to the strength of the metallic solution, while the printed figures will remain beautifully white. This experiment is designed to show the effect of acids in discharging vegetable colours.

Experiment 4. If lemon juice be dropped upon any kind of buff colour, the dye will be instantly discharged. The application of this acid by means of the block, is another method by which calico-printers give the white spots or figures to piece goods. The crystallized acid in a state of solution is generally used for this purpose.

SECTION XXIX. .

OF KINIC ACID.

Experiment 1. Dissolve the salt of Peruvian bark in water; add oxalic acid till no further precipitate is formed; separate the precipitate, and evaporate the liquor: crystals of kinic acid will be produced.

Rationale. The salt of bark contains lime united with kinic acid; the former is separated by oxalic acid in the form of oxalate of lime, and the latter is obtained by evaporating the fluid.

Remark. This acid was discovered by Vauquelin. Its colour is brown. Its taste is acrid. It is

very

soluble in water. It is decomposed by fire, charcoal remaining behind.

SECTION XXX.

OF MUCOUS OR SACCHOLACTIC ACID.

Experiment 1. Take one part of gum arabic reduced to powder, put it into a retort, and pour over it two parts of nitric acid; heat the mixture gradually, keep it boiling for about a quarter of an hour, and then suffer it to cool; a white powder will separate, which, after being washed, is mucous acid.

Experiment 2. Mucous acid may be obtained, by treating sugar of milk with nitric acid, in a similar manner.

Remark. This acid was discovered by Scheele, who obtained it by treating sugar of milk with nitric acid. Hence he called it, saccho-lactic acid.

Mucous acid exists in the form of a white gritty pow. der. It is soluble in eighty times its weight of boil

ing water, but more soluble in alcohol. Its taste is slightly acid, but it sensibly reddens tincture of cabbage. It scarcely acts upon any of the metals. It forms soluble salts with potash, soda, or ammonia : but insoluble compounds with most of the rest of the alkalies and earths.

SECTION XXXI.

OF URIC ACID.

This acid was discovered by Scheele in the urinary calculi; it exists also in human urine. That species of calculus which resembles wood in its colour and appearance, consists almost entirely of uric acid. It was formerly called lithic acid, a name now superseded by the judicious remarks of Dr. Pearson, to whom we are indebted for a thorough knowledge of it. It may be found by boiling calculi in a solution of potash, and precipitating by means of muriatic or acetic acid. The white powder which falls is uric acid.

Uric acid crystallizes in thin acicular and brilliant crystals, of a pale yellow colour. It has very little taste or odour. It is almost insoluble in cold, and sparingly soluble in 300 parts of boiling water; it then reddens delicate vegetable blues, but it becomes very soluble when combined with an excess of pota h or soda. It is not acted upon by muriatic acid. Sulphuric acid, assisted by heat, decomposes it. It is soluble in nitric acid, and communicates to it a pink colour. The solution tinges animal matter of the same colour. It combines readily with alkalies and earths, and forms salts, which are decomposable by most of the other acids. Uric acid combined with soda is found crystallized in the human body, forming the gouty concretions of those afflicted with the gout. Uric acid is decomposable at high temperatures, and

S

furnishes carbonate of ammonia, and carbonic acid, with very little oil or water. It is also decomposed by the nitric and oxygenized muriatic acids.

SECTION XXXII.

OF MALIC ACID.

Experiment 1. Take the juice of apples, Saturate it with potash, and then add a solution of acetate of lead till it no longer occasions a precipitate; wash this precipitate, which is malate of lead; pour over it sulphuric acid till the liquor acquires an acid taste without any mixture of sweetness, and filter the whole, in order to separate the malic acid from the sulphate of lead which is formed.

Experiment 2. Malic acid is also obtained by adding to the expressed juice of house-leek, a solution of acetate of lead, till no further precipitate ensues. The precipitate, after being washed, may be decomposed by sulphuric acid as before.

Experiment 3. Malic acid may likewise be produced by distilling equal quantities of nitric acid and sugar,

till the solution acquires a brown colour. The sugar is thus partly converted into oxalic, and partly into malic, acid. The oxalic acid may be separated by mingling the solution with lime-water, until no further precipitate ensues; the malic acid is then left behind. In order to obtain it in a pure state neutralize it with lime, filter the solution, and mix it with alcohol. A coagulation now takes place ; for the water separates, and the malic acid, united to the lime, may be obtained by filtration. Having done this, let it be dissolved in water, and add to it a solution of acetate of lead until no further precipitate ensues. This precipitate, which is malate of lead, may be decomposed by sulphuric acid, as before directed.

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