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the atmosphere or from oxidizing agents, with the greatest avidity, becoming brown and ultimately black.
THE HYDROGEN SALT, or gallic acid (H2, C,H, 03), is a solid body, crystallizing in silky needles, which have no odour, but an astringent taste: 1 part dissolves in 100 parts of cold, or in 3 of boiling water; it is very soluble in alcohol, but less so in ether. The gallates generally formed are those which have the formule
H, M, C, H, O, and H M,,C, H, Og; those represented by M, C, H, O, are but rarely produced.
THE POTASSIUM and Sodium Salts are soluble; those of BARIUM, STRONTIUM, and CALCIUM are but slightly soluble in water. They all have the general formula H, M, C, H, O..
The Magnesium and ZINC Salts are white precipitates, of the formula HM,, C, H, 0s, and very slightly soluble in water.
THE FERROUS Salt does not appear to have been obtained.
The Ferric Salt is produced by the addition of a ferric salt to a solution of a gallate; the liquid instantly becomes of a bluish black colour. If this liquid be boiled, it decolourizes with escape of carbonic acid gas and formation of a ferrous salt.
THE COPPER, MERCURY, and SILVER Salts appear to be unknown.
THE LEAD Salt is produced by the action of a solution of acetate of lead upon a solution of gallic acid, as a white precipitate having the composition HPb2, C, H, O., with water of crystallization. It dissolves easily (when recently precipitated) in concentrated acetic acid. Excess of this reagent is a perfect means of precipitating gallic acid.
This acid-radical may be still further recognized by
a. Its rapid oxidation, with formation of a brown colouring matter, when an alkaline solution containing it is freely heated in contact with air.
B. It may be distinguished from tannic acid by its not producing a precipitate in solution of gelatine, unless previously mixed with a solution of gum.
SALTS OF THE TANNIC RADICAL, OR TANNATES. Tannin or tannic acid is a name given to that large class of bodies which form the astringent principle in various parts of many plants. The individual acid here to be described, as most commonly met with, is that to which the name of gallotannic acid has been given, from its occurrence in nutgalls, from which it is always extracted by the joint action of ether and water.
THE HYDROGEN SALT, tannic or gallotannic acid (H,C,H,0,), is obtained as a white or slightly yellow spongy mass, glistening frequently from being an aggregation of crystals. It is very soluble in water, alcohol, or anhydrous ether.
The gallotannates generally contain 2 eqs. of metal, but sometimes 3 eqs.; consequently their radical must be regarded as tribasic.
THE POTASSIUM and SODIUM Salts are very soluble in water.
THE FERROUS SAlt is white and gelatinous, and sufficiently soluble in water not to be precipitated from dilute solutions.
The Ferric Salt is produced by adding a ferric salt to a solution of a gallotannate: it is a bluish black precipitate. Its formula is probably Fe, H, C, H, ,.
THE ZINC Salt is white. THE CUPRIC Salt is brownish yellow. THE SILVER Salt is reddish brown.
THE MERCUROUS Salt is yellow, and THE MERCURIC SALT brickred.
THE LEAD Salt is white. Its composition is probably Pbg, C2H,2017
This radical may also be recognized and distinguished from that of the gallates by its property of precipitating gelatine : when an aqueous solution of gallotannic acid is mixed with an aqueous solution of gelatine, a fine filmy or gelatinous precipitate immediately forms. This is the reason of the employment of oak-bark, and other substances which contain these astringent principles, in the process of tanning.
Of acid-radicals produced by the union of carbon with nitrogen, hydrogen, and oxygen, one only needs description here, viz. the uric.
SALTS OF THE URIC RADICAL, OR URATES. This radical is of animal origin: the chief source from which its salts are prepared is the excrement of serpents, which consists almost wholly of urate of ammonium. This radical is bibasic.
Uric acid and urates, when heated, in common with nearly all organic bodies containing nitrogen, evolve the odour of burning feathers.
aqueoediately to other
THE HYDROGEN SALT (H,C, N, H,O, or H, Ū), or uric acid, occurs in delicate white needles; it dissolves very sparingly in cold water; in sulphuric acid it is soluble, forming a brown solution, from which water reprecipitates the uric acid.
Urates, with the exception of THE POTASSIUM and Sodium SALTS, are almost all insoluble in water: it is very remarkable that THE AMMONIUM Salt is extremely insoluble in water; it is, however, dissolved by chloride or phosphate (Na, HPO ) of sodium, the sodium salt being formed.
THE CALCIUM SALT is white; THE FERRIC SALT brown; THE CUPRIC SALT green; THE SILVER SALT white, rapidly becoming black if the liquid be heated; THE MERCURIC and THE LEAD SALTS are white.
The uric radical may be easily identified by the following processes of decomposition :
a. Uric acid, if heated alone, evolves the odour of burnt hair together with that of cyanide of ammonium, a carbonaceous residue being left.
B. If uric acid or a urate be mixed with solid hydrate of potassium and heated in a narrow tube, ammonia will be evolved, and cyanide or cyanate of potassium remain in the tube. The two latter products may be identified in the manner already pointed out (pp. 338-341).
y. If uric acid be dissolved in dilute nitric acid, the solution evaporated just to dryness in a porcelain dish, and a glass stopper moistened with strong ammonia water be held over the residue in the dish, the magnificent purple colour of murexide is produced, which is quite characteristic.
In the Table which immediately follows, as indeed in all similar Tables, reference must be made to the reactions for confirmatory and discriminatory tests.