I am indebted chiefly to Dr. W. H. Willcox, B.Sc, A.I.C., for the details connected with chemistry in this section, which has been written for readers who are acquainted with that science.

Stable impurities may be classed as follows : -

1. Exhalations which are given off from the lungs and skin, and which consist chiefly of carbonic acid and water.

2. Urine, dung, and the products formed by their decomposition.

3. Bacteria (germs of decomposition and disease), which are adventitious bodies.

The decomposition of urine may be explained as follows : -

According to Is. Pierre the urine of the horse is composed, on an average, of 90.5 per cent. of water, 4 per cent. of mineral matter, and 5.5 per cent. of organic matter, which is composed chiefly of urea (CON2H4) and hippuric acid (C6H5CO. NHCH1COOH). Z. Roussin suggests that during work, the former is converted into the latter by the increased action of the organs of breathing; for at such times the percentage of the hippuric acid is raised and that of the urea diminished; the converse being the case during rest. The decomposition of both urea and hippuric acid is effected by bacteria.

The Urea (CON2H4) breaks up under the influence of the micrococcus urece, and uniting with a portion of the water (H10) becomes converted into ammonium carbonate (NH4)2C03. Thus : -

C0N2H4 + 2H10 = (NH4)2CO3.

The ammonium carbonate remains partly in solution, and being volatile, partly escapes into the air. In both cases it becomes decomposed into ammonia gas (NH2), carbonic acid gas, and water. Thus : -

(NH4)2C03 = 2NH2 + H10 + C02.

The hippuric acid is decomposed by bacteria probably as follows: -

C6H5CO. NHCH1COOH + H10 = C6H5COOH + NH1CH1COOH. (Hippuric acid ) (Water.) (Benzoic acid.) (Amido acetic acid.)

The benzoic acid. would unite with the ammonia derived from the urea, and would form ammonium benzoate; and the amido acetic acid, by the action of bacteria, would partly break up into ammonia and acetic acid (CH2COOH); and the acetic acid would become converted into ammonium acetate.

The principal products of the decomposition of urine would therefore be : Ammonium carbonate, ammonia, ammonium benzoate, amido acetic acid, and a small quantity of ammonium acetate.

As the dung consists chiefly of cellulose and other insoluble substances, its products of decomposition are unimportant; the chief ones being indole, skatole, lactic acid, and a trace of carbolic acid. Indole and skatole are complex ammoniacal bodies, from which the dung derives its smell. The dung, mixed with the alkaline fluid resulting from the decomposition of the urine, forms an excellent breeding ground for bacteria, the growth of which is unaffected by the small quantity of carbolic acid which is present. The ferments secreted by some of the bacteria have, like those of pus, the power of dissolving albumen, and consequently they eat away those portions of the horses' hoofs that come in contact with the decomposing urine and dung. The fact of this liquid manure being alkaline, also helps in the destruction of horn; for the matrix (chiefly mucin, which is nearly of the same composition as albumen) of the cells of the horn is soluble in alkaline fluids. Prolonged maceration, even in water, also causes disintegration of mucin, and consequent destruction of horn. As ordinary water is not alkaline, and as it is not a good medium for the cultivation of bacteria, its action on horn would be far less destructive than that of a mixture of decomposing urine and dung. Besides its comparatively slow action on mucin, it softens, loosens, and consequently favours the breaking down of horn cells.

The ammonia in the air of the stable is an irritant to the eyes and organs of breathing of horses.

The ammonia has a bad effect on carriages, harness, and saddlery, which are exposed to its influence, by dissolving resin and fat with which it forms a soap. It also, like soda and potash (p. 49) darkens brown leather, by combining with the tannic acid which is in the leather. The varnish on carriages is made by dissolving certain resins in spirit, or in fixed or volatile oil, so that when the solvent evaporates, it leaves a thin film of resin or shellac, for instance, behind.

As alkalies have the property of dissolving resins, waxes, and fat, all of which are chemically allied to each other; ammonia in the air will spoil the varnish of a carriage, and will tend to remove fat and bees'-wax from leather, and will consequently make it dry and liable to crack. Coachbuilders remove the varnish from carriages in the same way by using a solution of ammonia, caustic soda, or caustic potash.

To take the case of the removal by ammonia of bees'-wax (which consists mostly of myricyl palmitate), or of a varnish containing bees'-wax, we have the following equation: -

C30H61C16H2102 + NH4H0 = NH4C16H21O2 + C30H6lOH. Myricyl palmitate. Ammonium Ammonium Myricyl hydrate. palmitate. alcohol (a soluble soap). (soluble).

With the exception of the watery vapour, the products of the decomposition of urine and dung do not appear to have any injurious effect on steel. Besides the removal of fatty matters, ammonia has seemingly no destructive action on leather, which, however, falls a ready prey to bacteria, in the event of their reaching it, by, for instance, its becoming soiled with decomposing dung or urine.

In the manufacture of leather, the skins are placed for a brief period in "dung bate," which is a mixture of dung and water, and which, owing to the action of bacteria, will destroy the skins if they are left too long in it. This action of bacteria, and the property ammonia has of removing fat and darkening leather, account for the fact that keeping saddlery and harness exposed to the emanations of a stable has a bad effect on them.