The soluble mineral salts are detected by igniting the residue left after evaporation of a separate portion in a porcelain crucible.[K] From unweighted leather, the quantity is very small. The ash is exhausted with a few _c.c._ of distilled water, which will dissolve most sulphates and chlorides, which may be detected in small portions of the solution by baric chloride and silver nitrate respectively. Baric chloride and lead acetate are precipitated by a drop of sulphuric acid, and the latter is blackened with ammonic or sodic sulphide. Lime is precipitated by addition of ammonic chloride, ammonia, and ammonic oxalate; magnesia by the subsequent addition of sodic phosphate (see p. 109). The carbonates in the insoluble part (mostly derived from salts of organic acids) may be taken up by dilute hydrochloric acid and tested separately, or the acid may be used at first. Any residue undissolved by the acid is probably lead chloride, and will be dissolved by hot water. [Footnote K: A platinum crucible must not be used for fear of its destruction by lead, unless this metal has been proved absent.] _Estimation of ash._--The leather in small pieces (either after or before extraction with water) is incinerated in a porcelain crucible. The ash is extracted with hydrochloric acid. The insoluble portion may contain barium sulphate (barytes), lead sulphate, sand, clay, &c. For further examination, ordinary chemical text-books must be consulted. Any large amount of ash indicates weighting. Müntz found only about 0·5 per cent. of ash from bark-tanned leather. _Determination of hide substance._--It is sometimes of interest to determine the proportion of dry hide-substance in a sample of leather, but there is no known means of doing this directly. If, however, the leather be dried, finely powdered by rasping, and the nitrogen determined by combustion, either with soda-lime (Will and Varrentrapp's method), or with copper oxide (Dumas), the hide-substance may be calculated, since tannin contains no nitrogen. Müntz found unhaired skin dried at 230° F. (110° C.) to contain 51·43 per cent. of nitrogen (compare also p. 20). DETERMINATION OF FREE ACIDS IN TAN-LIQUOR. The lime-water method mentioned on p. 172 is, from its simplicity, well suited for daily use in the tannery as a control method for ordinary working; but where it is necessary to make very exact estimations, or to determine the various acids separately, it is not so satisfactory as one recently published by Kohnstein and Simand (Dingl. Polyt. Jour., 1885, cclvi. 38). The acids usually present in liquor consist of several members of the fatty or acetic group, which distil over with boiling water, of other non-volatile organic acids, and sometimes sulphuric acid, which is added to assist the swelling of the leather.