Eitner[124] has pointed out the important effect that differences of basicity have on the tanning properties of chrome solutions. Normal chrome sulphate or chrome-alum colours the leather quickly and equally throughout, and swells the pelt on account of its practically acid character, but gives a thin and lightly tanned leather, from which much of the chrome washes out, unless it is at once “neutralised” in alkaline solutions. As the chrome solution is made more basic, the tannage penetrates more slowly, but is heavier and more thorough, the colour is darker and bluer, and much less of the chromic salt is removed by washing with water. When the basicity becomes excessive, the solution becomes unstable, and decomposes on dilution with water into a very basic salt which is precipitated, and a more acid solution than that given by a moderately basic salt. The effect of such solutions on the leather is very unsatisfactory, producing the bad effects both of too acid and too basic salts. The pelt is apt to be swollen and lightly coloured by the more acid salt, but at the same time the actual tannage proceeds very slowly, and in extreme cases it is difficult to tan through, while the surface becomes over-tanned, and the grain often tender and even brittle from the incrustation of precipitated basic salt. Eitner likens the effect of the more acid liquors to the quickly penetrating and lightly tanning vegetable tans, such as gambier, and that of the more basic to the heavier tannages, such as valonia; and within limits, advantage may be taken of these facts in adjusting the liquors to the character of the leather it is desired to produce. In sulphate liquors, he considers the salt CrOH.SO₄ as most suited to general use, and in the case of chrome-alum, this is produced by the use of 286 parts of soda-crystals, or 106 parts of dry sodium carbonate (1 molecule) to 998 (or practically 1000) parts by weight (1 mol.) of chrome-alum. (In using washing soda, care must be taken to employ _clear_ crystals of the salt, and not those which have become white by loss of water.) In place of soda, Eitner makes a similar basic liquor by boiling 1000 parts of chrome-alum with 248 parts (1 mol.) of sodium hyposulphite until the whole of the liberated sulphurous acid is driven off, and the sulphur deposited. In comparative experiments by the Author, no difference could be detected between the tanning effects of the two solutions, and that with soda is both cheaper and more easily made. If the solution with hyposulphite is not boiled, a more acid liquor results, in which part of the chromium is combined with sulphurous acid, forming an unstable compound which may prove useful in certain cases. [124] Gerber, 1901, pp. 3 _et seq._