If, instead of using neutral iron salts, basic ferric salts (which may be obtained by dissolving ferric oxide in solution of neutral ferric salts, or by oxidising ferrous sulphate with manganese black oxide, or nitric acid) be employed, much larger quantities are absorbed by the hide, and if this be fixed with soap baths and finished with a moderate quantity of oil, a gain of weight--approaching 50 per cent. of the finished leather, or about the same as that given by bark, may be obtained. The leather, however, has by no means the same resistance to wet and decay as bark-tanned leather, and invariably has a tendency to crack when sharply bent. The process has been most carefully worked out by Professor Knapp, and was patented and worked commercially for a short time in Brunswick, but apparently without financial success. Professor Knapp's method is as follows:--The iron solution is prepared by adding nitric acid to a boiling solution of ferrous sulphate (green vitriol) till the iron is completely oxidised to the ferric condition. To this, ferrous sulphate is again added so long as it continues to cause effervescence. The resulting solution is a clear orange, and of more or less syrupy consistence, and may be evaporated without decomposition or crystallisation to a transparent varnish. The hides are unhaired and prepared for tanning in the usual way, and are then handled in solutions of the iron salt, which are at first weak, and are gradually strengthened. Skins are tanned in 2-3 days, and the heaviest hides in a week. After tanning, the hides are stuffed in a drum ventilated through the axes, very similar to that described under "crown leather," p. 213, with an insoluble iron-soap made by precipitating soap solution with the iron-liquor; or the iron-soaps may be formed in the hide by the alternate use of iron and soap solutions, as already described. The leather is finally saturated with a solution of stearin and paraffin, to render it waterproof.