The only other bye-product of the leather trades is waste leather itself. For small pieces of leather there is always some little opening in producing small articles, such as washers for taps, etc., and there is also the possibility of shredding or pulping and making an artificial leather. The best leather substitutes, indeed, are made from waste leather. Nevertheless, there is always a certain amount of waste leather which only finds an outlet in the fertilizer factory. Such material is usually steamed or roasted to make it brittle, and then crushed in a disintegrator. It is then mixed in with other materials, but is sometimes solubilized by the action of sulphuric acid. Leather seldom contains less than 30 per cent. protein. REFERENCES. "Chemical Fertilizers and Parasiticides," S.H. Collins, M.S., F.I.C. (Companion volume in this series on Industrial Chemistry.) "Wool Wastes," Part II., Section V., p. 75. "Hoofs, Horns, Leather," Part III., Section II., p. 115. "Gelatine, Glue, and Allied Products," T. Lambert. SECTION II.--BYE-PRODUCTS OF THE GELATINE AND GLUE TRADES From the skin gelatine and glue trades the most valuable bye-product is the grease, which is obtained from the "fleshings" of the adipose tissue. These fleshings are themselves a bye-product of the leather trades. The recovery and purification of this grease has been dealt with previously (Part V., Section II.). In the skin glue trade the only other bye-product is the residue from the extraction process (Part V., Section III.). This residue is known usually as glue "scutch," and is composed of the proteins of the skin which are insoluble in hot water. These insoluble portions are obtained from all layers of the skin. There is much hair often in scutch, the hyaline or glassy layer (grain), and the elastic fibres of the corium are also insoluble, and a proportion is derived from the fibres of the adipose tissue on the flesh side. All these portions are fairly rich in nitrogen, and the scutch has, therefore, considerable value to makers of fertilizers. It is liable to contain also a percentage of grease, which is usually removed by steaming under hydraulic pressure. This process recovers a valuable bye-product and increases the manurial value of the scutch. There is always left in scutch some of the gelatinous skin substance which, strictly speaking, should have been removed during extraction. There is, however, a practical limit beyond which it does not pay to do this. When this limit is reached the cost of steam in extracting, and also in evaporating and drying, together with the loss of time and labour involved by occupation of the plant, is greater than the value of the possible product.