The use of formalin for hardening gelatin has long been known, but it was left for Payne and Pullman to devise a commercial process for tanning pelt into leather by means of formaldehyde (H·CHO) solutions. Their process, which was patented, specified the use of alkalies in conjunction with formaldehyde or other aldehydes. The function of the alkalies is not very obvious, for it has been shown that formaldehyde will tan also in neutral and in acid solution. The precise action of the aldehydes is also as yet somewhat obscure, but it is noteworthy that very small proportions of formalin will give a complete tannage. It is probable that the action of formaldehyde is not perfectly analogous with that of its homologues, for it is a most reactive substance, and will certainly with proteids undergo reactions which are not analogous to those with other aldehydes. The leather obtained by tanning with formalin is quite white and resembles buff leather, but has advantages over the latter in that no bleaching is necessary. According to the patent specifications the pelt should be drummed in water and the tanning liquor--a solution of formalin and sodium carbonate--added gradually at 15-minute intervals. Up to 6 hours for light skins, and up to 48 hours for heavy hides, are required for complete tannage. The temperature is raised during the process from 100° to 118° F. The tanning liquor may be made from 16 lbs. of commercial formalin (36 per cent. formaldehyde) and 32 lbs. soda (80 per cent. Na{2}CO{3}) in 10-15 gallons of water. This should be added, one gallon at a time, to 4 cwt. pelt in 100-120 gallons of water. After tannage is complete the goods should be paddled with a 1-1/2 per cent. solution of ammonium sulphate to remove the soda, and "nourished" in a solution of soft soap and salt, about 2-1/4 per cent. of each on the weight of pelt. The goods are then dried out, and may be finished like chamois, buff, and buck leathers (Section III.). REFERENCES. Payne and Pullman, English Patent 1898, 2872. Bennett, "Manufacture of Leather," pp. 250 and 379. SECTION V.--SYNTHETIC TANNING MATERIALS In spite of much valuable work on the constitution of the vegetable tannins and the compounds usually associated with them, such as that of E. Fischer, K. Freudenberg and their collaborators on gallo-tannic acid, and that of A. G. Perkin on ellagic acid and catechin, we are still in the dark with respect to the constitution of the tannins which are of commercial importance, and any synthetic production of these materials is thus out of the question as yet. Attempts, however, have been made to produce artificially substances which possess similar properties to the tannins and which may be used for converting pelt into leather. Into this category fall some of the earlier attempts to synthesize gallo-tannic acid by heating gallic acid with condensing reagents.