The first commercial success in this direction was attained by Stiasny, who produced condensation products of the phenolsulphonic acids, to which products he gave the general name of "syntans" (synthetic tannins). The Badische Co. placed one of these products on the market as "Neradol D," and later took out subsidiary patents for the manufacture of similar products by slightly differing methods of productions. Since the outbreak of the European War such patent rights have been suspended, and several British firms have been manufacturing synthetic tanning materials by similar methods, but doubtless with developments and improvements of their own discovery. These products (_e.g._ Cresyntan, Maxyntan, Paradol, Syntan, etc.) are now in use in many factories, and assist rather than substitute the vegetable tannins in producing leather of the desired colour and quality. These synthetic tanning materials resemble the vegetable tannins in the following respects. They are organic acids containing phenolic groups. They are semi-colloidal, passing slowly through semipermeable membranes. They precipitate gelatin, basic dyestuffs and lead acetate, give a violet-blue colour with ferric salts, and convert hide into an undoubted leather. They differ from the vegetable tannins in that they contain sulphur and sulphonic acid groups, but they agree in that both are aromatic derivatives. In each case the tanning effect is diminished by alkalies, but the synthetic materials are the more sensitive. =Methods of Manufacture.=--There are, broadly speaking, three types of method by which these condensation products are produced, viz., condensation by formaldehyde, condensation by phosphorus trichloride or similar reagents, and condensation by heat alone. Illustrative methods will now be given.