i88 ANIMAL PROTEINS 



containing phenolic groups. They are semi-colloidal, passing 

 slowly through semipermeable membranes. They precipitate 

 gelatin, basic dyestufTs 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, condensa- 

 tion by phosphorus trichloride or similar reagents, and 

 condensation by heat alone. Illustrative methods will now 

 be given. 



Condensation by formaldehyde was the first method used. 

 The procedure is given by the Austrian patent 58,405. A 

 phenol, e.g. crude cresylic acid, is heated with the equivalent 

 amount of sulphuric acid for a few hours to ioo-2io C., 

 cooled, and formaldehyde added slowly whilst cooling and 

 stirring, in the proportion of one molecule of formaldehyde 

 to 2 molecules of phenol. The free mineral acid is neutralized, 

 and the resulting product is the syntan " Neradol." By 

 this procedure only water-soluble products are obtained, 

 but an alternative process is to heat the phenols in slightly 

 acid solution, and then to render soluble the resinous products 

 obtained by treating with sulphuric acid. The proportion 

 of formaldehyde to phenol used led Steasny to conclude that 

 the resulting products were diphenyl-methane derivatives 

 which polymerize to form molecules of considerable size. 

 The formaldehyde supplies the " carbon bridge." This 

 view was criticized by A. G. Green as too simple, and he 

 suggested the alternative theory that polymerization does 

 not take place at all, but that more advanced or higher 

 condensation products are formed ; he thought that o- 

 hydroxy-benzyl alcohols were first produced, that these 

 condensed with another molecule, and afterwards the process 

 was repeated. The result was a " colourless dyestuff," 



