_Weinmannia racemosa_ bark, the _tawhero towai_, or _kamai_ of New Zealand, contains 12-13 per cent. of tannin. CHAPTER IV. THE CHEMISTRY OF TANNINS. The essential constituents of tanning materials are various members of a large group of organic compounds called tannins or tannic acids.[D] [Footnote D: Ger. _gerbsäure_; in German the word _tannin_ denotes usually gallotannic acid only.] These bodies often differ widely both in chemical constitution and reaction, but have the common property of precipitating gelatin from solution, and forming insoluble compounds with gelatin-yielding tissues. By virtue of this power, they convert animal hide into the insoluble and imputrescible material called "leather." They are mostly uncrystallisable; and all form blackish-blue or blackish-green compounds with ferric salts, and in common with many other organic substances are precipitated by lead and copper acetates, stannous chloride, and many other metallic salts, and those of organic bases, such as quinine. In some cases, the tannin combines with the base only, liberating the acid; but frequently the salt as a whole enters into combination. This is the case with the precipitates formed with lead and copper acetates. With alkalies, the tannins and many of their derivatives give solutions which oxidise and darken rapidly, usually becoming successively orange, brown, and black. A. H. Allen has shown that these bodies also give instantaneously a deep-red coloration with a solution of potassium ferricyanide and ammonia. The reaction is one of considerable delicacy. Tannins are more or less soluble in water; and freely so in alcohol, mixtures of alcohol and ether, and ethyl acetate, but scarcely in dry ether alone, nor in dilute sulphuric acid; and insoluble in carbon disulphide, petroleum spirit, benzene, and chloroform. From their amorphous character, tannins are extremely difficult to purify; and when, as is frequently the case, two or more tannins occur in the same plant, it is often quite impossible completely to separate them. Owing to their considerable differences in character, no general method of purification can be given, but the following processes will be found in many cases to give good results. For the special methods adopted by different investigators, the original memoirs must be consulted, references to many of which will be found in the following pages. Preparation and Purification of Tannins.