These methods are applicable to the dried or highly concentrated extracts of many tanning materials. Many tannins may be separated from their strong aqueous solution in a state of considerable purity by first agitating with ether to remove gallic acid, and then saturating with common salt, and shaking well with acetic ether, which takes up the tannin. Another method is to extract with alcohol, evaporate to a small bulk at as low a temperature as possible, and treat at once with a considerable quantity of cold water. The infusion is then precipitated with successive small quantities of lead acetate; the first and last portions of the precipitate are filtered off and rejected as contaminated with colouring matters and other impurities, while the remainder, after rapid washing, is suspended in water and decomposed with sulphuretted hydrogen. The filtrate is shaken with ether to remove gallic acid, and the aqueous portion is evaporated at a low temperature in a partial vacuum to a thin syrup, and the drying completed over sulphuric acid in vacuo.
General Chemistry.
The natural tannins are all compounds of carbon, hydrogen and oxygen only. They all contain the benzene group of carbon atoms, but their ultimate structure is, except in the case of gallotannic acid, very imperfectly understood, and probably differs considerably in type in different members of the family.
In order to make clear to those readers who have not studied modern organic chemistry, what we do know on the subject, a few words of introduction will be necessary. All organic compounds contain carbon, in combination with hydrogen, and very frequently also with oxygen, nitrogen, and other elements. A single atom of carbon is able to combine with 4 atoms of hydrogen, as it does to form marsh gas, or methyl hydride, CH4. Other elements may be substituted for the hydrogen; for instance, if we replace 3 of the hydrogen atoms with chlorine, we obtain chloroform, CHCl3. Again an atom of oxygen may be inserted between the carbon atom and one of the hydrogen atoms, producing methyl hydroxide or wood spirit. The group CH3 is called methyl, and we may substitute in wood spirit this entire methyl group for one of the atoms of hydrogen, when we shall have ordinary alcohol, C2H5OH. This building-up process may be repeated almost ad infinitum, producing a whole series of alcohols of higher and higher boiling point as the atoms of carbon become more numerous. Again, if in wood spirit we substitute an atom of oxygen for 2 of the remaining atoms of hydrogen we obtain formic acid, CHO.OH, the first of a long series of acids, of which the second, corresponding to ordinary alcohol, is acetic acid, and the highest members, such as stearic acid, C18H35O.OH, are solid fats. Hence the whole series are commonly called the fatty acids. A few structural formulæ will serve to make these points clearer, but it may be well to say that such formulæ must be taken simply as indicating the order in which the different atoms are united, and in no sense their actual position in space. The atoms in a molecule are held together by attractions and are in continual motion, so that they are more comparable to the planets of the solar system than to a rigid shape.
| Methyl Hydride. | Methyl Alcohol. | Common Alcohol. |
| H | H──C──H | H | H | H──C──O──H | H | H H | | H──C──C──O──H | | H H |
| Chloroform. | Formic Acid. | Acetic Acid. |
| Cl | H──C──Cl | Cl | O ║ H──C──O──H | H O | ║ H──C──C──O──H | H |
In benzene, C6H6 we have a compound of another type. There is reason to think that the carbon atoms in this case are united in a ring, as shown,
H H H
| | |
C──C══C
║ |
C──C══C
| | |
H H H
This benzene group forms the foundation of an immense number of bodies known as the aromatic series, to which belong aniline, carbolic acid, picric acid, gallic acid, and a host of other compounds important alike in a scientific and commercial sense, and among which we may pretty safely group the whole of the tannins. Commencing with benzene, we may, by inserting atoms of oxygen, produce a series of alcohols or phenols, of which common phenol (usually but incorrectly called carbolic acid) is the first.
The following table gives a general view of some of these, so far as they are known, with their corresponding acids:—