The chief commercial source of this alkaloid is waste and damaged tea, from which it is prepared by extraction with boiling water, the tannin precipitated from the solution with litharge, and the solution then concentrated to crystallize out the caffein. It is further purified by sublimation or recrystallization from water. Coffee chaff and roaster-flue dust have been proposed as sources for medicinal caffein, but the extraction of the alkaloid from the former has not proven to be a commercial success. Several manufacturers of pharmaceuticals are now extracting caffein from roaster-flue dust, probably by an adaptation of the Faunce[134] process. The recovery of caffein from roaster-flue gases may be facilitated and increased by the use of a condenser such as proposed Ewe.[135] Pure caffein forms long, white, silky, flexible needles, which readily felt together to form light, fleecy masses. It melts at 235-7 deg. C. and sublimes completely at 178 deg. C., though the sublimation starts at 120 deg. Salts of an unstable nature are formed with caffein by most acids. The solubility of caffein as determined by Seidell[136] is given in Table I. TABLE I--THE SOLUBILITY OF CAFFEIN Solubility: Grm. Caffein per 100 Grm. of Sp. Gr. of Sp. Gr. of Temperature Saturated Saturated Solvent Solvent of Solution Solution Solution Water 0.997 25 2.14 Ether 0.716 25 0.27 Chloroform 1.476 25 11.0 Acetone 0.809 30-1 2.18 0.832 Benzene 0.872 30-1 1.22 0.875 Benzaldehyde 1.055 30-1 11.62 1.087 Amylacetate 0.860 30-1 0.72 0.862 Aniline 1.02 30-1 22.89 1.080 Amyl alcohol 0.814 25 0.49 0.810 Acetic acid 1.055 21.5 2.44 Xylene 0.847 32.5 1.11 0.847 Toluene 0.862 25 0.57 0.861 The similarity between caffein and theobromin (the chief alkaloid of cocoa), xanthin (one of the constituents of meat), and uric acid, is shown by the accompanying structural formulae. These formulae show merely the relative position occupied by caffein in the purin group, and do not in any wise indicate, because of its similarity of structure to the other compounds, that it has the same physiological action. The presence and position of the methyl groups (CH_3) in caffein is probably the controlling factor which makes its action differ from the behavior of other members of the series. The structure of these compounds was established, and their syntheses accomplished, in the course of various classic researches by Emil Fischer.[137]