It has been shown by Welbel and Zeisel,[132] that in the presence of twelve per cent of hydrochloric acid phloroglucin itself is condensed into dark insoluble compounds. When three molecules of furfurol and two molecules of phloroglucin are present, the bodies are both condensed and separated by continued action. When from one and a quarter to three parts of phloroglucin by weight are used to one part of furfurol, the weight of the precipitate obtained under constant conditions may serve sufficiently well for the calculation of the furfurol. The precipitates contain chlorin, which they give up even in the cold, to water. For these reasons the analytical data obtained by the method of Councler, given above, are apt to be misleading. It is probable also that similar conditions may to a certain extent prevail in the separation of furfurol with phenylhydrazin, and further investigation in this direction is desirable. For the present the very best method that can be recommended for the estimation of pentoses and pentosans is the conversion thereof into furfurol and the separation of the compound with phenylhydrazin acetate. =158. Estimation of Sugars by Fermentation.=—When a solution of a hexose sugar is subjected to the action of certain ferments a decomposition of the molecule takes place with the production of carbon dioxid and various alcohols and organic acids. Under the action of the ferment of yeast _Saccharomyces cerevisiae_ the sugar yields theoretically only carbon dioxid and ethyl alcohol, as represented by the equation: C₆H₁₂O₆ = 2C₂H₆O + 2CO₂. The theoretical quantities of alcohol and carbon dioxid obtained according to this equation are 51.11 percent of alcohol and 48.89 per cent of carbon dioxid. When the yeast ferment acts on cane sugar the latter first suffers inversion, and the molecules of dextrose and levulose produced are subsequently converted into alcohol and carbon dioxid as represented below: C₁₂H₂₂O₁₁ + H₂O = 2C₆H₁₂O₆ 2C₆H₁₂O₆ = 4C₂H₆O + 4CO₂. Cane sugar, plus the water of hydrolysis, will yield theoretically 53.8 per cent of alcohol and 51.5 per cent of carbon dioxid. In practice the theoretical proportions of alcohol and carbon dioxid are not obtained because of the difficulty of excluding other fermentative action, resulting in the formation especially of succinic acid and glycerol. Moreover, a part of the sugar is consumed by the yeast cells to secure their proper growth and development. In all only about ninety-five per cent of the sugar can be safely assumed as entering into the production of alcohol. About 48.5 per cent of alcohol are all that may be expected of the weight of dextrose or invert sugar used. Only sugars containing three molecules of carbon or some multiple thereof are fermentable. Thus the trioses, hexoses, nonoses, etc., are susceptible of fermentation, while the tetroses, pentoses, etc., are not.