The carbohydrates of chief importance, from an agricultural point of view, are starch and sucrose; while the alternation products of chief importance, derived therefrom by chemical and biological means, are dextrin, maltose, dextrose and invert sugar. =246. Sucrose, Levulose, and Dextrose.=—The purely chemical methods of separating these three sugars have been investigated by Wiechmann.[204] They are based on the data obtained by determining the percentage of reducing sugars, both before and after the inversion of the sucrose, and before and after the removal of the levulose. For the destruction of the levulose, the method of Sieben is employed, and attention is called to the fact that the complete removal of the levulose by this process is difficult of accomplishment, and is probably attended with alterations of the other sugars present. =247. Sieben’s Method of Determining Levulose.=—The decomposing action of hot hydrochloric acid on levulose, and its comparative inaction on dextrose are the basis of Sieben’s process.[205] The hydrochloric acid employed should contain about 220 grams of the pure gas per liter, that is, be of twenty-two per cent strength, corresponding to 1.108 specific gravity. If the substance acted on be invert sugar, its solution should be approximately of two and a half per cent strength. To 100 cubic centimeters of such a solution, sixty of the hydrochloric acid are added, and the mixture immersed in boiling water for three hours. After quickly cooling, the acid is neutralized with sodium hydrate of thirty-six times normal strength. Ten cubic centimeters of the hydrate solution will thus neutralize the sixty of hydrochloric acid which have been used to destroy the levulose. The work of Wiechmann discloses the fact, easily prevised, that the method used for destroying levulose is not always effective and that action of the reagent is not exclusively confined to the levogyrate constituent of the mixture. Nevertheless, data of reasonable accuracy may be secured by this process, which is best carried out as described by Wiechmann. In this connection the possibility of the polymerization of the dextrose molecules, when heated with hydrochloric acid, must not be overlooked. =248. The Analytical Process.=—The total quantity of invert sugar in a given solution is determined by the methods already given (=136, 141=.) After this has been accomplished, the levulose is destroyed as described above, and the dextrose determined by any approved method (=136, 140=). In the presence of sucrose, the sum of the reducing sugars is first determined as in =136, 142=. After the inversion of the sucrose, the invert sugar is again determined, and the increased quantity found, calculated to sucrose. The levulose is then destroyed by hydrochloric acid, and the dextrose determined as described above. The quantity of sucrose may also be determined by an optical method (=91, 92, 94=.).