The sugars that are of interest here belong altogether to the pentose and hexose groups; _viz._, C₅H₁₀O₅ and C₆H₁₂O₆, respectively. Of the hexobioses, sucrose, maltose, and lactose are the most important, and of the hexotrioses, raffinose. In this manual, unless otherwise stated, the term dextrose corresponds to _d_ glucose, and levulose to _d_ fructose. In this connection, however, it should be noted that the levulose of nature, or that which is formed by the hydrolysis of inulin or sucrose is not identical in its optical properties with the _l_ fructose of Fischer. =46. Preparation of Pure Sugar.=—In using the polariscope or in testing solutions for the chemical analysis of samples, the analyst will be required to keep always on hand some pure sugar. Several methods of preparing pure sugar have been proposed. The finest granulated sugar of commerce is almost pure. In securing samples for examination those should be selected which have had a minimum treatment with bluing in manufacture. The best quality of granulated sugar when pulverized, washed with ninety-five per cent and then with absolute alcohol and dried over sulfuric acid at a temperature not exceeding 50° will be found nearly pure. Such a sugar will, as a rule, not contain more than one-tenth per cent of impurities, and can be safely used for all analytical purposes. It is assumed in the above that the granulated sugar is made from sugar cane. Granulated beet sugars may contain raffinose and so may show a polarization in excess of 100. This sugar may be purified by dissolving seventy parts by weight in thirty parts of water. The sugar is precipitated by adding slowly an equal volume of ninety-six per cent alcohol with constant stirring, the temperature of the mixture being kept at 60°. While still warm the supernatant liquor is decanted and the precipitated sugar washed by decantation several times with strong warm alcohol. The sugar, on a filter, is finally washed with absolute alcohol and dried in a thin layer over sulfuric acid at from 35° to 40°. By this process any raffinose which the sugar may have contained is completely removed by the warm alcohol. Since beet sugar is gradually coming into use in this country it is safer to follow the above method with all samples.[25] In former times it was customary to prepare pure sugar from the whitest crystals of rock candy. These crystals are powdered, dissolved in water, filtered, precipitated with alcohol, washed and dried in the manner described above.