=Fructose= (levulose, honey sugar, or "diabetic" sugar) occurs along with glucose in the juices of many fruits, etc. It is a constituent of sucrose, of raffinose, and of the polysaccharide inulin, from which it may be obtained by hydrolysis. It is a ketose sugar, reduces Fehling's solution, forms the same osazone as glucose, and is easily fermentable by yeast. Its sweetness is slightly greater than that of ordinary cane sugar. _d_-fructose (the ordinary form) is easily soluble in water, and is strongly levorotatory, its specific rotatory power at 20° C. being -92.5°; it is unique in the very large effect which is produced in its rotatory power by increasing the temperature of the solution; at 87° its specific rotatory power is reduced to -52.7°, exactly equal to but in the opposite direction of the effect of glucose; hence, _invert sugar_, which is a mixture of an equal number of molecules of glucose and fructose, and which has a specific rotatory power of -19.4° at 20° C., becomes optically inactive at 82° C. =Sorbose= is the only other ketohexose which has any importance in plant chemistry. It does not occur free in plants, but is the first oxidation product from the hexatomic alcohol, sorbitol, which is present in the juice of the berries of the mountain-ash. Sorbose is a crystalline solid, which is not fermentable by yeast, but which otherwise closely resembles fructose. DISACCHARIDES The disaccharides, having the formula C_{12}H_{22}O_{11}, may be regarded as derived from the monosaccharides by the linking together of two hexose groups with the dropping out of a molecule of water, in the same way that many other organic compounds form such linkages. That this is a perfectly correct conception, is shown by the fact that, when hydrolyzed, the disaccharides break down into two hexose sugars, thus C_{12}H_{22}O_{11} + H_{2}O = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}. With all known disaccharides, at least one of the hexoses obtained by hydrolysis is glucose; hence all disaccharides may be regarded as glucosides (C_{6}H_{12}O_{5}·R) in which the R is another hexose group.