HO-C-H H-C-OH / \ / \ / \ / \ H-C-OH \ H-C-OH \ | O | O HO-C-H / HO-C-H / \ / \ / \ / \ / C-H C-H | | H-C-OH H-C-OH | | CH_{2}OH CH_{2}OH [alpha]-Glucose [beta]-Glucose It is assumed that the [alpha] modification (with its specific rotatory power of +105°) is the normal form for crystalline glucose, but that when dissolved in water it is changed into an _aldehydrol_, i.e., a compound containing two additional OH groups, which later breaks down again, into the [beta] modification (with its specific rotatory power of +22°). When dissolved in alcohol, this change does not take place because of the absence of the excess of water necessary to produce the intermediate aldehydrol form. There are other examples of the existence of the [alpha] and [beta] modification of glucose. For example, [alpha]-methyl-glucoside and [beta]-methyl-glucoside (specific rotatory powers, +157° and -33°, respectively) are both known, as well as several other similar glucoside arrangements. =Mannose.=--This sugar does not occur as such in plants; but complex compounds which yield _d_-mannose when hydrolyzed, known as "mannosans," are found in a number of tropical plant forms. The mannose which is obtained from these by hydrolysis is very similar to glucose in its properties, forms the same osazones as do glucose and fructose, exhibits mutarotation, etc. Mannose may also be obtained by oxidizing mannitol, a hexatomic alcohol, known as "mannite," which occurs in many plants, especially in the manna-ash (_Fraxinus ornus_), the dried sap from which is known as "manna." =Galactose= occurs in the animal kingdom as one of the constituents of lactose, or milk-sugar. It is also one of the constituents of raffinose, a trisaccharide sugar found in plants, and occurs as "galactans" in many gums and sea-weeds. The _d_-galactose, obtained by the hydrolysis of any of these compounds, is a faintly sweet substance which resembles glucose in many of its properties; having one characteristic difference, however, in that it forms mucic acid instead of saccharic acid when oxidized by concentrated nitric acid. These oxidation products are very different in their physical properties and this difference serves to distinguish between the two sugars from which they are derived.