Page:Popular Science Monthly Volume 21.djvu/518

504 being only a little greater. It is much less soluble and much less sweet than cane-sugar.

Both lactose and dextrose, where freshly dissolved from the crystalline state, have a rotatory power nearly double the normal. This peculiarity is called "birotation." Milk-sugar ferments when mixed with yeast, but not so readily as grape-sugar or dextrose. The fermented milk forms a mild alcoholic beverage much prized in some countries.

Most sugars readily combine with lime, and with the alkalies, and also with many of the ordinary salts. Cane-sugar especially combines easily with bases almost like an acid, forming salts which are called sucrates.

Many metallic compounds help the crystallization of the sugars, and such salts have been used in the refining of sugar for this purpose. Owing to the difficulty, however, of removing these compounds completely, the practice has been generally abandoned.

The action of sugars on copper compounds is of especial interest, because it is used as a means of estimating the quantity of sugar present in a substance.

Alkaline copper solutions, when heated with most sugars, have their copper reduced to the form of a suboxide (Cn2O). Of the sugars which act in this way, I may mention grape-sugar, lactose, dextrose, and maltose. Pure cane-sugar does not act upon copper solutions until after it has been converted into invert-sugar. Dextrine or starch gum is likewise inactive. The copper solution generally employed for the estimation of sugar contains the copper in the form of a tartrate, with some sulphate of sodium and an excess of sodium hydrate in the mixture. It is called "Fehling's solution."

The specific rotatory power of a sugar is its property of twisting the plane of polarized light either to the right or left. The instrument used to determine this is called a polariscope, or saccharimeter. The instrument in more common use has an ordinary oil or gas lamp as the source of light. By quartz plates this light is modified in character so as to produce a tint most sensible to change. This is called the transition tint, or teinte de passage. It is a purplish color, which on the one side changes to blue, and on the other to a rose-red. In the last few years instruments using a monochromatic light are coming into use, and they have some advantages over the other kind. The one-color light is produced by passing the rays from a sodium-flame through a crystal of bichromate of potassium, by which a pure yellow is obtained.

The field of view in these instruments has only half its area filled by a quartz plate. When the instrument is adjusted to zero the quartz semi-disk offers no opposition to the passage of the light. Interposing, however, a tube containing a sugar solution, one half the field is darkened. The analyzer is then turned until the field is equally illuminated