5 c 4 THE POPULAR SCIENCE MONTHLY. 



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 crys- 

 talline 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 com- 

 pletely, 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 pres- 

 ent in a substance. 



Alkaline copper solutions, when heated with most sugars, have 

 their copper reduced to the form of a suboxide (Cn a O). 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 dark- 

 ened. The analyzer is then turned until the field is equally illuminated 



