502 CHAPTER XXV 



flame, instead of using exhaust steam ; and a water vacuum pump may be 

 used in place of a connection to the evaporator. 



Determination of Cane Sugar. — The method adopted generally for deter- 

 mining the quantity of sugar present in a material is based on the measure- 

 ment of the rotation of a ray of polarized light. Within the limits specified in 

 detail below, this rotation is proportional to the concentration of the solution 

 through which the ra}^ passes, and to the length of the column of the solution. 

 Hence, if the rotation for any one concentration and for any one length of 

 column be known, an unknown concentration can be estimated when the rota- 

 tion for that concentration and for a known length of column is determined. 



Specific Rotation. — The specific rotation is that rotation expressed in 

 angular degrees when the light passes through a column of length lo cms. of 

 solution containing one gram in one cubic centimetre. This rotation is 

 referred to some source of monochromatic light, the yellow line of the sodium 

 spectrum or the green line of that of mercury being selected for use. Obser- 

 vations made on these bases are referred to as [a]'^ and [w]*/,^. Rotations 

 measured with ordinary white light are referred to as [a]_, (French 

 jaune yellow, referring to the elimination of tlie yel'ow rays. V. page 477). 



For convenience of reference, the specific rotations of some important 

 sugars are collected here, where p is the percentage, and c the concentration 

 in grams per 100 c.c. The rotations are referred to [a]"^ 



Sucrose^ = 66-386 -f- 0-015035^ — • 0003986 /)2. 



Sucrose^" — 66-438 +0*010312^ —0-003545/)^. 



Sucrose^i = 66-529 (for c = 26). 



Glucose^^ __ 52-50 +0-018796/) + 0-00051683 /)2, 



Fructose^^ = — [101-38 — 0-56 ^ + 0-108 (c — 10)]. 



Raffinose 5F2O 14 = 104-5 [for c == 16-6]. 



Maltose^-^ = 140-375 — 0-01837/) — 0-095 t. 



Lactose^^ = 52-5 for t = 20° C. 



Mannose^'^ = 12-96 for t — 20° C. 



Galactose^^ = 83-883 + 0-0785 p — 0-209 ^• 



for values /> 5 to 35 and t 10° C. to 30° C. 



Arabinose^'' = 105-4 for ^ = 18° C. 



Xylose'-^" = 18 - 095 + o - 06986 /) for /) 3 to 34. 



Dextran^i = 230. 



Levulan^^ _ — 221 for / = 20° C. 



Xylan23 = 70 to 85. 



Normal Weight. — In the process of saccharimetry a certain weight of 

 cane sugar observed in the polarimeter under fixed conditions gives on the 

 arbitrary scale a reading of 100 degrees. This weight is called the normal 

 weight. The original normal weight was devised by Biot,^* who took as a 

 standard of rotation that afforded by a plate of quartz i mm. thick and cut 

 perpendicular to the optic axis. Transformed into sugar terms he found 

 that this rotation was that produced by 16-47 grams of sugar dissolved in 

 100 c.c. and observed in a tube 20 cms. long. In combination with Clerget 

 he reduced this figure to 16-35 grams, a figure for which the older Soleil- 

 Duboscq instruments are graduated. Girard and Lunes^-^ found 16 - 19 grams, 

 a figure altered to 16 - 29 grams in the determinations of Mascart and Benard,^^ 

 which now is the accepted standard. This weight has always been referred 

 to 100 metric centimetres. 



