Tl{ANi!«ACTIONS OF SECTION B. 541 



3. On the Specific Rotatory Foiver of Cane and Invert Sugar. 

 By Alfeed H. Allen, F.G.S. 



The angular rotation produced by a plate of quartz of 1 mm. in thickness is 24 

 degrees for the mean yellow ray or transition tint. In Soleil's polarising sacehari- 

 meter the 24 angular degrees are graduated into 100 divisions, and in using the 

 instrument, a solution of cane sugar is employed of such concentration that a column 

 of 2 decimetres in length shall cause a de^"iation of exactly 24 degrees, or 100 

 di^Tsions. 



If S be the apparent specific rotatory power of an optically active substance in 

 solution ; a the angular rotation observed ; I the thickness in decimetres of the 

 solution traversed by the ray of polarised light ; and c the number of grammes of 

 solid in each 100 c.c. of solution, the -salue of S can be fovmd by the folio-wing 

 equation : — 



S= ^ 



7 "^ 



100 



It is agreed by nimierous observers that the apparent specific rotatory power of 

 cane sugar in aqueous solutions, containing at least 10 per cent, of the solid, is 

 + 73-8° for the transition tint. Substituting this value for S in the above equation, 

 24° for a, and 2 for I, we obtain — • 



yo.g, 24 



1 T; whence c = 16-26. 



2 X — 

 100 



Hence the proper weight of sugar to be taken for use with Soleil's saccharimeter is 



16-26 grammes, and not 16-19, 16-35 grammes, or any different weight. If it be 



contended that either of these alternative quantities is the right one to employ, it 



follows that + 73-8° is not the correct apparent specific rotatory power of cane 



sugar. 



According to Tuschmidt, Casamajor, and many other observers, a solution of 

 cane sugar which, before inversion, shows a deviation of + 100 Soleil di\'isions, 

 gives after inversion a negative rotation of 44 divisions at 0° C, decreasing by 1 

 division for each rise of 2° 0., so that the inverted solution -will show a deviation of 

 - 37 at 14° C, and - 36-5 at 15° C. 



Manj' writers on the rotatory power of invert si;gar have overlooked the fact 

 that inversion causes an increase in the weight of solid matter in the solution, 95 

 parts of cane sugar yielding 100 parts of invert sugar. This increase of weight 

 ought to be taken into account in calculating the specific rotatory power of invert 

 sugar, which at 15° C. is really — 25-6° : — 



^ -86-5x -24 __ 

 ^ 16-26 :^5-^ 



95 



This number corresponds to a value of — 25-94° for Sj at 14° 0., instead of — 25-0, 

 as generally stated. If 16-19 grammes be adhered to as the normal weight of 

 sugar per 100 c.c, the value of Sj at 14° C. becomes — 26*05°, against — 25-0° as 

 usually taken. 



If the value of Sj for invert sugar be taken at — 26° (the mean of the above 

 values), and O'SuUivan's figure + 57-6° be adopted as the value of Sj for dextrose, 

 then the specific rotatory power of Isevulose at 14° C. is — 109-6°, instead of — 106°, 

 as usually taken. 



26 X 2 + 57-6 = 109-6. 



To sum up, the corrected values of Sj are as follow : — 



Cane Sugar + 73-8. 



Invert Sugar - 25-6 at 15° 0. 



Dextrose + 57-6. 



L^^ndose - 108-8 at 15° 0. 



