SUGAR 



i29 



may be varied at pleasure, by rotating the Nicol's prism , by means of the milled 

 head b. If, however, we interpose between p' and p", the tube c,fg. 1919, filled with 



a solution of cane-sugar, and the ends closed with glass, the semi-discs will be 

 differently coloured. Cane-sugar, possessing the power of circular polarisation, 

 combines with the rotating power of the half-disc which turns the ray to the right, 

 but tends to neutralise the half-disc, whose direction is the reverse. By increasing 

 or diminishing the thickness of the wedges of quartz 1 1', to the extent required for 

 counteracting their rotation to the right, and causing the semi-discs to reassume the 

 same colours, we have a means by the graduated scale e d , v i/, of measuring the 

 rotating power, which is exactly proportional to the amount of cane-sugar, tempera- 

 tures being equal, and no foreign substance having the power of circular polarisation 

 being present. 



To apply this method, the deviation must be known which is produced by a solu- 

 tion of sugar of known strength. For this purpose a given weight, e, of sugar is 

 dissolved in such a quantity of distilled water that the solution occupies a given 

 volume, V. Sufficient of this solution is taken to fill a tube of certain length, and 

 the deviation suffered by the plane of polarisation of the luminous ray passing through 

 this tube is measured. Let this deviation be a. Let then other quantities of sugar 

 be dissolved in sufficient water to give the same volume of solution, V ; and let the 

 deviations produced by these solutions in the same tube be a', a", a'", &c. ; then 

 the quantities of sugar contained in the volume, V, of these liquids will be repre- 



a! a" a'" 

 sented by the products e , e , e , &c., respectively. If the sugar examined, 



instead of being pure, is mixed with other but inactive substances, it is evident that 

 these same products express the absolute weights of pure sugar contained in tho 

 weights of substances employed in the formation of the liquids of the given volume, 

 V. It is possible to employ proof-tubes of different lengths ; but it is then necessary 

 to reduce by calculation the observed deflections to those which would have been 

 produced in the same tube. 



It often happens that solutions of sugar which have to be examined are turbid or 

 strongly coloured. When this interferes with the examination, they must be clarified 

 and rendered either quite colourless, or when this is* not possible the colour must 

 be at least reduced. This is often effected by precipitating the colouring-matter of 

 the syrups with subacetate of lead ; but the most accurate method is by a filter of 

 animal-charcoal. The filtrates are then examined. When syrups contain, besides 

 cane-sugar, other constituents which exert an action upon the plane of polarisation, 

 the amount of cane-sugar present may be determined by inverting, by means of 

 hydrochloric acid, the rotatory power of the cane-sugar. No other saccharine sub- 

 stance is, in fact, known which suffers a similar change under the same circumstances. 



If, for instance, the liquid under examination contains besides cane-sugar, glucose, 

 whose rotatory action on the plane of polarisation is in the same direction as that of 

 cane-sugar ; if a' be the deviation observed to be produced by the liquid, then a' is 

 evidently the sum of the separate deflections of the cane-sugar x, and of the glucose, 

 y. About one-tenth of its volume of hydrochloric acid is added to the syrup, and it 

 is kept for ten minutes at a temperature of 140 154. The cane-sugar is thereby 

 completely transformed into noncrystallisable sugar, which turns the plane of polarisa- 

 tion to the left, while the rotatory power of the glucose undergoes no alteration. When 

 this change has been effected, the new deviation, a", of the liquid is observed. It is 

 now the difference between the deviation y, of the glucose and that of the noncrys- 

 tallisable sugar derived from the cane-sugars. But the degree of dilution of the 



Vol. III. 3 



