resorted to, except in cases in which the other processes are inapplicable. The most ready method of estimating the percentage of pure cane-sugar in raw sugars, whether derived from beet or cane, is by the polariscope. The principle of this instrument is that a plane polarised ray of light may always be considered as made up of two circularly polarised rays, and if these pass through a medium, such as sugar, tartaric acid, etc., which retards the one more than the other, the plane of polarisation of their resultant when they leave the medium will in general not be the same as that of the incident ray—or, in other words, it will have been caused to rotate through a certain angle, sometimes to the right and sometimes to the left. This rotation varies in the different descriptions of sugar, both in regard to the angle and the direction. If a tube I decimetre long be filled with a solution of pure cane-sugar, containing 1 gram in every cubic centimetre of fluid, it will rotate the plane of polarisation 73.8 degrees to the right, and this is called the specific rotatory power of pure cane-sugar. Rotation is in proportion to the length of the tube, and the mass of substance possessing the rotatory power, water being quite neutral. It follows, therefore, that if we take a solution, containing a decigram of pure cane-sugar in every cubic centimetre of fluid, the tube being the same length as before, we obtain a rotation of 7.38°. If we then take an impure cane-sugar, and make a solution such that it shall contain 1 decigram in every cubic centimetre of liquid, fill a tube, 1 decimetre in length, with such solution, and find the rotation to be 6'3°, we should, supposing no invert-sugar to be present, find the percentage of sugar by the following proportion: as 7'38: 6'3 :: 100 x The rule for finding the specific rotation from the observed rotation is: Divide the observed rotation by the length of the tube, multiplied by the weight of sugar in each cubic centimetre of liquid, I gram being the unit of weight, and I decimetre the unit of length. Thus if a solution, containing o'150 gram of sugar in every cubic centimetre of fluid, has an observed rotatory power of 16° in a tube 2 decimetres long, the specific rotatory power would be (1) 16 2x0*150 53'33° and if this were a cane-syrup, the percentage of sugar would be as 738 53'33 :: 100: x. But raw sugars generally contain more or less invert-sugar; and as glucose has a specific rotatory power of 56° to the right, while lævulose, at a temperature of 57°2° F. (14° C.), rotates 106° to the left, the specific rotation of invert-sugar at 57°2° F. must consequently be to the left. If, therefore, at the temperature of 57°2° F. (14° C.), we obtain a solution of sugar which produces a specific rotatory power of 67o, and we find by Fehling's test that it contains 4 per cent. of invert-sugar, we have the data necessary for estimating the cane-sugar. Let a the percentage of invert-sugar by Fehling's test, b the specific rotatory power of the sugar examined, and x the percentage of crystallized cane-sugar, = The polariscopes now in most general use are those in which the scale, instead of being marked with the angle, has upon it the percentage of crystallized sugar corresponding to that angle, the quantity of sugar used, and the volume to which it is made up, the length of tube being always the same. In the trade the percentage of crystallized sugar is not regarded as the sole criterion of value. The percentage corresponding to the angle, given by the mixed sugars, is what is called by sugar merchants the percentage of crystallized sugar; and the percentage of ash, as well as the appearance of the sugar, is taken into account along with this indication in fixing the price. It should also be remarked that beet-sugar, as will be seen by the analyses of various samples, contains very little invert-sugar, so little indeed that it is disregarded on the Continent. The following table exhibits the specific rotatory power of commercial sugars, with the amount of cane-sugar calculated directly therefrom, and also the quantities of invert-sugar, ash, and moisture: If the formula (3) given above be applied to these results, the proportion of cane-sugar indicated will be from 1 to 2 per cent. more than the amount calculated directly from the angle of rotation. It is sometimes useful to determine the approximate value of a sugar from the specific gravity of its aqueous solution; and for this purpose the following table, which shows the specific gravities ADULTERATION. Owing probably to the low price of cane-sugar, and to the difficulty of finding a suitable cheap adulterant, it is remarkably free from sophistication. If glucose or starch sugar were suspected to have been added, the quantity present might be estimated by Fehling's test, or by the polariscope, according to the methods previously described. Perhaps the most serious deceit now practised upon the consumer of sugar is the sale of the lower products of the refiner as raw sugar. These products, technically known as "pieces," are caused to crystallize in very small crystals, and thus to hold a comparatively large percentage of water as well as of invert-sugar. They possess much less sweetening power than raw sugar, but having generally less colour are erroneously supposed by the public to combine cheapness with superiority of quality. |