of strong sulphuric acid, and heating the solution in the water-bath for 30 minutes, the water lost by evaporation being from time to time replaced. The free acid is next neutralised by a little sodium carbonate, its volume made up to 200 c.c., and the invert sugar now contained estimated by Fehling’s solution. The difference in the two determinations represents the glucose formed by the conversion of the cane sugar; 100 parts of the glucose so produced is equivalent to 95 parts of cane sugar. Commercial cane sugar is, however, generally estimated by the instrument known as the saccharimeter or polariscope. In order to convey an intelligent idea of the physical laws which govern the practical working of the polariscope, it will first be necessary to refer to the subject of the polarisation of light. The transformation of ordinary into polarised light is best effected either by reflection from a glass plate at an angle of about 56°, or by what is known as double refraction. The former method can be illustrated by Fig. 1, Plate X., which represents two tubes, B and C, arranged so as to allow the one to be turned round within the other. Two flat plates of glass, A and P, blackened at the backs, are attached obliquely to the end of each tube at an angle of about 56°, as represented in the figure. The tube B, with its attached plate, A, can be turned round in the tube C without changing the inclination of the plate to a ray passing along the axis of the tube. If a candle be now placed at I, the light will be reflected from the plate P through the tube, and, owing to the particular angle of this plate, will undergo a certain transformation in its nature, or, in other words, become “polarised.” So long as the plate A retains the position represented in the figure, the reflected ray would fall in the same plane as that in which the polarisation of the ray took place, and an image of the candle would be seen by an observer stationed at O. But, suppose the tube B to be turned a quarter round; the plane of reflection is now at right angles to that of polarisation, and the image will become invisible. When the tube B is turned half-way round, the candle is seen as brightly at first; at the third quadrant it disappears, until, on completing the revolution of the tube, it again becomes perfectly visible. It is evident that the ray reflected from the glass plate P has acquired properties different from those possessed by ordinary light, which would have been reflected by the plate A in whatever direction it might have been turned. PLATE X. [Illustration: Fig. 1. Fig. 2. Fig. 3. Fig. 4. Polariscope.] ARTOTYPE. E. BIERSTADT N. Y.