An interesting fact is observed in cooling solutions of honey to 0°. The maximum left hand rotation is not reached as soon as the temperature reaches 0° but only after it has been kept at that temperature for two or three hours. The line representing the change in rotatory power in solutions of honey between 10° and 88° is practically straight but from 10° to 0°, if measured by the readings taken without delay, it is decidedly curved; the reading being less at first than it is afterwards. After three hours the 0° becomes sensibly constant and then the whole line is nearly straight, but still with a slight deficiency in the reading at the 0°. For this reason the computations should be based on readings between 10° and 88° rather than on a number covering the whole range of temperature. Nevertheless, if the solution be kept at 10° for three hours before the final reading is taken, no error of any practical magnitude is introduced.

The calculations given above, for the cane sugar scale, can also be made in an exactly similar manner for angular rotation. The angular variation produced by one gram of levulose for 1° of temperature is 0°.01256. For 88° this would become 1°.10528. Suppose the total observed angular deviation in a given case between 0° and 88° to be 10°.404, then the weight of levulose present is 10.404 ÷ 1.10528 = 9.413 grams.

In the case mentioned 26.048 grams of honey were taken for the examination. The percentage of levulose present, therefore, was 9.413 × 100 ÷ 26.048 = 36.13.

241. General Formula for the Calculation of Percentage of Levulose.—Let K = deviation in divisions of the cane sugar scale or in angular rotation produced by one gram of levulose for 1° temperature.

Let T and tʹ = temperatures at which observations are made.

Let R = observed deviation in rotation.

Let W = weight of levulose obtained.

Let L = per cent of levulose required.

Then L = R÷ W.
K(T - tʹ)

In most genuine honeys the value of R between 0° and 88° is approximately thirty divisions of the cane sugar scale or 10° angular measure for 26.048 grams in 100 cubic centimeters, read in a 200 millimeter tube, or, for 13.024 grams in 100 cubic centimeters read in a 400 millimeter tube.