=102. Formulas for Calculating Specific Rotatory Power.=—In order to determine the specific rotatory power (gyrodynat[64]) of a given substance it is necessary to know the specific gravity and percentage composition or concentration of its solution, and to examine it with monochromatic polarized light in an instrument by which the angular rotation can be measured. The gyrodynat of any body changes with its degree of concentration, in some cases with the temperature, and always with the color of the light. With the red rays the gyrodynat is least and itprogressively increases as the violet end of the spectrum is approached. In practice the yellow ray of the spectrum has been found most convenient for use, and in the case of sugars the gyrodynat is always expressed either in terms of this ray or if made with color compensating instruments in terms of the sensitive or transition tint. In the one case the symbol used is (_a_)_{D} and in the other (_a_)_{j}. From this statement it follows that (_a_)_{D} is always numerically less than (_a_)_{j}. Unless otherwise specified the gyrodynat of a body is to be considered as determined by yellow monochromatic light, and therefore corresponds to _a__{D}.[65] =103. Variations in Specific Rotatory Power.=—The gyrodynat of any optically active body varies with the nature of the solvent, the strength of the solution, and the temperature.[66] Since water is the only solvent of importance in determining the gyrodynat of sugars it will not be necessary here to discuss the influence of the nature of the solvent. In respect of the strength of the solution it has been established that in the case of cane sugar the gyrodynat decreases while with dextrose it increases with the degree of concentration. The influence of temperature on the gyrodynat of common sugars is not of great importance save in the case of levulose, where it is the most important factor, the gyrodynat rapidly increasing as the temperature falls. It is of course understood that the above remarks do not apply to the increase or decrease in the volume of a solution at changed temperatures. This influence of temperature is universally proportional to the change of volume in all cases, and this volumetric change is completely eliminated when the polarizations are made at the temperatures at which the solutions are completed to standard volumes. =104. Gyrodynatic Data for Common Sugars.=—In the case of cane sugar the gyrodynat for twenty-five grams of sugar in 100 grams of solution at 20° is [_a_]_{D} = 66°.37. This is about the degree of concentration of the solutions employed in the shadow lamplight polariscopes. For seventeen grams of sugar in 100 grams of solution the number is [_a_]_{D} = 66°.49. This is approximately the degree of concentration for the laurent instrument.