1 In admixture with different inactive liquids. In these, the value [a] exhibits very different rates of increase or decrease for increasing percentages of solvent q, the law of variation, when shown graphically, appearing in some cases as a straight line, in others as a curve. the first place a formula was deduced for each, embracing all the curve lying between the most concentrated and the most dilute of the solutions used; and, as the observations for the most part started with solutions containing about 90 per cent. of active substance, it was to be expected that at this point the value deduced for the constant A would approximate very closely to the true specific rotation of the absolute substance. Next it was sought to determine to what extent the calculated differs from the true specific rotation when only dilute solutions are used, as would be the case with substances but sparingly soluble. As already stated (§ 23), we find that when an active substance is dissolved in different liquids, different values of [a] are always obtained, even when the degrees of concentration are the same, whence it has been often inferred that for each solvent any substance has a different constant of specific rotation. Experiment in this direction must decide whether the true rotatory power of a substance does indeed suffer an immediate definite alteration under the influence of small proportions of inactive solvent, which subsequently proceeds at a different rate on further dilution, or whether the values obtained for constant A with different solvents do sufficiently agree with each other. Biot has recorded a solitary experiment of this kind, in which he worked with solutions of camphor in acetic. acid and in alcohol. The results for the red ray (§ 18) gave the formula The two values for A, instead of agreeing as expected, here exhibit a by no means insignificant difference, and Biot assumes, in explanation, that the law of variation of the specific rotation is not expressed with sufficient exactness by the foregoing interpolationformulæ. The observations appended were made by the methods described at length in Chapter V. 1 Liebig's Ann., 189, 241. 2 Biot: Ann. Chim. Phys. [3], 36, 301, 307. F The following abbreviations have been used : P, percentage of active substance in 100 parts by weight of solution. d, specific gravity of the solution at 20° Cent., that of water at 4° being taken as unity. c=dp, the concentration, i.e., the number of grammes of active substance in 100 cubic centimetres of solution. L, the length of tube in millimetres. a, observed angle of deviation of rayD, at a temperature of 20° Cent., expressed in degrees and decimals (circular measure). α. 104 the specific rotation for ray D, at a temperature of 20° Cent. I. LEFT-HANDED OIL OF TURPENTINE. § 30. Two kilogrammes of Bordeaux oil, after standing for several weeks over calcium chloride, were submitted to distillation. Nearly the whole came over between 160° and 162° Cent. Height of barometer = 737 millimetres. Specific gravity 0·86290. The rotation was observed with a Wild's instrument, having two bath-tubes of different lengths. It should be remembered that when oil of turpentine is kept in vessels containing air, it undergoes a process of oxidation, whereby the specific gravity of the oil is increased and the rotatory power diminished. A portion of the above oil, after standing thus for four weeks, gave a specific gravity of 0.86779, and with a 219.79 millimetre tube showed a deviation of 68.144, whence [a] = 35.728°. (a.) Mixtures with Alcohol. Alcohol, as nearly as possible anhydrous, with a specific gravity 0.7957, was used. The following mixtures were examined with Wild's polariscope : Like the oil itself its alcoholic solutions suffer a gradual decrease of rotatory power when kept in incompletely filled vessels. Mixture II. at the end of eight days gave [a] = 37·164°. (b.) Mixtures with Benzene. Crystallizable benzene with a boiling point 80.4° Cent. (barometer, 755 millimetres), and a specific gravity of 0.88029 was used. Observations were taken with Wild's instrument : The glacial acetic acid used was rectified by two fractional crystallizations, and had a specific gravity 1.0502 at 20° Cent., corresponding to 99.8 to 99.9 per cent. of acetic acid. Wild's polariscope was used: In these mixtures also an increase of density along with diminution of rotatory power could be detected by keeping. After standing for three days, the following results were obtained :— By reason of this liability to change under the influence of oxidation, oil of turpentine is not altogether a suitable substance for experiments of the above kind, and inattention to this point at the outset entailed the necessity of repeating several of our experiments. As the observations show, the specific rotation of the oil of turpentine rises with the addition of increasing quantities q of all three solvents, the graphic representations taking the form of curves, of which that for acetic acid appears steepest, that for benzene less so, and for alcohol least (see Fig. 15). Although the curvature in each case is slight, the deviation from a straight line is too great to admit of application of the formula [a] = A + B q. If, taking this equation, we proceed to determine the constant A from two mixtures, values will be obtained, which will always be less than the true specific rotation of pure oil of turpentine (37·01), differing from the latter in proportion to the diluteness of the solutions employed in its determination. Thus, with alcohol as the inactive On the other hand, if we adopt the formula [a] = A + B q + C q2, and compute the values of the constants A, B, and C from the solutions containing the smallest, mean, and largest proportions respectively of the inactive constituent, we get for A a value approaching very near to that of the specific rotation of pure oil of turpentine. Moreover, the formula agrees sufficiently well with the experimental curve throughout (from q = 10 to 90). The mixtures specified furnished the results shown below: 1. Alcohol (computed from solutions I., III., and V.) : |