These data may be recorded as follows:

Cane-sugar (solution in water, p = 16.993, t = 20°), [a]D = + 66·49°.

Ordinary camphor (solution in alcohol of specific gravity 0·796, at 20°, p = 15·092, t = 20°), [a]D = +43.66°.

Santonin (solution in alcohol of 97 per cent. by volume, c = 2, 1 = 15°), [a]D = - 174.00.

Quinine hydrate, C20 H24 N2 O2 + 3 H2O (solution in alcohol of 80 per cent. by volume, c = 1, t = 15°), [a]D=158.63°.

(solution in alcohol of 80 per cent. by volume, c = 6, t = 15°), [a]»

114.92°.

(solution in a mixture of 2 volumes chloroform + 1 volume alcohol of 97 per cent. by volume, c = 5, t = 15°), [a]D = 140.50°. In this way Hesse1 has estimated the specific rotation of a great number of optically-active substances dissolved in different liquids, thus supplying data which, as constant marks of the several substances, are of great value in determining the identity or purity of different preparations.

In all cases it is advisable to record the per cent. composition p, rather than the concentration c of the solutions, and so to calculate the specific rotation by the formula [a]

=

α. 100

i.d.p

which, moreover,

renders it necessary to determine the specific gravity of the solutions. The resulting values, at least in cases where several solutions have been observed, can then be used in determining the specific rotation of the absolute substance. This, as frequently already mentioned, is not the case when only the concentration is determined by means of a graduated vessel, and the specific rotation calculated by the otherwise more convenient formula [a]

=

α

100 l.c

$ 41. Molecular Rotation.-Specific rotation [a] is frequently referred to by Biot under the name of molecular rotation, indicating, as observed (§ 10), that the rotatory power of liquids is a property resident in the molecules.

of

1 Hesse: Liebig's Ann.176, 89, 189; 178, 260; 182, 128. Hesse indicates the number grammes of active substance in 100 cubic centimetres solution by p. It is, however, much better to employ c for this purpose (concentration) and let p denote the true per cent. composition (or number of parts by weight of active substance in 100 parts by weight of solution).

But this expression has been applied by Wilhelmy,1 HoppeSeyler, and more recently by Krecke,3 to a different value, viz., to the number obtained by multiplying the specific rotation of any substance into its molecular weight P. The values thus obtained being inconveniently large, Krecke has proposed to divide them uniformly by 100. The molecular rotation [M] of a given substance then appears as

P [a]
100

which expresses the angles of rotation produced by passage of the ray through layers 1 millimetre thick of substances when the unitvolumes contain the same number of molecules.

It has been attempted, by means of this formula, to discover relations between an active substance and its derivatives in respect to rotatory power, and the existence of certain multiple relations has been supposed to have been detected (Krecke,3 Landolt1). But the observations on which these comparisons were based were made, as was formerly the practice, with a single solution of each substance, whereas we have seen (§ 34) that the constant A of the pure substance should alone have been employed. Before, therefore, the hypothetical so-called law of multiple rotation is ripe for discussion a much more extensive series of experiments is necessary.

1 Wilhelmy: Pogg. Ann. 81, 527.

2 Hoppe-Seyler: Journ. für prakt. Chem. [1], 89, 273.

3 Krecke: Journ. für prakt. Chem. [2] 5, 6.

4 Landolt: Ber. der deutsch. chem. Gesell. 1873, 1073.

V.

PROCESS OF DETERMINING SPECIFIC ROTATION.

§ 42. In calculating specific rotations by the formulæ given in §§ 20, 21, viz.,

the following data must be obtained by direct experiment :—

1. The measurement of the angle of rotation a for a given ray. 2. The measurement of the length of the experimental-tube, in millimetres.

3. The weight p of active substance in 100 parts by weight of solution.

4. The specific gravity d of the active liquid.

5. The concentration c-i.e., the number of grammes of active substance in 100 cubic centimetres of solution.

If the object of determining the specific rotation of a solution of a solid substance, is merely to obtain a characteristic of its presence in solution, formula III., based on the knowledge of its concentration c, will suffice. But if, on the contrary, it is desired to ascertain the actual specific rotation of the substance itself, from observations on a number of different solutions, it is necessary (see § 25) to employ formula II., involving a knowledge of the percentage composition, and specific gravity of the several solutions.

A. Determination of the Angle of Rotation.

POLARISCOPIC APPARATUS.

§ 43. Apparatus for the Qualitative Examination of Rotatory Power.-To determine merely whether a given substance is or is not optically-active, and, if active, the direction in which the

rotation takes place, the instrument here represented (Fig. 201), which is delicate enough to detect even feeble degrees of rotatory power, may be used.2

A brass trough a b, of semi-circular section, fitted with a cover c so as to form a tube, carries at the extremity a, in a fixed case, a polarizing Nicol d. In front of the latter is placed the convex lens e, and on the other side of the polarizer at f, a so-called Soleil double-plate, formed of two plates, one of dextro-rotatory, the other of lævo-rotatory quartz, fitted vertically together and ground to a uniform thickness of either 3.75 or 7.5 millimetres.

The opposite end of the brass tube holds the movable Nicol g, 1[f is given apparently out of proper section, representing a front view, whilst the rest of the figure shows a longitudinal section.-D.C.R.]

2 The instrument shown above is manufactured by F. Schmidt and Haensch, Stallschreiberstrasse 4, Berlin. Instruments on the same optical principle, but of simpler construction (described by C. Neubauer in Fresenius' Zeitschr. für analyt. Chem. 16, 213) intended for determining grape-sugar in wine, but equally applicable for all other active substances, are procurable from the Optical Instrument Works of Dr. Steeg and Reuter, Homburg v. d. Höhe.

besides a small Galilean telescope, consisting of an object glass h, and an ocular i. The Nicol is turned by the handle k, which moves round the face of a small graduated disc 7, so as to allow the amount of rotation to be determined, at least approximately. The brass trough receives the glass tube p p (the ends of which may be closed by glass plates fixed with brass screw-caps) containing the liquid to be examined. The whole rests on a stand o. As a considerable depth of liquid is requisite for the detection of feeble rotatory power, the brass case is so constructed as to take glass tubes 5 or 6 decimetres in length. It is to this, and the introduction of the Soleil double-plate, that the sensitiveness of this instrument is due.

In using the instrument, the glass tube is at first left out whilst the extremity is directed towards a bright flame, for which purpose the gas-lamp, shown in Fig. 25, will be found best. The eyepicce of the telescope is then adjusted so that the vertical division of the double-plate appears sharply defined. By turning the analyzer g, a certain position will readily be found in which the two halves of the field of vision exhibit a perfectly uniform purplish tint, which the least turn of the Nicol to the right or left changes, one half becoming red, the other blue. Further particulars of this so-called sensitive tint will be given later on (§78) in speaking of the Soleil saccharimeter. Having thus established perfect uniformity of colour in the two halves of the field of vision, with the index standing at the zero-point on the scale of the analyzer, the glass tube containing the liquid to be tested is laid in the trough, when its optical activity will at once be declared by inequality of tint in the field of vision. To know whether the rotation be right-handed or left-handed, it is requisite, in the first place, to determine in the instrument, once for all, what relative positions the red and blue take up when some substance of known rotatory power, such as a (dextro-rotatory) solution of canesugar, is inserted. If the substance under examination shows the colours in the same relative order in which they are shown by the sugar, it likewise is dextro-rotatory; if the positions are interchanged, it must be lævo-rotatory. Further, with dextro-rotatory substances uniformity of tint in both halves of the field of vision is restored by turning the analyzer to the right, or in the direction of the hands of a watch, and with lavo-rotatory substances, to the left. The position of the index on the graduated disc of the analyzer shows the angle of rotation in each case.

H