143. We have not yet sufficient knowledge to enable us to use the refraction-equivalent of a compound, otherwise than very tentatively, as a help in deciding between the possible structural formulæ assigned to that compound. Brühl has employed the observed values of the constant (R) as an argument in favour of, or against, certain formulæ, but not, it appears to me, with much success1.

From comparing the values of (R) with the structural formulæ of certain carbon compounds, the same naturalist has drawn conclusions regarding the meanings of the symbolical expressions 'single' and 'double bonds'; but, as seems always the case in attempts to deal with 'bonds', the foundations of the reasoning are shifting, and the superstructure is untrustworthy2.

144. If a ray of plane polarised light is passed through a plate of quartz cut at right angles to its optical axis, the position of the plane of polarisation of the emergent ray does not coincide with that of the incident ray; the plane has been rotated through a certain angle, called the angle of rotation. If the rotation takes place in the same direction as that in

1 See Ber. 12. 2146, and 14. 2736.

2 See Book II. chap. IV. par. 258. Kanonnikow (original paper in Russian; see abstract in Ber. 16. 3047) has found the refraction-equivalents of a number of solid carbon compounds, by dissolving them in chemically inactive solvents, and measuring the refractive indices of the solutions, the values of the indices of the solvent being known; it is then easy to find the refractionequivalent of the dissolved substance, if it be granted that the refraction-equivalent of a mixture is the sum of the equivalents of its components (see ante, p. 307-9). Kanonnikow concludes that neither the degree of concentration of the solution, nor the physical condition of the solid, exerts any marked effect on the refractive power of the dissolved substance.

Conclusions are drawn as to the structural formulæ of various carbon compounds; Brühl's generalisations, on the whole, are confirmed.

The same chemist (see abstract in Ber. 17. ref. 157. [the abstracts, referate, in the Berichte beginning with vol. 17 are paged separately from the original papers]) has attempted to determine r for various metals, by finding (RA) for various salts of carbon acids, and deducting (RA) for the acids. His numbers point to the conclusion that in a 'group' of metals (as 'group' is used in the classification based on the periodic law), A increases as the atomic weights of the metals increase. Kanonnikow also tries to deduce values for (RA) for the groups NOS, SO4, &c., and so to find the distribution of the interatomic actions in sulphates, nitrates, &c.

which the hands of a watch appear to move as we look at the face, the quartz is said to exhibit dextrorotatory power; this is expressed by prefixing + to the value of the angle of rotation. If the rotation takes place in the direction opposite to that in which the hands of a watch appear to move as we look at the face, the quartz is said to exhibit lævorotatory power; this is expressed by prefixing to the value of the angle of rotation.

Optically active transparent media are those which rotate the plane of polarisation of a ray of light passed through them; they are divided into dextrorotatory substances, e.g. some specimens of quartz, sugar in aqueous solution, &c., and lævorotatory substances, e.g. other specimens of quartz, turpentine, quinine in alcoholic solution', &c.

To determine the amount of rotation caused by any substance, it is necessary to have an instrument wherein a ray of light may be polarised, and the position of the plane determined; the polarised ray may be passed through a known quantity of the medium under examination; and finally the position of the plane of the emergent ray may be determined. Such instruments, known as polarimeters or polaristrobometers, are described in detail in various text-books".

10 16*

Let us consider a liquid carbon compound, say C1H The angle of rotation, a, depends on (1) the thickness of the layer of liquid through which the light passes, (2) the wavelength of the ray of light employed, and in most cases, (3) the temperature at which the observation is made. The first of these conditions will be determined if we know the

1 For details concerning polarised light, and circular polarisation considered from the physical stand-point, see Glazebrook's Physical Optics, chaps. XI.

and XIV.

2 See especially Armstrong and Groves, Organic Chemistry, 569 et seq.; and also Watts's Dictionary, 3rd Supplt. 1198–1207.

3 The angle of rotation, a, was supposed by Biot to vary inversely as the square of the wave-length of the light employed, but Boltzmann has shewn that the

B C

expression a= + where B and C are constants determined experimentally, is

a nearer approximation to the law: but the values of B and C appear to differ slightly for different media. See Watts's Dict. 3rd Supplt. 1207.

length of the column of liquid employed, and the second is rendered definite by making use of monochromatic light.

Let /= length of column of liquid in decimetres, d= density of liquid referred to water at 4o, and a= angle of rotation of plane of polarisation of light of given wave-length; then

=3

a

[a] = specific rotatory power of the liquid, for the given ray, = d.

That is to say, the specific rotatory power of an optically active substance is the angle through which the plane of polarisation of a given ray is rotated by passing through a column I decimetre long of a liquid containing I gram of the substance in I cubic centimetre. If the substance to be examined is a solid, it must be dissolved in an optically inactive menstruum. In such a case, let /= length in decimetres of column of solution employed, p= grams of optically active substance in 100 grams of solution (i.e. grampercentage composition), and d density of solution referred to water at 4°; then p. d = c = concentration, i.e. grams of active substance in 100 c.c. of solution.

=

100 α

[a] = 1.p.d=l.c

As the value of [a] generally rises as temperature rises', thermometric observations must be made. The value of [a] also varies with variations in (1) the nature, and (2) the quantity, of inactive solvent employed; the preceding formula therefore gives only the apparent specific rotatory power of the solid substance.

That [a] varies according to the nature of the solvent, is shewn by Hesse's observations on turpentine oil';

[a],

pure turpentine (C10H18)

oil + benzene

oil + alcohol oil + acetic acid (amount of solvent varied in each case from 10 per cent. to 90 per cent.)

37°01; 37°035 to 38°486; 37°*194 to 39°449; 37°148 to 40°222.

1 For numbers illustrative of this in the case of aqueous solutions of tartaric acid see Dict., 3rd Supplt. 1209.

2 Hesse, Annalen 176. 89 and 189: see also Landolt's Handbook of the Polariscope (English translation), 54-94. This book presents a very complete view of the whole subject of circular polarisation, chemically considered.

The following numbers1 illustrate the dependence of [a] on the amount of solvent employed;

Landolt (loc. cit.) has shewn that the true value of [a] may generally be found from a number of observations made with solutions of varying concentration; the more concentrated the solution the more nearly does the value found for [a] approach the true value, i.e. the more nearly does the observed, agree with the true specific rotatory power. It is better to use several solvents, and make a series of observations with each; the value deduced for [z] is generally the same for each solvent.

The nature and extent of the variations in [a] caused by varying the quantity of solvent appear to differ for each optically active solid substance2; in some cases the relation is very complicated, in others it may be expressed by a comparatively simple formula".

That the observed and calculated values of [a] agree

1 Landolt, loc. cit.

2 The following numbers illustrate this statement (Landolt, loc. cit. 82):

3 Thus, for solutions of turpentine in alcohol, Landolt gets the formula

[a]=36°*974+00481649+000133192

where 9 percentage of inactive solvent. (For more details see Landolt, loc.

cit. 81-94.)

For a fuller treatment of the methods employed for finding the true value of [a] from observations on solutions, see Dict. 3rd Supplt. 1212—1213.

M. C.

21

closely, provided a sufficient number of observations is made, is evident from these results (Landolt).

The true specific rotatory powers of camphor, cane sugar, and dextroglucose have been determined by Landolt, Tollens, and Schmitz'. But I think it should be noted that the observations on which the method for determining [a] is based, were necessarily made with solutions of liquid compounds in inactive solvents, whereas in the cases of camphor and sugar we have to deal with solutions of solid substances; it is possible that the value of [a] for liquid camphor may vary very much from that for solid camphor. It should also be observed that any deductions concerning the relations between specific rotatory power and molecular structure, drawn from a study of liquid compounds, could not be applied in a precise manner to solid compounds, assuming the true value of [a] for these compounds to be known.

145. In attempting to trace the relations which undoubtedly exist between the specific rotatory power and the structure of compounds, we must distinguish relationships between this constant and the composition of molecules whose empirical formulæ at least are known, from those between the same constant and such mixtures of molecules in varying proportions as are presented by solutions of known concentration.

1 See Landolt, loc. cit. 84-92: Tollens, and Schmitz in Ber. 9. 1531: 10. 1403 and do. 1414.

2 Biot states that fused liquid tartaric acid is markedly dextrorotatory, but the solidified acid is feebly lævorotary (Dict. 3rd Supplt. 1209). Landolt's value of [a] for solid camphor is 55°6 (see Dict., loc. cit. 374): while Gernez obtained the value 70°.33 for fused camphor (do. do. p. 1209).