Encyclopaedia Britannica, 11th Edition, 'Châtelet' to 'Chicago' / Volume 6, Slice 1 - Various

The empirical formula (n² - 1)/(n² + 0.4)d apparently gives more constant values with change of temperature than the Lorenz-Lorentz form. The superiority of the Lorenz-Lorentz formula over the Gladstone and Dale formula for changes of state is shown by the following observations of Brühl (Zeit. f. phys. Chem., 1891, 71, p. 4). The values are for the D line:—

Substance.	Temp.	Gladstone and Dale.		Lorenz and Lorentz.
Substance.	Temp.	Vapour.	Liquid.	Vapour.	Liquid.
Water	10°	0.3101	0.3338	0.2068	0.2061
Carbon disulphide	10°	0.4347	0.4977	0.2898	0.2805
Chloroform	10°	0.2694	0.3000	0.1796	0.1790

Landolt and Gladstone, and at a later date J.W. Brühl, have investigated the relations existing between the refractive power and composition. To Landolt is due the proof that, Additive relations. in general, isomers, i.e. compounds having the same composition, have equal molecular refractions, and that equal differences in composition are associated with equal differences in refractive power. This is shown in the following table (the values are for Hα):—

Substance.		Mol. Refract.	Substance.	Mol. Refract.	Diff. for CH2.
Ethylene chloride	C2H4Cl2	20.96	Acetic acid	12.93
Ethylidene chloride	C2H4Cl2	21.08	Propionic acid	17.42	4.49
Fumaric acid	C4H4O4	70.89	Butyric acid	22.01	4.59
Maleic acid	C4H4O4	70.29
o-Cresol	C7H8O	32.52	Acetaldehyde	11.50
m-Cresol		32.56	Propionaldehyde	15.93	4.43
p-Cresol		32.57	Butylaldehyde	20.52	4.59

Additive relations undoubtedly exist, but many discrepancies occur which may be assigned, as in the case of molecular volumes, to differences in constitution. Atomic refractions may be obtained either directly, by investigating the various elements, or indirectly, by considering differences in the molecular refractions of related compounds. The first method needs no explanation. The second method proceeds on the same lines as adopted for atomic volumes. By subtracting the value for CH2, which may be derived from two substances belonging to the same homologous series, from the molecular refraction of methane, CH4, the value of hydrogen is obtained; subtracting this from CH2, the value of carbon is determined. Hydroxylic oxygen is obtained by subtracting the molecular refractions of acetic acid and acetaldehyde. Similarly, by this method of differences, the atomic refraction of any element may be determined. It is found, however, that the same element has not always the same atomic refraction, the difference being due to the nature of the elements which saturate its valencies. Thus oxygen varies according as whether it is linked to hydrogen (hydroxylic oxygen), to two atoms of carbon (ether oxygen), or to one carbon atom (carbonyl oxygen); similarly, carbon varies according as whether it is singly, doubly, or trebly bound to carbon atoms.

A table of the atomic refractions and dispersions of the principal elements is here given:—

Element.	Hα	D.	Hγ	Dispersion Hγ - Hα.
Hydrogen	1.103	1.051	1.139	0.036
Oxygen, hydroxyl	1.506	1.521	1.525	0.019
Oxygen, ether	1.655	1.683	1.667	0.012
Oxygen, carbonyl	2.328	2.287	2.414	0.086
Chlorine	6.014	5.998	6.190	0.176
Bromine	8.863	8.927	9.211	0.348
Iodine	13.808	14.12	14.582	0.774
Carbon (singly bound)	2.365	2.501	2.404	0.039
Double linkage of carbon	1.836	1.707	1.859	0.23
Triple linkage of carbon	2.22		2.41	0.19
Nitrogen, singly bound and only to carbon	2.76		2.95	0.19

Dispersion and Composition.—-In the preceding section we have seen that substances possess a definite molecular (or atomic) refraction for light of particular wave-length; the difference between the refractions for any two rays is known as the molecular (or atomic) dispersion. Since molecular refractions are independent of temperature and of the state of aggregation, it follows that molecular dispersions must be also independent of these conditions; and hence quantitative measurements should give an indication as to the chemical composition of substances. This subject has been principally investigated by Brühl; he found that molecular dispersions of liquids and gases were independent of temperature, and fairly independent of the state of aggregation, but that no simple connexion exists between atomic refractions and dispersions (see preceding table). He also showed how changes in constitution effected dispersions to a far greater extent than they did refractions; thus, while the atomic dispersion of carbon is 0.039, the dispersions due to a double and treble linkage is 0.23 and 0.19 respectively.

Colour and Constitution.—In this article a summary of the theories which have been promoted in order to connect the colour of organic compounds with their constitution will be given, and the reader is referred to the article [Colour] for the physical explanation of this property, and to [Vision] for the physiological and psychological bearings. A clear distinction must be drawn between colour and the property of dyeing; all coloured substances are not dyes, and it is shown in the article [Dyeing] that the property of entering into chemical or physical combination with fibres involves properties other than those essential to colour. At the same time, however, all dyestuffs are coloured substances.

A survey of coloured substances led O.N. Witt in 1876 to formulate his “chromophore-auxochrome” theory. On this theory colour is regarded as due to the presence of a “chromophore,” and dyeing power to an “auxochrome”; the latter by itself cannot produce colour or dyeing power, but it is only active in the presence of a chromophore, when it intensifies the colour and confers the property of dyeing. The principal chromophores are the azo, -N=N-, azoxy, =N2O, nitro, -NO2, nitroso, -NO, and carbonyl, =CO, groups. The azo-group is particularly active, both the aliphatic and aromatic compounds being coloured. The simplest aliphatic compounds, such as diazo-methane, diazo-ethane, and azo-formic acid, are yellow; the diamide of the latter acid is orange-red. Of the aromatic compounds azo-benzene is bright orange-red, and α-azo-naphthalene forms red needles or small steel-blue prisms. The azo-group, however, has little or no colouring effect when present in a ring system, such as in cinnolene, phthalazine and tolazone. The nitro group has a very important action mainly on account of the readiness with which it can be introduced into the molecule, but its effect is much less than that of the azo group. The colour produced is generally yellow, which, in accordance with a general rule, is intensified with an increase in the number of groups; compare, for example, mono-, di-and tri-nitrobenzene. The nitroso group is less important. The colour produced is generally of a greenish shade; for example, nitrosobenzene is green when fused or in solution (when crystalline, it is colourless), and dinitrosoresorcin has been employed as a dyestuff under the names “solid green” and “chlorine.” The carbonyl group by itself does not produce colour, but when two adjacent groups occur in the molecule, as for example in the a-diketones (such as di-acetyl and benzil), a yellow colour is produced. It also acts as a chromogenic centre when double bonds or ethylenic linkages are present, as in fluorene ketone or fluorenone.