Many organic liquids exhibit a feeble double-refraction when they are placed in a strong magnetic field and a beam of light traverses the substances in a direction transverse to the lines of force. The magnitude of this effect, which was discovered in 1907 by Cotton and Mouton, depends very largely on the chemical structure of the molecule. Hydrocarbons belonging to the aliphatic series and the aromatic series are strikingly different in their behaviour; hexane, for instance, showing no detectable effect, while benzene is an example of a liquid showing a measurable double-refraction. We propose in the series of papers of which this is the first, to discuss this phenomenon in its relation to the structure of molecules and their magnetic properties. For this purpose we shall use the theory of Langevin, which explains magnetic double-refraction as an effect arising from the orientative action of the field on the molecules (assumed to be magnetically and optically anisotropic) and connects the absolute value of the Cotton-Mouton constant with the values of the optical refractivity and of the magnetic susceptibility of the molecule along three mutually perpendicular axes. To enable the formula of Langevin to be used for the purpose of calculating the absolute value of the Cotton-Mouton constant, it is necessary to have data concerning, firstly, the magnetic character of the molecule, and, secondly, its optical anisotropy. In regard to the latter, we propose to utilise the data obtained from observations on light-scattering in the liquids concerned. In regard to the magnetic anisotropy of the molecules, we shall endeavour to connect the indications furnished by the data on magnetic double-refraction with considerations of atomic and mole-cular structure and the well-known theory of diamagnetism, also due to Langevin. We shall here merely quote the formula due to Langevin, the derivation of which is very conveniently set out in a recent article by Debye. The Cotton-Mouton constant Cm of double-refraction is given by the relation C
m
═n
p
─n
q
/λH
2
═3(n
0
─1
2
/80Пn
0
λKT
v
.[(A─B)(A'B')+(B─C) (B'─C')+(C─A)(C'─A')/(A+B+C)
2
,1 Where A, B, C are the moments induced along the three mutually perpendicular axes of the optical ellipsoid of the molecule by unit electric force in the incident light-waves, acting respectively along the three axes, and A', B', C' are the magnetic moments induced in the molecule by unit magnetic force acting in the same three directions. H is the acting magnetic field, λ is the wave-length of the light,
k
is the Boltzmann constant, T the absolute temperature, v the number of molecules per unit volume, n
0
the refractive index of the liquid outside the field, and n
p
and n
q
are the principal refractive indices in the field. The quantities A, B, C are connected with the refractive index n
0
of the liquid by the relation.
Two types of diamond
