Page:Philosophical magazine 23 series 4.djvu/115

of refraction of that ray in the body,

$\lambda ={\frac {\Lambda }{i}}.$

(162)

Also $v$, the velocity of light in the substance, is related to V, the velocity of light in air, by the equation

$v={\frac {V}{i}}.$

(163)

Hence if $z$ be the thickness of the substance through which the ray passes, the angle through which the plane of polarization will be turned will be in degrees,

$\theta ={\frac {180^{\circ }}{\pi }}qz;$

(164)

or, by what we have now calculated,

$\theta =90^{\circ }{\frac {1}{\sqrt {\pi }}}\cdot {\frac {r}{s^{\frac {3}{2}}}}{\frac {\mu iZz}{\Lambda ^{2}V}}.$

(165)

In this expression all the quantities are known by experiment except $r$, the radius of the vortices in the body, and $s$, the density of the luminiferous medium in air.

The experiments of M, Verdet^[1] supply all that is wanted except the determination of Z in absolute measures and this would also be known for all his experiments, if the value of the galvanometer deflection for a semirotation of the testing bobbin in a known magnetic field, such as that due to terrestrial magnetism at Paris, were once for all determined.

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XV. On the Composition, Structure, and Formation of Beekite.

By Arthur H. Church, B. A. Oxon, F.C.S.^[2]

[With a Plate.]

THERE occurs in the triassic red conglomerate of Torbay and its neighbourhood, an interesting siliceous substance (generally considered to be a variety of hornstone), which offers a problem not only to the geologist and palaeontologist, but also to the chemist. The Beekite is, in fact, not a mineral merely, but a fossil which has been more or less completely mineralised, the mineralization having, however, been effected in a way not very easy to understand. In the present paper, after having quoted some authorities in order to show the geological character and position of Beekites, I shall endeavour to throw some light,

1. Annales de Chimie et de Physique, ser. 3, vol. xli p. 370.
2. Communicated by the Author.