On the Composition , Structure, and Formation of Beehte. 9o 



of refraction of that ray in the body, 



\=^. . . . . . . . (162) 



Also v, the velocity of light in the substance, is related to V, the 

 velocity of light in air, by the equation 



v=Z. . . . ,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, 



6=^qz; ...... (164) 



or, by what we have now calculated, 



*= 90 °^iw- • • • < 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* supply all that is wanted 

 except the determination of Z in absolute measure; 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 mag- 

 netism at Paris, were once for all determined. 



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



By Arthur H. Church, B.A. Oxon, F.C.S.f 



[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 mineralized, 

 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, 



* Annates de Chimie et de Physique, ser. 3. vol, xli. p. 3/0. 

 t Communicated by the Author. 



