Michelson and Morley — Influence of Motion, etc. 377 



The metamorphism is therefore quite complete. On boiling 

 the powdered mineral with sulphuric acid and oxidizing with 

 potassium permanganate an amount of the standard solution 

 was required corresponding to 0*91 per cent FeO as given. 

 The ratio of FeO to S is very nearly 1 : 1. The traces of CaO 

 and MgO are probably foreign. If the silica is combined with 

 a part of the iron in the form of a silicate as usually considered, 

 the water and the remaining ferric oxide are in approximately 

 the ratio represented by the formula assigned to limonite, 14*4 

 per cent H 2 0, 85*6 percent Fe 2 3 . Kobell* gives an analysis 

 of a similar pseudomorph from Minclen, Si0 2 4*50, H 2 13*26, 

 Fe 2 O s 82*24 = 100*00 per cent. The loss on ignition, as deter- 

 mined in three different samples, gave 11*72, 11*90 and 11*15 

 per cent respectively. The last determination was of the 

 powder from several small quite perfectly developed crystals. 

 Beloit College Chemical Laboratory, Beloit, Wisconsin, March, 1886. 



Art. XXXVI. — Influence of Motion of the Medium on the 

 Velocity of Light; by Albert A. Michelson and Edward 

 W. Morley. f 



The only work of any consequence, on the influence upon 

 the velocity of light of the motion of the medium, through 

 which it passes, is the experiment of Fizeau. He announced the 

 remarkable result that the increment of velocity which the light 

 experienced was not equal to the velocity of the medium, but 

 was a fraction x of this velocity which depended on the index 

 of refraction of the medium. This result was previously ob- 

 tained theoretically by Fresnel, but most satisfactorily demon- 

 strated by Eisenlohr,;j: as follows : 



Consider the prism AC in motion relatively to the ether in 

 direction AB with velocity d. Suppose the density of the ex- 

 ternal ether to be 1 and of the ether within the prism, 1+J. 

 In the time dt the prism will advance a distance ddt=B~B / At 



* J. pr. Ch., 1-319 (Rammelsberg, Mineral Chemie, 185). 



f This research was carried on by the aid of the Bache Fund. 



% Verdet. Conferences de Physique, ii, 687. 



