Relation of Volta Electromotive Force to Pressure Sfc. 97 

 direction <f> is B(cos£) l +«. 



It is then easy to show that the total amount of light given 

 out per unit area is equal to 



ttB — . 



In all the above experiments the actual measurements have 

 referred to the light receding from the diffusing substance at 

 inclinations all practically normal to its surface. The quantity 

 B has been measured, the total light has been calculated as 



7rB, and has been overestimated in the ratio 1 +^: 1. 



To account for an error of 5 per cent., the quantity e need 

 only be 0*1. The brightness of the surface (in the physio- 

 logical sense) would, when viewed at an inclination (/>, be 

 proportional to 



B (cos $) 1+e /cos </>, or B(cos(/>) e . 

 This quantity is practically constant (if e = 0'l) until <f> 

 becomes very large. Its value is '994 lor </> = 20° and -974 

 for </> = 40°, and the change in the brightness of the surface 

 would hardly be perceptible to the eye. 



This correction applies to the coefficients rj 2 and r 2 ; it does 

 not affect the values of 77!, a, or t v In some of the first tests 

 of reflecting-power the inclination of the light-rays to the 

 surface was considerably less than 90 degrees, and the values 

 of 7) obtained were less than those given above. These tests 

 confirm the idea that the cosine law is not strictly fulfilled, 

 but they were not accurate enough to be conclusive. 



The above measurements were all made in the Optical 

 Laboratory of the Central Institution, and the writer has had 

 the benefit of the assistance of some of the students of that 

 College in re-testing and confirming the results given in the 

 foregoing tables. 



XIII. Relation of Volta Electromotive Force to Pressure $r. 

 By Dr. G. Gore, F.R.S.* 



EORTY-TWO years ago I made several experimental 

 attempts to discover a difference of molecular state of 

 the upper and lower ends of a vertical column of solution of 

 cupric sulphate: — 1st. By suddenly reversing the ends of a 

 gutta-percha tube, about 6 feet high and 6 inches diameter, 

 filled with the liquid, by swinging the tube in a vertical 



* Communicated by the Author. 



Note. Compare Wild's experiment (Wiedemann's Galvanismus, 1872, 

 vol. i.p. 77G; Pogg. Ann. 18o5, vol. cxxv. p. 119). 



Phil. Mag. 8. 5. Vol. 35. No. 213. Feb. 1893. 11 



