392 Scientific Intelligence. 



ing of the cathode fall when glowing metallic oxides are used is 

 due to emission of negative ions. By means of this property of 

 emission very strong currents can be sent through a gas even at 

 low pressures.— Ann. der Physik, No. 8, 1904, pp. 425-468. 



J. T. 



10. Electro-chemical Equivalent of Silver. — Determinations' of 

 this constant has been made by the following observers: 



Mascart 0-011156 



F. and W. Kohlrausch 0-011183 



Lord Rayleigh and Mrs. Sidgwick 0-01 1 179 



Pellat and Potier 0-01 1 1 92 



Kahle.. _ 0-011183 



Patterson and Guthe 0*01 1 1 92 



Pellat and Leduc,__ 0*011195 



The last determination with, it is claimed, more accurate determi- 

 nations of the horizontal intensity of the earth's magnetism, the 

 strength of the electrical current and the time, has been made by 

 G. Van Dijk and J. Kunst, who have obtained the value 



a = 0, 0111823 ± 0-0000004 (m.F.) 

 From* the agreement of observations, this result is claimed to be 

 accurate to tovWo- — Ann. der Physik, No. 8, 1904, pp. 569-577. 



J, T. 



1 1. Electric Effect of Rotating a Dielectric in a Magnetic 

 Field. — According to Maxwell's Electromagnetic Theory, an 

 electromotive force should be induced in a dielectric and its 

 amount— according to Lamor and Lorentz — should be (1— K"™ 1 ) 

 times that in a conductor, K being the permittivity of the dielec- 

 tric. H. A. Wilson has shown the existence of this effect by 

 rotating a hollow cylinder of ebonite in a magnetic field, parallel 

 to the axis of the cylinder, with suitable conducting brushes, the 

 results of the experiment were as follows : 



(1) A radial electric displacement, is produced in the dielectric 

 when it is rotated in a magnetic field parallel to the axis of revo- 

 lution. 



(2) The direction of the displacement is the same as is pro- 

 duced in a conductor. 



(3) The displacement is proportional to the magnetic field and 

 to the rate of revolution. 



(4) The amount of the displacement agrees with that calcu- 

 lated on the assumption that the induced E.M.F. in the dielectric 

 is equal to that in a conductor multiplied by (1— K -1 ).- — Proc. 

 Roy. Soc, June 22, 1904, pp. 490-492. J. t. 



12. A Radio-active Gas from Crude Petroleum.-— &. F. Burton 

 shows that fresh crude petroleum contains a strongly radio-active 

 gas similar in its rate of decay and in that of the induced radio- 

 activity to the emanation from radium and to the emanations 

 obtained by various experimenters from mercury and from certain 

 waters fresh from the earth. This radio-active gas decays 

 approximately according to an exponential law, falling to half 



