Chemistry and Physics. 283 



netic field would be established at right angles to the incoming 

 cathode rays when a steady current flowed through the solenoid. 

 These rays were spread out into a " magnetic spectrum " since 

 they possessed different speeds ; each speed corresponding to a 

 circular trajectory lying in a vertical plane and having its center 

 in a vertical line tangent to the cylinder near the mouth of the 

 inlet tube. The cylinder was provided with a vertical, radial out- 

 let tube which opened into the cylinder at its lowest element of 

 surface and in the plane of the magnetic fan or spectrum. Con- 

 sequently only such rays as were deviated through about one 

 right angle could escape from the cylinder. By varying the cur- 

 rent in the solenoid and by altering the hardness of the discharge 

 tube it was possible to send through the outlet tube a pencil of 

 cathode rays containing approximately one speed, this speed 

 being of practically any desired value. The emergent pencil 

 entered a Faraday cylinder, impinged upon an inclined, silver 

 anticathode and excited the primary X-rays. A beam of these 

 Rontgen rays was allowed to pass through a thin aluminium 

 window and to fall upon a plate of the secondary radiator under 

 investigation. The actual measurements were not novel and 

 therefore further experimental details would be superfluous. 



The elements employed as secondary radiators were aluminium, 

 chromium, iron, nickel, copper, zinc, and selenium. For each 

 secondary radiator a curve was plotted with abscissae proportional 

 to the fourth power of the speed of the cathode rays which gave 

 rise to the primary X-rays. The ordinates were proportional to 

 the energy of the secondary Rontgen rays. In general, the por- 

 tions of the graphs nearest to the axis of abscissas are decidedly 

 curved, being concave towards the axis of energy. As soon as 

 the energy attains a certain value, depending upon the material 

 of the secondary radiator, the graphs change into straight lines 

 and continue to be straight for all higher values of the speeds of 

 the cathode rays. The curved portions of the graphs are ascribed 

 to scattered Rontgen radiation whose absorption coefiicient dimin- 

 ishes as the speed of the cathode rays increases. Special care 

 was taken to determine from the graphs the critical values of 

 the speeds, that is, to determine the points at which the graphs 

 change from curves into straight lines. 



The results of this series of experiments may be summarized as 

 follows : (a) u The energy emitted in the form of Rontgen radi- 

 ation by a cathode particle when suddenly stopped is proportional 

 to the fourth power of its velocity." (b) " The primary rays from 

 a Rontgen ray tube can only excite the radiation characteristic 

 of a radiator of atomic weight 10 when the velocity of the parent 

 cathode rays exceeds wx 10 8 cm/sec." "This law holds fairly 

 closely for the radiators Al, Cr, Fe, N"i, Cu, Zn, and Se." 



Result (a) was deduced theoretically by Sir J. J. Thomson 

 and published in the Philosophical Magazine for August, 1907.- — 

 Proc. Roy. Soc, vol. lxxxv, p. 323, 1911. h. s. u. 



10. Weitere Messungen uber Wellenlangennormale im Eisenspek- 

 trum. — It was resolved in 1907, at the Meudon meeting of the 



