Chemistry and Physics. 401 



6. The Ionizing Potential of Sodium Vapor. — Several experi- 

 menters have shown that, when the vapors of cadmium, mag- 

 nesium, mercury, and zinc in vacuo are bombarded by electrons 

 from a hot cathode, a single-line spectrum is emitted, provided 

 the kinetic energy of the electrons does not exceed a certain 

 critical value. The line appeared when the potential difference 

 involved attained the value required by the quantum relation 

 and the frequency of the radiation; for example, 4.9 volts for 

 the wave-length 2536.7 in the spectrum of mercury. 



An extended series of experiments on the vapor of sodium has 

 been carried out recently by R. W. Wood and S. Okano. Since 

 they used three or four different forms of apparatus and varied 

 the experimental conditions in many ingenious ways, in order to 

 eliminate the hypothetical sources of error as far as possible, it 

 will not be feasible, for lack of space, to do justice to the experi- 

 mental details of the investigation. By making visual observa- 

 tions with a very efficient Schmidt and Haensch pocket 

 spectroscope, it was found that the red and green lines of the 

 subordinate series faded gradually as the voltage was decreased 

 and finally disappeared at 2.3 volts. The quantum relation leads 

 to the expectation that the D-lines would vanish at about 2.1 

 volts. As a matter of fact the yellow radiation could be detected 

 until the potential difference had dropped as low as 0.5 volt. 

 The lack of agreement between the predicted and observed 

 values may be due to the presence in the cathode stream of a 

 relatively small number of electrons having a much higher speed 

 than the mean value corresponding to 2.1 volts. The authors 

 suggest that this possibility may be tested by separating the 

 electrons magnetically into beams each of which is composed of 

 corpuscles moving with sensibly equal speeds.— P/iiL Mag., xxxiv, 

 p. 177, September, 1917. h. s. u. 



7. Penetrating Power of X-Rays from a Coolidge Tube. — The 

 paper under consideration was written by Rutherford and it 

 contains an account of some experiments made to determine the 

 maximum penetrating power of the X-rays excited by high 

 voltages in a Coolidge tube, lead being used as the absorbing 

 material. 



The radiation was excited by a large induction-coil, actuated 

 by a mercury motor-break in an atmosphere of coal-gas, and 

 capable of giving 20 inch sparks. The heating current through 

 the tungsten spiral of the tube was adjusted to give a radiation 

 of maximum intensity at the voltage required. This voltage was 

 fixed by an alternative spark-gap between points. The radia- 

 tion was found to be most constant when a fairly rapid stream 

 of sparks passed between the points. The voltage corresponding 

 to the alternative spark-gap was determined by comparison with 

 the sparking potential between two brass spheres 20 cm. in 

 diameter. The ionization current was measured by means of 



