1913] on the Method of Positive Rays 597 



as are used for Rontgen ray focus tubes, and tlie cathode rays focus 

 on the platform on which the substance to be bombarded is placed. 

 [It is not absolutely necessary to focus the cathode rays in this way, 

 but it makes the supply of the gas X3 more copious.] After the 

 metal or other solid to be examined has been placed on the platform, 

 the taps between A and B being turned so as to cut oflf the connexion 

 between them, A is exhausted until the vacuum is low enough to 

 give the cathode rays ; the discharge is then sent through A, and 

 the cathode rays bombard the solid. The result of this is that in a 

 few seconds so much gas, mainly CO^, and hydrogen, is driven out 

 of it that the pressure gets too high for the cathode rays to be formed, 

 and unless some precautions to lower the pressure were taken the 

 bombardment would stop. To avoid this, a tube containing charcoal 

 cooled by liquid air is connected with A, and this absorbs the CO2 

 and enough of the hydrogen to keep the vacuum in the cathode ray 

 state. To see what new gases are given ofP in consequence of the 

 bombardment, a photograph is taken while the connexion between 

 A and B is cut off. After this is finished, and when the bombardment 

 has gone on for about four hours, the tap is turned and a little of 

 the gas from A is allowed to go into B ; another photograph is 

 taken, and those lines in the second photograph which are not in the 

 first represent those gases which are liberated by the bombardment, 

 and which have escaped being absorbed by the charcoal. I have 

 here a slide (Fig. 5) representing the result of bombarding nickel. 

 There are two photographs, one (a) before turning the tap and the 

 other (/3) after ; in the second you see the three line very distinctly, 

 while it is absent from the first, showing that the gas giving the 

 three line has been liberated by the bombardment. I have got similar 

 results to these when, instead of nickel, iron, copper, lead, zinc have 

 been bombarded. I have tried two specimens of meteorites kindly 

 lent to me from the Mineralogical Museum, Cambridge, and found 

 there the three line. Nearly every substance I have tried gives, the 

 first time it is bombarded, the helium line as well as this line due to 

 X3 ; if, however, the same substance is bombarded a second time, 

 the helium line is in general absent (occasionally it is still to be 

 detected, though exceedingly faint) ; and on the third bombardment 

 is invisible in all the substances I have tried except monazite sand, 

 where it is given off in exceedingly large quantities as long as the 

 bombardment continues. It is remarkable that monazite sand, which 

 contains so many elements, gives no trace of the three line when 

 bombarded. 



I have also obtained the X3 line and also the helium line when 

 the tube A was replaced by one containing a Wehnelt cathode ; with 

 this the current of cathode rays through the tube was much larger 

 than with the other cathode, though the velocity of the rays was 

 smaller. The Wehnelt cathode gives the line without placing pieces 



