different Metals by Rontgen Rays. 287 



let the metal to be examined form one end of the ionization 

 chamber, and let the primary beam of Rontgen rays pass 

 through the ionization chamber and fall on the metal. Then 

 all the corpuscles coming from the metal would pass into 

 the ionization chamber, producing ions in the air, and if the 

 ionization due. to the Rontgen rays was known, that due to the 

 corpuscular secondary rays could be deduced. Bat it was 

 found that the ionization due to the secondary rays was such 

 a small fraction of that due to the primary, that this method 

 was not accurate. 



The apparatus as finally used and shown in fig. 1 was 

 designed to eliminate the ionization due to the primary rays. 

 Two ionization chambers, " A " and " B," were used, consisting 

 of hollow brass cylinders about 5 cm long and about 10 om in 

 diameter, each encased in thick lead. The cylinders were 

 mounted on vulcanite insulators, u f /r ," with their axes in a 

 horizontal plane and parallel to each other and about 14 cm 

 apart. Above the cylinder "A" and supported on it was a 

 gold leaf electroscope consisting of a lead box, " a," with win- 

 dows for observing the gold leaf, and a narrow strip of gold 

 leaf hung from the upper end of a vertical brass rod, " b." 

 The brass rod, " b," passed through the bottom of the electro- 

 scope box into the interior of " A," being insulated from "A" 

 and "a" by vulcanite rings, " f," and its lower end termina- 

 ting in a ring, " c," about 4 cm in diameter, made of fine brass 

 wire. This ring was placed concentric with the axis of " A " 

 and in a vertical plane parallel to and midway between the 

 ends of "A." A brass rod, "g," insulated from "A" and 

 "B" by vulcanite rings, " f '," connected "c" to a similar 

 ring, '' c'," similarly placed with respect to " B." 



The Rontgen ray bulb was about 60 cm in front of the two 

 ionization chambers with its target midway between their axes, 

 so that the beam would pass through each cylinder longitudi- 

 nally. The ends of " A " and " B " on the side toward the tube 

 were covered with very thin aluminium leaf, and the rear end 

 of " B " was covered in a similar manner. The rear end of 

 " A " was left open, but provided with clamps by means of 

 which disks of the metal to be tested could be clamped over 

 this end. 



Both cylinders stood in a thick lead box, open at the rear, 

 and provided with windows in the side for observing the elec- 

 troscope. In the front of the box were two circular openings, 

 " W " and " W 7 ," corresponding respectively with the ends of 

 "A" and "JB." Both openings were just small enough to 

 prevent the beam of Rontgen rays from falling on the walls 

 of either cylinder. Lead screens with different sized holes 



