Rays of Positive Electricity. 



229 



they are all on one plate. For example, the magnetic 

 deflexion of the hydrogen atom is about fourteen times that 

 of the mercury one ; thus if the deflexion of the hydrogen 

 atom is within the limits of the plate, that of the mercury 

 atom will be too small to be measured with accuracy. When 

 three photographs are taken, however, we can use a small 

 magnetic force for the first and get the light atoms on the 

 plate, then a larger magnetic field for the second for the 

 study of the atoms and molecules of oxygen and other 

 elements of a similar atomic weight, while we (an use a still 

 stronger field for the third for the study of very heavy atoms 

 such as those of mercury. 



Method of exhausting the Tube. — The tube was exhausted 

 by a Gaede pump and charcoal cooled with liquid air. The 

 narrow tube through which the rays passed fitted at ihe 

 end next the photographic plate into a plug of ebonite, and 

 the joint was made air-tight with a little wax, the joint 

 between the ebonite plug and the glass tube into which it 

 fitted was also made air-tight in the same way ; thus the only 

 way gas could get from the discharge-tube into the space 

 through which the positive rays passed after they left the 

 tube was through this long narrow tube through which the 

 gas only filtered slowly ; two large tubes containing charcoal 

 immersed in liquid air led into this part of the apparatus and 

 kept the pressure much lower than it was in the discharge- 

 tube. 



The communication with the Gaede pump was made 

 between the cathode and the ebonite plug as shown in fig. 3. 



Fi»\ 3. 



^^^ 



To Gaede Pujnh 



essksw 



T „.. 



The cathode fits fairly tightly into the tube, so that there 

 may be a considerable difference of pressure between the 

 discharge-tube and the space 0. 



Method of filling the Tube with Gases. — When the rays m 

 some particular gas were under examination a constant stream 

 of that gas was kept flowing through the tube. This was 

 done by fusing on the discharge-tube a glass tube, whose end 



