on Ionization Currents. 125 



quantities of gas let into the ionization chamber. The pres- 

 sure obtained after a given number of manipulations of the 

 stopcocks " e " and " f " is estimated by means of a special 

 series of experiments in which the pressure has been measured 

 by the MacLeod gauge attached to the mercury pump. 



The method of blowing the little glass spheres containing the 

 emanation was perfected in collaboration with Mr. S. C. Lind.* 

 It may be more or less similar to the method previously used 

 by Rutherford and Roydsf in making the thin -walled tubes 

 used in their experiments on the a-rays. We have not seen, 

 however, a description of their method. A glass tube is 

 drawn out in or near a flame until it has a diameter of from 

 0-2-l mm and a wall thickness of from O02-0-l mm . The size 

 and wall thickness of this tube determine the size of the 

 sphere that can be blown from it. The end of the tube being 

 sealed in the flame, it is inserted inside a larger Jena-glass tube 

 (1 — 2 cra in diameter), which is clamped in a horizontal position 

 and heated on the outside by the hot flame of a large Bunsen 

 burner. By means of a foot-blower a considerable pressure is 

 maintained inside the capillary tube> and when the tip of this 

 tube arrives at the part of the Jena-glass tube heated by the 

 flame a little sphere begins to grow slowly. If the walls of 

 the capillary tube are not thin enough to begin with, they may 

 be drawn out thinner in the Jena-glass oven, the foot-blower 

 being used to create a small pressure inside the capillary tube. 

 By this method tubes may be drawn so thin that they show 

 brilliant interference colors, and yet stand the pressure of the 

 atmosphere with a vacuum inside. The sphere also can be 

 made as thin as this, but they do not stand the pressure of the 

 atmosphere as well as the tubes. 



The glass at the end of the capillary tube, when it was 

 sealed, leaves a thick spot on the surface of the sphere, and for 

 this reason it is better to use the a-rays passing out at the side 

 as represented at A in fig. 1. To bend the capillary tube at 

 the proper angle a tiny gas jet about l mm long at the end of a 

 fine glass tube may be used. 



The condensation of the emanation into the small glass 

 sphere presents some difficulties, for it is desirable, on account 

 of the decay of the emanation with the time, to remove the 

 gases, oxygen, hydrogen, carbonic acid gas, etc., given off by 

 the solution of radium salt and the vessel containing it, as 

 quickly as possible. I tried a number of methods of puri- 

 fying the emanations and finally adopted a slight simplification 

 of the method employed by M. Debierne. M. Debierne^: per- 



* Sitzungsber. Akad. Wiss. Wien, cxx, Ha, December, 1911. 

 fPhil. Mag., xvii, 281, 1909. 



% Ramsay and Soddy, Nature, July 16, p. 246, 1903 ; Proc. Roy. Soc, lxxii, 

 204, 1903 ; lxxiii, 346, 1904. Debierne, Comptes rendus, 14 Aug. 1905. 



