656 



Mr. J. A. Crowther on the Secondary 



for containing the gases under investigation. They were 

 made equal and similar in every respect, and were arranged 

 symmetrically with respect to the anticathode o£ the focus 

 tube F. It will be sufficient, therefore, to describe one of 

 them. 



The X-rays enter the box through a thin aluminium window 

 c, 5 cms. square, in its end, while the secondary rays pass 

 out through a second aluminium window, d, 6 cms. square in 

 the roof. Secondary rays are given off not only by the gas 

 in the box, but also by the aluminium window e, and by the 

 far end of the box upon which the primary rays finally fall. 

 As shown in figure 2, however, the apparatus is so arranged 

 that none of these secondary rays can enter the ionization- 

 chambers, B, B. A certain amount of tertiary radiation, 

 however, enters the ionization-chambers, mainly from the 

 aluminium of the windows d. This was found to amount to as 

 much as 20 per cent, of the total radiation emerging from the 

 box. The effect was, however, as will be shown later, com- 

 pletely eliminated by the methods of measurement employed. 



The boxes were made air-tight by careful soldering, and 

 finally by covering all the joints, while hot, with sealing-wax. 

 An exit-tube connected each box to a pressure-gauge and a 

 large vessel which could be exhausted by means of a water- 

 pump. Other side-tubes served for the admission of gas, 



Fii 



7b EiEcrffoscops 



iv, id (fig. 2) illustrates in section a useful device for 

 rendering the aluminium window air-tight. A brass frame, 



