352 



Sir W. Bamsay and Mr. F. Soddy. 



[Apr. 14, 



it was then mixed with a large excess of oxygen, and sparked in 

 presence of caustic soda for some hours to remove nitrogen. The 

 oxygen was next withdrawn by means of phosphorus, and the minute 

 bubble left was mixed with a bubble of oxygen, in order to wash it 

 into the apparatus to which the vacuum tube was sealed. As already 

 described, this apparatus did not differ from that shown in fig. 2, 

 except for the fact that the tube containing the copper spiral was 

 replaced by one containing a fragment of phosphorus, in order to 

 withdraw the oxygen. The phosphorus was warmed and withdrew 

 the oxygen. The gas was then forced by means of mercury through 

 a cooled U-tube, and a portion reached the vacuum tube. On passing 

 a current for an instant, the D 3 line was plainly seen, but nitrogen 

 was also present in small amount. The tube was then sealed off. 



The volume of the spectrum tube, including the U-tube, had been 

 previously estimated by filling it twenty times with air and pumping 

 out each time ; from this measurement the total volume was found 

 to be - 310 c.c, and after the U-tube had been sealed off, the same 

 process was repeated with the U-tube and connections, minus the 

 spectrum tube. The volume of the spectrum tube was thus found to 

 be 0-165 c.c. 



An exactly similar spectrum tube made of the same glass and 

 having the same length was attached to a bulb tube, from which it 

 could be cut off by turning a stop-cock; the bulb tube in its turn 

 could be cut off from the pump by a stop-cock. The capacity of the 

 spectrum tube as veil as of the bulb tube was known. A known 

 amount of helium was introduced into the bulb tube and the spectrum 

 tube by means of an inverted syphon furnished with two stop-cocks ; 

 the volume between the stop-cocks was 0'0268 c.c. As the volume 

 of the spectrum tube and connections was 1*68 c.c, and that of the 

 bulb was 1*25 c.c, when the gas contained in the spectrum tube was 

 allowed to expand into the evacuated bulb tube, its volume was 

 reduced in the ratio 1-68/1*25 + 1-68, or 0'57. The spectrum tube 

 containing the fraction of the helium from 60 days' accumulation was 

 placed in series with that containing helium, so that the same current 

 traversed both, and their spectra were compared as regards luminosity 

 of the D 3 line. It was necessary to divide the contents of the helium 

 tube seven times before the D 3 line could be regarded as of com- 

 parable intensity in both spectrum tubes. Multiplying this ratio by 

 the volume of the helium admitted at atmospheric pressure into the 

 apparatus, the volume remaining in the apparatus after rarefaction is 

 given : 



(0-57) 7 x 0-0268 = 0-000517 c.c 



Now, the volume of the helium tube and connections was by chance 

 practically ten times that of the spectrum tube alone (1*65 and 0T65), 



