224 SCIENCE PROGRESS 



10 milligrams of bromide placed in a vacuum tube, which was 

 then sealed off. He stated that after fourteen days he observed 

 the red, green, blue, and violet lines of the helium spectrum, 

 although the yellow line was absent. Such a conjunction seems 

 unlikely ; Himstedt and Meyer consider it probable that the 

 lines observed belonged to the secondary spectrum of hydrogen. 



These physicists carried out in 1904-5 a series of most 

 exhaustive tests ; the results of their work supplied any final 

 proof, if that were required. In their first experiment 50 milli- 

 grams of radium bromide were placed in a U-shaped glass tube, 

 to which was joined a vacuum tube containing aluminium 

 electrodes. The tubes were joined to a pump, and the whole 

 apparatus completely evacuated during seventy-two hours. The 

 tubes were then sealed off. Initially only the spectrum of 

 hydrogen and carbon monoxide were visible. After fourteen 

 months the yellow and green helium lines were seen, but could 

 not be photographed. Three months later most of the strong 

 helium lines were visible. The tubes employed in this, and in 

 most of Himstedt and Meyer's experiments, were comparatively 

 large, which probably accounts for the length of time before 

 sufficient helium had accumulated to give a visible spectrum. 



The authors considered it unlikely that the helium could 

 come from the electrodes, but carried out two experiments to 

 test this point specifically. Twenty-five milligrams of the 

 bromide were placed in a small U-tube, to which was joined a 

 small spectrum tube. The whole was connected to a pump, 

 evacuated, and washed out with hydrogen ten times, and during 

 each operation the current was passed to remove completely 

 any helium from the electrodes. Finally, the tubes were 

 exhausted completely and sealed off. After three months the 

 helium spectrum was definitely identified. 



The remaining 25 milligrams of bromide were converted 

 into sulphate, and this dried and ignited. It was then placed 

 in a tube of quartz, to which a small electrodeless tube was 

 joined with marine glue. The tubes were connected to a pump, 

 evacuated, and rinsed with hydrogen as usual, and finally sealed 

 off. A Tesla coil was used to light up the spectrum tube. 

 After three weeks the four principal helium lines were observed, 

 and two weeks later they were identified conclusively. 



Later the same workers carried out a series of negative tests ; 

 in all cases the results were satisfactory. 



