1896.] Argon and Helium submitted to Electric Discharge. 265 



of this method of viewing the spectrum is the very small absorption 

 of light ; the spectrum is exceedingly brilliant. 



The secondary spectrum of hydrogen was very conspicuous in 

 these experiments ; it consists of a great number of lines in the 

 red, red-orange, and yellow. It appears to be brought out by the 

 electrodeless tube. 



3. 0*066 helium; 0264 hydrogen in 57*5 c.c. Helium invisible. 



P = 0'73 mm. Helium yellow just visible ; green fairly strong. 

 On reducing pressure, the helium gained in strength. The 

 red came out at a pressure of 0*12 mm., but it did not 

 become equal to the hydrogen-red in intensity on further 

 reduction of pressure. 



4. 0*066 helium; 0'66 hydrogen. ISTo helium visible. 



P = 145 mm. Helium green faintly visible ; yellow almost 

 extinct. 



P = 0"24 mm. A little more distinct, but still very faint. 



It appears possible, therefore, to detect 10 per cent, of helium in 

 hydrogen at low pressures ; but a smaller percentage would not be 

 seen. Interposing a spark gap or jars made no appreciable differ- 

 ence in these spectra. 



The second set of experiments had for its object to ascertain 

 how much hydrogen could be detected in helium. Without enter- 

 ing into details, it may be stated that the smallest trace of hydro- 

 gen can be detected in helium. Actually the proportion was reduced 

 to about 1 part of hydrogen in 100,000 of helium, and the hydrogen 

 lines were still easily recognised. 



The third set of experiments related to a mixture of nitrogen and 

 helium. Knowing that a very small amount of nitrogen could be 

 detected in a large amount of helium, the first charge of nitrogen was 

 reduced to 0"002 c.c. in 57 - 5 of space by emptying the pump, after 

 the stopcock had been left open for about ten minutes ; for experience 

 has shown that the flow of gas from one part of the apparatus to 

 another is very slow at small pressures. The results were obtained 

 in the same manner as before, and are tabulated below. 



Nitrogen, 3'0 p. c. at 0*90 mm. pressure. Strong. 



„ 0-4 „ 1-02 



0-4 „ 0-17 



„ 0-07 ., 1-04 „ Nitrogen still visible. 



07 „ 0-17 



„ 0011 „ P02 



„ O'Oll „ 0*17 „ Almost gone. 



The next charge of helium reduced the percentage of nitrogen 



VOL. LIX. T 



