1883.J 



On the Reversal of Hydrogen Lines. 



75 



The light of the narrow part of the tube is, under these circum- 

 stances, very brilliant, while the spark in the broad ends is wider and 

 less bright, but does not fill the tube. On viewing such a tube end 

 on, and projecting the image of the narrow part of the tube on to the 

 slit of the spectroscope, a continuous spectrum, of the width of the image 

 of the narrow part of the tube, is seen, besides the lines of hydrogen 

 given by the discharge in the wide part of the tube. These lines ex- 

 tend above and below the narrow continuous spectrum if the electrode 

 is well placed so that half-an-inch or so of the spark in the wide part 

 of the tube may intervene between the narrow part of the tube and the 

 spectroscope. The continuous spectrum of the narrow part of the 

 tube seems due chiefly to the expansion of the hydrogen lines when 

 the discharge occurs in so confined a space, and it is much brighter 

 than the lines given by the spark in the wide part of the tube. 

 Where the latter cross the continuous spectrum a very evident 

 absorption occurs. We have observed it with a diffraction grating. 

 The C line in the third order falls so near the F line in the fourth, 

 that both may be observed together. The appearance presented in 

 our spectroscope is shown in the accompanying drawing ; F is much 

 more expanded than C, and the reversal consequently less marked 

 though quite plain. The other lines being still more diffuse their 

 absorption could not be traced. 



F C 



We have before observed (" Proc. Roy. Soc," vol. 30, p. 157) that 

 the C and F lines of hydrogen are visible in the arc of a De Meritens 

 magneto-electric machine taken in hydrogen ; though in the arc of 

 a Siemens machine the C line can only be detected at the instant of 

 breaking the arc, the F line hardly at all. When, instead of taking 

 the arc in hydrogen, small drops of water are allowed to fall from a 

 fine pipette into the arc taken in air in a lime crucible, each drop as 

 it falls into the arc produces an explosive outburst of the hydrogen 

 lines. Generally the outburst is only momentary, but occasionally a 

 sort of flickering arc is maintained for a second or two and the 

 hydrogen line C is visible all the time. The lines (0 and F) are 

 usually much expanded, but are frequently very unequally wide in 

 different parts of the line. F is weaker, more diffuse, and more 

 difficult to see than C, and is visible for a shorter time. There is no 

 sign of reversal. In the explosive character of the outburst and the 



