212 Liveing and Dewar — Separation of the Least Volatile 



bright, while other rays are very few, and those few barely 

 visible. Putting a jar into the circuit makes very little differ- 

 ence ; the three rays above mentioned remain much the 

 brightest, nearly, though not quite, so bright as before, nnd the 

 blue rays, so conspicuous in other tubes, though strengthened 

 by the use of the jar, are still very weak. In other tubes the 

 extreme red ray is invisible,* the rays at X 5571 and 5871 abso- 

 lutely, as well as relatively, much feebler, while the strong blue 

 rays are bright, even brighter than the green and yellow rays 

 above named. In one tube the blue rays could be seen, though 

 not the others. This looks very much as if two different gases 

 were involved, but we have not been able to assure ourselves 

 of that. The case seems nearly parallel with that of hydrogen. 

 There are some hydrogen tubes which show the second spec- 

 trum of hydrogen very bright, and others which show only 

 the first spectrum ; the second spectrum is enfeebled or extin- 

 guished by introducing a jar into the circuit, while the first 

 spectrum is strengthened ; and the conditions which determine 

 the appearance of the ultra-violet series of hydrogen rays have 

 not yet been satisfactorily made out. 



It is to be noted that putting the jar out of circuit does not 

 in general immediately reduce the brightness of the rays which 

 are strengthened by the jar discharge. Their intensity fades 

 gradually, and is generally revived, more or less, by reversing 

 the direction of the current, but this revival gets less marked 

 at each reversal until the intensity reaches its minimum. The 

 rays strengthened by the jar discharge also sometimes appear 

 bright, without a jar, on first passing the spark when the elec- 

 trodes are cold, and fade when the electrodes get hot, reappear- 

 ing when the tube has cooled again. Moreover, if the dis- 

 charge be continued without a jar, the resistance in the 

 krypton tubes increases rather rapidly, the tube becomes much 

 less luminous and finally refuses to pass the spark. With an 

 oscillatory discharge the passage of the spark and the bright- 

 ness of the rays are much more persistent. This seems to 

 point to some action at the electrodes, which is more marked 

 in the case of krypton than in that of xenon. 



The wave-lengths of the xenon and krypton rays in the tables 

 below were determined, in the visible part of the spectrum, 

 with a spectroscope having three white flint-glass prisms of 60° 

 each, by reference to the spark spectrum of iron, except in the 

 cases of the extreme red ray of krypton, which was referred to 

 the flame spectrum of potassium, and its fainter neighbor, 

 which we saw but did not measure. The indigo, violet, and 

 ultra-violet rays were measured in photographs, taken with 

 quartz lenses and two calcite prisms of 60° each. The spec- 

 trum of the iron spark was photographed at the same time as 



