394 Profs. G. D. Liveing and J. Dewar. On the 



discharge when the slit of the spectroscope is wide. Eunge used a 

 grating, whereas we have, for the sake of more light, used a prism 

 spectroscope throughout, and were therefore able to observe many 

 more rays than he. 



There is one very remarkable change in the xenon spectrum pro- 

 duced by the introduction of a jar into the circuit. Without the jar 

 xenon gives two bright green rays at about A 4917 and A 4924, but on 

 putting a jar into the circuit they are replaced by a single still stronger 

 ray at about A. 4922.* In no other case have we noticed a change so 

 striking as this on merely changing the character of the discharge. 

 Changes of the spectrum by the introduction of a jar into the circuit are, 

 however, the rule rather than the exception, and there are changes in 

 the spectrum of krypton which seem to depend on other circumstances. 

 In the course of our examination of many tubes filled with krypton 

 in the manner above indicated, we have found some of them to give 

 with no jar the green ray A 5571, the yellow ray A 5871, and the red 

 ray A 7 600 very 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, and 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 A 5571 and 5871 absolutely, 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 ass are our- 

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

 There are some hydrogen tubes which show the second spectrum of 

 hydrogen very bright, and others which show only the first spectrum; 

 the second spectrum is enfeebled or extinguished by introducing a jar 

 into the circuit, while the first spectrum is strengthened ; and the con- 

 ditions 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 dis- 

 charge also sometimes appear bright, without a jar, on first passing 

 the spark when the electrodes are cold, and fade when the electrodes 

 get hot, reappearing when the tube has cooled again. Moreover, if 



* This line is almost identical with a strong helium line, but the yellow line of 

 helium was not seen. 



