1904.] The Spectrum of the Radium Emanation. 473 



hydrogen. They were found to lie exactly on a scale which had 

 previously been constructed. The new lines were read as rapidly as 

 possible, an operation which required about half a minute. During a 

 second reading many of the lines had faded, and the secondary spectrum 

 of hydrogen began to appear, and rapidly grew stronger. It was 

 identified by throwing into a field a hydrogen spectrum through the 

 small prism ; and it soon became so powerful as to mask the spectrum 

 of the emanation completely. In order to attempt to recover it, the 

 mercury was again drawn down to a, and liquid air again poured into 

 the jacket ; the emanation again condensed, and the tap to the pump 

 was opened, and the hydrogen removed by the pump until its 

 spectrum was again hardly visible. On repeating the series of 

 operations already described, the spectrum of the emanation was seen 

 a second time, but it was so transient that only the position of some of 

 the lines could be confirmed. 



Next day, only the spectrum of hydrogen was visible ; its secondary 

 spectrum was strong. The day after, the same was the case : but 

 interposing a jar and spark-gap brought out two lines which had 

 previously been mapped ; they were very feeble. 



In the table which follows, all the strong lines which were read are 

 given ; the degree of coincidence of those which are of known wave- 

 length shows the approach to accuracy obtained ; the error is probably 

 less than five Angstrom units. 



Wave-length. Kemarks. 



6567 Hydrogen C ; true wave-length, 6563 ; observed 

 each time. 



6307 Observed only at first; evanescent. 



5975 ,, ,, ,, 



5955 ,, ,, ,, 



5805 Observed each time ; persistent. 



5790 Mercury; true wave-length, 5790. 



5768 „ „ „ 5769. 



5725 Observed only at first ; evanescent. 



5595 Observed each time; persistent and strong. 



5465 Mercury; true wave-length, 5461. 



5105 Not observed at first; appeared after some seconds; 



persisted, and was visible during the second 



examination. 



4985 Observed each time ; persistent and strong. 

 4865 Hydrogen F; true wave-length, 4861. 

 4690 Observed only at first. 



4650 Not observed when the emanation was examined 



again. 

 4630 Ditto. 



4360 Mercury ; true wave-length, 4359. 



