258 SCIENCE PROGRESS. 



lines. Which of these we shall see in a tube will depend 

 upon the pressure of the gas and the electric current used. 

 The fluted spectrum of nitrogen is very bright and full of 

 beautiful detail in the orange part of the spectrum ; the line 

 spectrum, on the other hand, is almost bare in that region. 



It is important to note that it so happened 'that the pressure 

 and electric conditions employed by Dr. Hillebrand enabled 

 him generally to see the fluted spectrum. This however 

 was not always the case. In an interesting letter to Pro- 

 fessor Ramsay he writes (Proc. Roy. Soc, vol. lviii., p. 81): — 



" Both Dr. Hallock and I observed numerous bright 

 lines on one or two occasions, some of which apparently 

 could be accounted for by known elements — as mercury, or 

 sulphur from sulphuric acid ; but there were others which I 

 could not identify with any mapped lines. The well-known 

 variability in the spectra of some substances under varying 

 conditions of current and degree of evacuation of the tube 

 led me to ascribe similar causes for these anomalous appear- 

 ances, and to reject the suggestion made by one of us in a 

 doubtfully serious spirit, that a new element might be in 

 question." 



Dr. Hillebrand concludes his paper as follows : — 



"The interest in the matter is not confined merely to a 

 solution of the composition of this one mineral ; it is broader 

 than that, and the question arises : May not nitrogen be a 

 constituent of other species in a form hitherto unsuspected 

 and unrecognisable by our ordinary chemical manipulations? 

 And, if so, other problems are suggested which it is not now 

 in order to discuss." 



D 3 AND OTHER UNKNOWN LINES IN NEBULA, 1890. 



A negative of the nebula of Orion, taken at my observatory 

 at Westgate-on-Sea in 1890, contains fifty-six lines, and of 

 course by determining, as we have been able to do approxi- 

 mately, the wave-lengths — the positions of these lines in 

 the spectrum — we can determine the exact light notes 

 represented, and therefore the substances which produce 

 them. In this spectrum of the nebula of the Orion were 



