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ON OUR KNOWLEDGE OF SPECTRUM ANALYSIS. 289 



synonymous ; but those who are accustomed to work with a lens between 

 the spark and the slit, will be able to give many instances where a faint 

 line is seen to stretch nearly across the whole field of view, while a strong- 

 line may be confined to the neighbourhood of the pole, and is reduced 

 sometimes to a brilliant point onlj^. 



We give a few conspicuous examples, to which we shall have occasion 

 to refer again. The remarks refer to the length of the lines in sparks 

 condensed by means of a Leydeu jar. 



Lithium : The blue line (46027) is brilliant but short. It is given by 

 Thalen as stronger than the orange line, which is much longer. 



Lead: 4062'5, one of the longest lead lines, but faint, and according" 

 to Lockyer difiicult to observe. 



Tin : 56300 is the longest tin line, but it is faint, while the stronger 

 lines near it (5588'5 and 5562'5) are shorter. 



Zinc: The zinc lines (4923-8, 4911-2, 4809-7, 4721-4, 4679'5) are 

 given by Thalen as of equal intensity, but the three more refrangible oneg, 

 are longer. 



As a first result, Mr. Lockyer found that, by a reduction of pressui-e,, 

 some of the shorter lines rapidly decreased in length and disappeared, 

 Avhile the longer lines remained visible and were sometimes hardly affected. 

 It follows, therefore, that a reduction of pressure may change the relative 

 intensity of the lines ; for a stronger, but shorter, line may disappear, 

 while a weaker and longer one remains. We may quote, for instance, 

 Mr. Lockyer's remark on the behaviour of the zinc lines, when the pres- 

 sure is reduced. 



' In the case of zinc, the effect of these circumstances was very marked,^ 

 and they may be given as a sample of the phenomena generally observed. 

 When the pressure-gauge connected with the Sprengel pump stood at from 

 35 to 40 mm., the spectrum at the part observed was normal, except that 

 the two lines 4924 and 4911 (both of which, when the spectrum is observed 

 under the normal pressure, are lines with thick wings) were considerably 

 reduced in width. On the pump being started, these lines rapidly decreased 

 in length, as did the line at 4679, — 4810 and 4721 being almost unaffected ; 

 at last the two at 4924 and 4911 vanished, as did 4679, and appeared 

 only at intervals as spots on the poles, the two 4810 and 4721 remaining 

 little changed in length, though much in brilliancy. This experiment 

 was repeated four times, and on each occasion the gauge was found to bo 

 almost at the same point, "viz. : — 



1st observation, when the lines 4924 and 491 1 were 



gone, the gauge stood at . 30 millimetres. 



2nd „ „ „ „ . 29 



3rd „ „ „ „ . 29 



4tli ,, „ „ „ . 31 



'A rise to 34 milhmetres was sufficient to restore the lost lines.' 



Mr. Lockyer next examined the spectra given by chemical compounds r 

 ' It was found in all cases that the difference between the spectrum of 

 the chloride and the spectrum of the metal was : That under the same 

 spark conditions the short lines vjere obliterated, ivhile the air lines remained 

 unchanged in thichness.' 



Thus, for instance, when the spark was taken from zinc chloride, it 

 did not show the lines 4923 and 4911, which, though of equal brightness 

 with 4809, 4721, 4679, are shorter, and disappear, as we have seen, when 

 the pressure is reduced. The three last-mentioned lines were seen. 

 1880. xj 



