1882.] On the Disappearance of some Spectral Lines, Sfc. 431 



Some further observations on this group of lines are contained in 

 the sequel. 



The effects of mixtures of metallic vapours in developing bright 

 lines are equally marked, and in general more easily observed. We 

 have before noticed (" Proc. Roy. Soc," vol. 30, p. 97) " that certain 

 lines of metals present in the crucible are only seen, or come out with 

 especial brilliance, when some other metal is introduced. This is the 

 case with some groups of calcium lines which are not seen, or barely 

 visible, in the arc in a lime crucible, and come out with great brilliance 

 on the introduction of a fragment of iron, but are not developed by 

 other metals such as tin." Effects of this kind are most frequent in 

 the case of metals which produce a large number of lines. They are 

 specially noticeable in the case of nickel and titanium. Both these 

 metals produce many lines, but a comparatively large quantity of 

 nickel may be introduced into a crucible of magnesia, through which 

 the arc of a powerful Siemens dynamo-electric machine is passing, 

 without the lines of nickel being strongly developed; they show 

 steadily as sharp but not specially bright lines ; but after several other 

 metals — iron, chromium, &c. — have been put in in succession, the 

 nickel lines frequently come out with great brilliance and considerably 

 expanded, and remain so for a long time. The titanium lines are 

 generally very persistent when that metal (as cyanide or oxide) has 

 been introduced into the crucible, but are subject to continual varia- 

 tions of intensity ; sometimes they are twinkling, at other times 

 steady ; but they can frequently be brought out with great brilliance 

 by dropping in iron or other metals. In such cases the metals put 

 into the arc can hardly be supposed to increase the resistance or the 

 temperature, but they may assist the volatilisation of each other, and 

 may also act by reduction, and so by increasing the incandescent 

 mass strengthen the weaker lines. Chlorides, however, which seem 

 to have the effect of helping the volatilisation and diminishing the 

 resistance so that the arc can be drawn out to a greater length, usually 

 sweep out the fainter lines. 



In many cases when a fragment of a metal is dropped into the cruci- 

 ble brilliant lines, hitherto unrecorded, come out for a short time and 

 quickly die out. It is hardly possible in such cases to say without 

 prolonged observations whether these lines belong to the newly intro- 

 duced metal or to some of those previously put in and developed by 

 the presence of the new metal. How much remains to be done in the 

 study of these lines, and how much light this may throw on the 

 phenomena of the solar lines, will be seen from the following account 

 of our observations of some very small portions of the spectrum of the 



paper on the spectra of sun-spots showing " what happens with regard to three ad- 

 jacent iron lines under different solar and terrestrial conditions," is at variance with 

 our observations, in so far as the line at 4923 is represented as absent from the arc. 



