1882.] On an Arrangement of the Electric A rc, fyc. 121 



under easy control. We have taken photographs of the violet and 

 lower part of the ultra-violet spectrum given by the tube at succes- 

 sive intervals while the temperature was rising, and noted the follow- 

 ing results. When commercial carbons were used the first lines to be 

 seen as the temperature rose were the potassium lines, wave-length 

 4044r-6, next the two aluminium lines between H and K became con- 

 spicuous, then the manganese triplet about wave-length 4034, and the 

 calcium line, wave-length 4226, then the calcium lines near M and an 

 iron line, probably M, between them, and then gradually a multitude 

 of lines which seem to be all the conspicuous iron lines between O 

 and h. At this stage, when the small rod c is used to give a back- 

 ground, the bright continuous spectrum is crossed by a multitude of 

 sharp dark lines, vividly recalling the general appearance of the solar 

 spectrum. In the higher region the continuous spectrum extends 

 beyond the solar spectrum, and the magnesium line, wave-length 

 2852, is a diffuse dark band, while all the strong iron lines about T, 

 and the aluminium pair near S, are seen as dark lines. The be- 

 haviour of the calcium lines H and K is peculiar. These lines are 

 often absent altogether, when the line wave-length 4226 and the two 

 near M are well seen, and when the two aluminium lines between 

 them and many of the iron lines are sharply reversed. Even the 

 introduction of a small quantity of metallic calcium or calcium 

 chloride into the tube did not bring them out reversed. They were 

 only seen as bright lines, not very strong, when the small rod c was 

 removed. 



In some of the photographs H is visible as a bright line without K. 

 We have formerly observed that K shows reversal in the electric arc 

 spectrum taken in a lime crucible on the addition of aluminium, when H 

 remains bright, and such a condition as that shown by the hollow carbon 

 tube where H is present without K, might legitimately have been 

 predicted. The lithium lines at 4603 and 4131 are often bright when 

 many other lines in the neighbourhood are reversed, and must, there- 

 fore, be regarded as relatively difficult of reversal. As a rule the lines 

 less refrangible than 4226 are balanced as to their emissive and 

 absorptive power, and, therefore, disappear, while the more refrangi- 

 ble are reversed. The cyanogen group at 3883 remain bright when 

 the iron lines on either side are reversed ; they often, however, dis- 

 appear on the continuous spectrum. Many lines about P and Q of 

 the solar spectrum are reversed. The cyanogen band above K is 

 generally to be found in the photographs of the spectrum when only 

 air is in the tube. It is then very faint, and is the only cyanogen 

 group visible. If ammonia is passed into the tube the fine set above 

 K, the N group, and, although less plainly marked, the set at 4218, 

 appear. In one plate the three lines at 4380 and the group of seven 

 at 4600 appear along with the blue hydrocarbon set. It is well 



