190 



Profs. Liveing and Dewar. 



[Mar. 12, 



described by us (" Proc. Roy. Soc," vol. 30, p. 96), in the spectrum 

 of the flame ; but we have never observed that the appearance of these 

 lines, or of the strong line with which they begin, is connected with 

 the non-appearance of & 4 . Indeed, we can almost always see all three 

 lines of the b group in the flame, though as £ 4 is the least strong of 

 the three, it is likely to be most easily overpowered by the continuous 

 spectrum of the flame. The new observations recorded below leave, 

 we think, no room for doubt that the series of lines beginning at 

 wave-length 5210 are due to a combination of hydrogen with mag- 

 nesium, and are not dependent solely on the temperature. 



The wave-lengths of the strong triplet near M are about 3720, 3724, 

 3730, and of the defined edges of the bands about 3750, 3756, 3765,. 

 3772, 3777, 3782, 3790, 3799, 3806, 3810, 3815, 3824, 3841, 3845, 

 3848, 3855, 3858, 3860, 3865. 



Burning magnesium in oxygen instead of atmospheric air does not 

 bring out any additional lines ; on the contrary, the continuous spec- 

 trum from the magnesia overpowers the line spectrum, and makes it 

 more difficult of observation. 



Magnesia heated in the oxyhydrogen jet does not appear to give 

 the lines seen in the flame. 



We have left out of the figure and from the enumeration of lines 

 the well-known bands of the oxide. 



Spectrum of the Arc. 



By examining the arc of a battery of 40 Grove's cells, or that of a 

 Siemens' machine, taken in a crucible of lime, under the disper- 

 sion of the spectrum of the fourth order given by a Rutherford 

 grating of 17,296 lines to the inch, we are able to separate the iron 

 and magnesium lines which form the very close pair & 4 of the solar 

 spectrum. Either of the two lines can be rendered the more prominent 

 of the pair at will, by introducing iron or magnesium into the crucible. 

 The less refrangible line of the pair is thus seen to be due to iron, the 

 more refrangible to magnesium. Comparison of the solar line and 

 the spark between magnesium points confirms this conclusion, that the 

 magnesium line is the more refrangible of the two. 



In the ultra-violet part of the spectrum photographs show several 

 new lines. First, a pair of lines above U at wave-lengths about 2942, 

 2938'5.* These lines are a little below a pair of lines given by the 

 spark for which Cornu has found the wave-lengths 2934" 9, 2926*7. 

 The latter pair are not seen at all in photographs of the arc, nor the 

 former in those of the spark. The strong line, wave-length about 



* [Many of our photographs show besides these two lines a third line wave-length 

 about 2937'5, but we hare not been able to determine certainly that it is due to' 

 magnesium. If so this group probably belongs to the series of triplets. — June 2.] 



