61) 6 Professor Deicar [June 10, 



At a pressure of 2 J atmosj^Leres, with a jar, the ultra-violet maguesium 

 triplet near L was very well reversed, and the two pairs of lines on its 

 less refrangible side (shown in Plate III., Fig. 3) were expanded into 

 two diffuse bands. 



In nitrogen and in carbonic oxide the general effects of increased 

 pressure on the magnesium lines (not the magnesium-hydrogen bands) 

 seemed to be much the same as in hydrogen. Without a jar the blue 

 and yellow lines were enfeebled, and at the higher pressures dis- 

 appeared, while the h group was very brilliant but not much ex- 

 2>auded. With the jar all the lines were expanded, and all three lines 

 of the h group strongly reversed. The bands of the oxide (wave- 

 length 4930-5000) were not seen at all in hydrogen or nitrogen ; 

 they were seen at first in carbonic oxide, but not after the sparking 

 had been continued for some time. 



The disappearance of certain lines at increased pressure is in har- 

 mony with the observations of Cazin,* who noticed that the banded 

 spectrum of nitrogen, and also the lines, grew fainter as the pressure 

 was increased, and finally disappeared. When a Leyden jar is em- 

 ployed there is a very great increase in the amount of matter 

 volatilised by the spark from the electrodes, as is shown by the very 

 rapid blackening of the sides of the tube with the deposited metal, 

 and this increase in the amount of metallic vapour may reasonably be 

 supposed to affect the character of the discharge, and conduce to the 

 widening of the lines and the reversal of some of them. Without a 

 jar the amount of matter carried off the electrode also doubtless 

 increases with the pressure and consequent resistance, and may be the 

 cause of the weakening, as Cazin suggests, of the lines of the gas in 

 which the discharge is passed. It is to be noted, moreover, that the 

 disappearance of the hydrogen lines depends, in some degree, on the 

 nearness of the electrodes. The lines C and F which were, as above 

 stated, sometimes invisible in the spark when the electrodes were 

 near, became visible, under circumstances otherwise similar, when the 

 magnesium points had become worn away by the discharge. 



Comparison of the Spectra. 



When we compare the spectra of magnesium in the flame, arc, and 

 spark, we observe that the most persistent line is that at wave-length 

 '2850, which is also the strongest in the flame and arc, and one of the 

 strongest in the spark. The intensity of the radiation of magnesium 

 at this wave-lenf^th is witnessed by the fact that this line is always 

 reversed in the flame as well as in the arc when metallic magnesium 

 is introduced into it, and in the spark between magnesium electrodes 

 when a Leyden jar is used. It is equally remarkable for its power of 

 expansion. In the flame it is a broad band, and equally so in the arc 

 when magnesium is freshly introduced, but fines down to a narrow 

 line as the metal evaporates. 



* ' Phil. Mag.' 1877, vol. iv. p. 154. 



