1881.] Investigations on the Spectrum of Magnesium. 



199 



and do not always extend quite across the field. As the pressure is 

 increased, however, they increase in brilliance and soon extend per- 

 sistently from pole to pole, and go on increasing in intensity, until, at 

 fifteen and twenty atmospheres, they are fully equal in brilliance to 

 the b group, notwithstanding the increased brightness these have 

 acquired by the higher temperature, due to the increased pressure. 

 The second set of flutings, those in the yellowish-green, come out as 

 the pressure is increased, and, in fact, at twenty atmospheres only 

 the b group and the flutings are noticeable ; if the yellow magnesium 

 line be visible at all it is quite lost in the brilliance of the yellow 

 flutings. The tail of fine lines of these flutings extend at the high 

 pressure quite up to the green, and those of the green flutings quite 

 up to the blue. On again letting down the pressure the like phe- 

 nomena occur in the reverse order, but the brilliance of the flutings 

 does not diminish so rapidly as it had increased. If, now, when the 

 pressure has again reached that of the atmosphere, a large Leyden jar 

 be interposed in the circuit, on passing the spark the flutings are still 

 seen quite bright, and they continue to be seen with gradually 

 diminishing intensity until the sparks have been continued for a con- 

 siderable time. It appears that the compound, which had been 

 formed in large quantity by the spark without jar at the higher 

 pressures, is only gradually decomposed, and not re-formed, by the 

 high temperature of the spark with jar. This experiment, which was 

 several times repeated, is conclusive against the supposition that the 

 flutings are merely due to a lower temperature. When the pressure 

 was increased at the same time that the jar was employed, the flutings 

 did not immediately disappear, but the expansion of the magnesium 

 lines and the increase of the continuous spectrum seemed to over- 

 power them. 



When nitrogen was substituted for hydrogen, the strongest lines of 

 the green flutings were seen when the spark without jar was first 

 passed at atmospheric pressure, probably from hydrogen occluded, as 

 it usually is, in the magnesium electrodes. As the pressure was 

 increased they speedily disappeared entirely and were not again seen 

 either at high or low pressures. 



With carbonic oxide the same thing occurred as with nitrogen ; but 

 in this gas the flutings due to the oxide of magnesium (wave-length 

 4930 to 5000) were, for a time, very well seen. 



Ciamician (" Sitzungsber. Akad. Wissensch.," Wien, 1880, p. 437) 

 has described a spectrum of magnesium of the first order (in 

 Plucker's nomenclature) obtained by taking sparks from an induc- 

 tion coil, without a jar, between magnesium electrodes in an atmo- 

 sphere of hydrogen. He gives a figure to a scale of this spectrum, 

 but it is not to a scale of wave-lengths, so that exact comparison 

 of his observations with ours is difficult. The least refrangible set of 



VOL. XXXII. p 



