450 MR. A. FOWLER ON THE SPECTRUM OF MAGNESIUM HYDRIDE. 



1880, these observers stated that " if it (the line 5210 at the head of the green 

 fluting) be not due to a compound of magnesium with hydrogen, at any rate it 

 occurs with special facility in the presence of hydrogen, and ought to occur in the 

 sun if the temperature were not too high. We have been careful to ascribe this 

 line and its attendant series to a mixture of magnesium and hydrogen rather than 

 to a chemical compound, because this expresses the facts, and we have not yet 

 obtained any independent evidence of the existence of any chemical compound of 

 these elements." 



Further important evidence was obtained later by LIVEING and DEWAR from a 

 series of observations of the magnesium spark, without jar, in hydrogen at pressures 

 above that of the atmosphere.* It was found that the flutings increased in brilliancy 

 with the pressure, until at 1 5 to 20 atmospheres they were fully equal to the b group, 

 notwithstanding that these had also increased in brightness. On letting down the 

 pressure, the same phenomena occurred in reverse order, but the brightness of the 

 flutings did not diminish so rapidly as it had increased. On introducing a large 

 Leyden jar, when the pressure had again reached that of the atmosphere, the flutings 

 were still visible, but with gradually diminishing intensity until the spark had 

 continued for some time. In explanation of these observations, they wrote : " 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." Hence it was concluded that " the series of lines beginning 

 at wave-length 5210 are due to a combination of hydrogen with magnesium, and are 

 not dependent solely upon the temperature." 



Since then the spectrum in question has been generally known as that of 

 " magnesium hydride," and as no sufficient reason has appeared for assigning a 

 different origin, this designation has been retained in the present paper. 



I have at various times repeated many of the experiments described by LIVEING 

 and DEWAR (but not yet including the spark under pressure), and have obtained 

 results in complete accordance with those to which they referred. Additional 

 observations made during the present investigation may also be mentioned. For 

 example, when the arc was passed between magnesium poles in a nearly exhausted 

 globe, no trace whatever of the hydride flutings was seen on first starting the arc, 

 but the oxide group beginning at 5007 was a conspicuous feature of the spectrum. 

 After 2 or 3 minutes' running, however, the oxide group entirely disappeared, while 

 the hydride flutings gradually made their appearance and finally became very 

 brilliant. These observations are most simply explained by supposing that the 

 residual oxygen present in the globe at the beginning of the experiment was 

 gradually consumed, forming magnesium oxide, while hydrogen was liberated when 



* ' Roy. Soc. Proc.,' vol. 32, p. 198 (1881). 



