448 MR. A. FOWLEK ON THE SPECTRUM OF MAGNESIUM HYDRIDE. 



magnesium hydride spectrum, and includes a table of wave-lengths for the three 

 principal flutings, which it is believed will be of service to those who are engaged in 

 the reduction of photographs of the sun-spot spectrum. 



Methods of Producing the Spectrum. 



The magnesium hydride spectrum appears to have been first observed and 

 investigated by LIVEING and DEWAR in 1878.* In the course of their experiments 

 on the reversal of metallic lines, these observers recorded an absorption " line " near 

 5210 (corresponding to the first head of the green fluting), and concluded that it must 

 be attributed either to magnesium or to magnesium together with hydrogen. The 

 same " line " was afterwards observed in the spectrum of the spark between magnesium 

 points in an atmosphere of hydrogen,t the line being most constantly visible when no 

 jar was employed. AVith a small Leyden jar in the circuit, the line was still visible, 

 but it was not seen continuously when a large jar was used until the pressure of the 

 hydrogen, and its resistance, were very much reduced. In these experiments the 

 fluted character of the spectrum was clearly brought out.| 



In another paper it was further remarked that the line 5210 and the series of fine 

 lines which accompany it were sometimes seen in the flame of burning magnesium, 

 but appeared with increased brilliance if the burning magnesium were held in a jet ot 

 hydrogen, of coal gas, or of steam. 



The spectrum has also been investigated to some extent by Sir NORMAN LOCKYER, 

 whose observations were chiefly made on magnesium ribbon burning in the interior of 

 a Buusen flame. This is, perhaps, the readiest method of producing the spectrum for 

 observation with small instruments, but is unsuitable as a source intended for 

 photography with high dispersion. 



The most convenient source which I have yet found is the electric arc between 

 magnesium rods in an atmosphere of hydrogen, preferably at from 1 to 3 inches 

 pressure. The spectrum is also well seen if the arc be simply passed in an exhausted 

 globe, as a sufficient supply of hydrogen is liberated from the heated poles after a few 

 minutes' running. |j In this way the spectrum is obtained sensibly free from impurity 

 lines, and shows little more than the lines of magnesium (and sometimes those of 

 hydrogen) in addition to the flutings of the hydride. 



The construction of the apparatus employed for this experiment will be gathered 

 from fig. 1. A glass receiver of 2 or 3 litres capacity was selected, having 2 necks, 

 and a side tube about 6 inches in length, which was closed by a plate of glass or 

 quartz. The lower pole-piece was attached to a narrow brass tube passing through a 



* 'Roy. Soc. Proc.,' vol. 27, p. 352 (1878). 



t 'Roy. Soc. Proc.,' vol. 27, p. 494 (1878). 



t 'Roy. Soc. Proc.,' vol. 30, p. 96 (1880). 



'Roy. Soc. Proc.,' vol. 30, p. 27 (1879); vol. 43, p. 122 (1887). 



]| FOWLER and PAYN, 'Roy. Soc. Proc.,' vol. 72, p. 254 (1903). 



