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



449 



rubber stopper which fitted tightly into the lower neck of the flask. The upper pole 

 was attached to a similar brass tube and rubber stopper, but in order to allow of the 

 manipulation of the arc, the stopper was not placed directly into the upper neck of 

 the flask, but was connected with it by a wide rubber tube about 4 inches in length, 

 kept stretched by a spiral spring inside it. The upper tube was connected with the 



Fig. 1. Apparatus employed for arc in vacua, hydrogen, or other gas. 



pump and the lower one with the gas supply, communication with the interior of the 

 globe being made through small apertures drilled in the brass tubes near the pole- 

 pieces. This arrangement was found quite satisfactory so long as the apparatus was 

 not allowed to become overheated, and intermittent exposures extending over several 

 hours could be made before the window through which the arc was viewed became 

 too clouded for further use. The current ordinarily employed was from 2 to 6 amperes, 

 derived from the 110-volt lighting circuit, and showing a potential difference of 

 40 volts between the poles. 



A convenient method of producing the spectrum has also been described by 

 Mr. BROOKS,* who has obtained the flutings very brilliantly from a " high-frequency 

 flame discharge " between magnesium ribbon spirals in hydrogen at low pressures. 

 In a later papert Mr. BROOKS refers to other experiments on the mode of production 

 of this spectrum. 



Considerations as to the Origin of the Flutings. 



The outcome of the numerous experiments made by LIVEING and DEWAR was to 

 indicate that hydrogen, in some form or other, was as necessary as magnesium for the 

 production of the spectrum under consideration. Summarising their earlier results in 



VOL. CCIX. A. 



* 'Roy. Soc. Proc.,' A, vol. 80, p. 218 (1908). 

 t ' Astrophys. Jour.,' vol. 29, p. 177 (1909). 

 3 M 



