of Magnesium, Copper, and Manganese. 309 



excitation, do not show in the reproduction on account of 

 their great feebleness. It will, however, suffice to point 

 out the sudden cessation of the electrically controlled 

 emission at the lower border of the fringe, which, like that 

 of the C bands and Ti lines, is in striking contrast to the 

 more gradual fading away of the lines of Fe and Ca under 

 the control of the plate temperature. As compared with the 

 other two spectra t> and c on PI. IX., the fringe emission of 

 magnesium extends to a relatively greater distance from the 

 graphite plate. This is probably due to the presence of 

 iron vapour, which, as already explained in § 2, seems to be 

 a particularly good conductor of the thermelectronic 

 current at high temperatures. 



§ 7. Copper. 



The metal as well as its oxide and chloride were employed 

 both with and without a protecting layer of carborundum. 

 The best results were, however, obtained with pieces of 

 metallic copper on an unprotected plate of graphite. Since 

 the metal boils already at about 2300° C. it became necessary, 

 in order to prevent the boiling mass from being thrown off 

 the plate, to adopt special precautions, and the following 

 method was found to work satisfactorily. The pieces of 

 metal, instead of being placed near the middle of the plate 

 as sketched in fig. 2, were brought close up to the end faces 

 of the clamping bars as shown in plan view, fig. 3. The 



Fig. 3. 



Upper surface 

 of oraphite p/ate 

 Clamping rod , y damping rod 



b 



Metal 



Metal 



Direction of 

 collimator axis 



Method of obtaining spectrum of a metal having low boiling-point. 



temperature here is considerably lower than near the middle 

 of the plate, and the metal does not boil too violently. The 

 lumps of copper were held in position by small pieces of 



