the Occurrence of Enhanced Lines in the Arc. 291 



and heating the arc vapours or the electrodes, and upon that 

 of varying the nature of the medium in which the arc is 

 established. The present paper deals more particularly with 

 the arc spectra of lead and tin. 



§ 2. A convenient arrangement for obtaining the arc spectra of 

 metals having low melting-points. 



The usual way of obtaining the arc spectrum of a metal is 

 to place small lumps of it into the hollowed out positive 

 crater of the lower carbon rod. The inconveniences attending 

 this method are well known ; the temperature of the crater 

 is so high that metals of low boiling-point are almost 

 immediately thrust out again. The remedy generally 

 applied is to use some compound of the element 

 in place of the metal. It is obviously impossible 

 under these conditions to carefully examine the spectrum 

 radiations emitted by the vapour in the immediate vicinity 

 of the metal, and it is probably for this reason that the 

 emission of spark lines by the arc has passed unnoticed 

 for so long. For the purpose of the present research it 

 was essential to be able to explore the region around the 

 metal from which the arc was made to pass, and the 

 following simple method was found to fulfil these require- 

 ments. The piece of metal is laid upon a plate of copper 



Fio-. 1. 



Carbon rod. 



Copper plate 



Metal. 

 Method ot obtaining arc spectra of volatile metals. 



about 5 by 10 cm. in area and not less than one milli- 

 metre thick. This plate remains fixed in position and is 

 connected to the positive conductor of the electric supply 

 circuit (fig. 1). A pointed carbon rod, which can be 



U2 



