52 
PROFESSOR W. N. HARTLEY ON SPECTRUM PHOTOGRAPHY 
connected with these. Capillary attraction keeps the lower electrode moist at its 
upper surface. Such electrodes have been constantly in use for producing sparks 
from which photographs of the spectra of saline solutions have been taken. Fresh 
electrodes have generally been made for each solution, but they may be used over 
again repeatedly provided they be well washed with hydrochloric acid and water. 
They wear away rather quickly, but their durability may be indicated by the fact 
that the same points have actually been employed continuously for a period of ten 
hours. Each of the two pieces of graphite should be sharpened so as to resemble 
a chisel, the edge of the one being exactly superposed above that of the other, while 
both are placed in a line with the slit; the movement of the spark is thus restricted 
to a direction backwards and forwards, but always in front of the slit and not to one 
side or the other. 
Graphite electrodes (actual size). 
The only two lines of a foreign element plainly visible in the spectrum of graphite 
are the first and third of the quadruple group in the magnesium spectrum, with wave¬ 
lengths 2794*4 and 2801*1. Faintly seen are the second and fourth lines, wave¬ 
lengths 2796*9 and 2789*6. This specimen of graphite came from Ceylon, and it was 
thought worth while to submit it to the ordinary method of analysis. Accordingly a 
considerable quantity was incinerated, and the ash was found to contain only 
magnesium and a trace of iron. After some hundreds of electrodes had been cut from 
