Hartley — On the Constitution of Electric Sparks. 373 



Now we know that there is no alternation of the current, and that 

 the passage of the spark is from below upwards ; accordingly, the 

 spark to produce this effect must oscillate. Obviously, however, 

 if the spark is passing upwards from below, and the oscillations 

 are very numerous, the aggregate effect will be to make the lower 

 electrode more largely contribute vapour to the spark. Photo- 

 graphs of lines in the spectrum indicate this. To this is to be 

 attributed the fact that lines of impurities present in minute pro- 

 portions, and likewise the lines of metals present in very dilute 

 solutions, are seen only at the lower or negative electrode, that is 

 to say, they proceed from the point at which the initial discharge 

 takes place. 



If now this view cannot be accepted without further proof, it 

 is to be obtained by placing a thin film of mica in the path of the 

 spark. The mica, after it has been perforated, and the spark has 

 passed for an interval of about four minutes, has a much larger 

 deposit upon its lower than on its upper surface, though the 

 electrodes are of the same metal. If the spark is reversed the 

 positions of the larger and smaller deposits are reversed. To put 

 this proof to a still more rigid test the lower electrode should be 

 of graphite, and the upper one of gold. Under these circum- 

 stances, the graphite being the negative, gold is found to be 

 deposited upon the upper surface of the mica, and carbon only 

 upon the lower. Reverse the current and much more gold is 

 deposited upon the upper surface of the mica, and little or nothing 

 on the lower, because the carbon is combustible and but slightly 

 volatile. Now in the former instance the deposit of gold could 

 only be formed by the rebound, so to speak, of the electrically- 

 charged vapour. 



It will thus be seen that the oscillations of the spark discharge 

 can fully account for the following facts : — 



1st. The occurrence of lines in the spectra of the metals with a 

 nimbus surrounding each electrode. 



2nd. The extension of the lines of both electrodes. 



3rd. The formation of deposits on both surfaces of a non- 

 conductor interposed in the path of the spark ; the deposit being 

 more copious on the surface nearest the negative electrode. 



4th. The appearance of lines due to impurities or traces of 

 metals in the spectrum of the negative electrode only. 



