374 EXPERIMENTAL INVESTIGATIONS. 



passed, the plate being positive, and a round dark spot of 

 oxide formed, represented at fig. 8 ; the platinum sealed in 

 glass was then removed and the steel needle substituted for it, 

 all else, viz. plate, gas, barometer height, &c., being the same. 

 The system of rings represented at fig. 9 was now produced. 



Another experiment was made, directed to the same point : 

 a wire of copper 0*04 inch diameter, and a thread of glass of 

 the same diameter, were attached by sealing-wax at their 

 extremities in a horizontal position 0^025 of an inch from dif- 

 ferent parts of a silver plate, being insulated from the silver 

 by the wax interposed at the extremities. The gaseous mix- 

 ture and barometric height being the same as in the last ex- 

 periment, and the silver plate made positive, when the platinum 

 wire sealed in glass was brought near the plate, and the dis- 

 charges passed, a spot similar to fig. 8 was formed ; but when 

 the coated point of platinum was brought over the copper 

 wire at 0*02 inch distance, a figure consisting of two separated 

 semicircles was formed, having spots in the bisection of the 

 chords, as shown at fig. 10, the portion between the spots and 

 the semicircular line of oxide being of polished silver. With 

 the glass thread the effect was the same, but produced with 

 greater difficulty and not so well defined. 



In many repetitions of these experiments which I have 

 made I have invariably produced the alternately polished 

 and oxidated rings from "the bare wire, and have not procured 

 them from the coated wire, except to a very slight degree, 

 and under certain circumstances, which, as far as I could trace, 

 were as follows : 



ist. When the extremity of the wire was very near the 

 plate, so that it had a sensible magnitude with reference to the 

 intervening space, a slight formation of minute rings could be 

 detected at the commencement of the experiment. 



2nd. When the experiment was long-continued, or when 

 the coated platinum wire had been used for previous experi- 

 ments, a set of rings, not consisting of an alternation of 

 oxidated and polished rings, but of annuli of different degrees 

 of oxidation, were formed. 



When the experiment is continued for some time a dark 

 deposit is formed on the glass around the extremity of the 



