J. W. Fewkes—Brush Discharge from the Holtz Machine. 497 
the remarks of Faraday (Experimental Researches in Electri- 
city, vol. i, § 1526) on this glow discharge. ‘“ Diminution of the 
charging surface will produce it; thus when a rod 0°8 of an inch 
in diameter, with a rounded termination, was rendered positive 
in free air, it gave fine brushes from the extremity ; but occa- 
sionally these disappeared, and a quiet phosphorescent, contin- 
uous glow took their place, covering the whole end of the wire, 
and extending a very small distance from the metal into the air. 
§ 1528. Increase of power in the machine tends to produce the 
glow; for rounded terminations, which will give only brushes 
when the machine is in weak action, will readily give the glow 
when it is in good order.” 
It is evident that diminution of the charging surface in these 
experiments of Faraday resulted in a greater electric density at 
the termination of the conductor used, which was also the effect 
with an increase in power of the machine. In our experiments 
the effect of condensers is analagous to those produced by Far- 
Yin diminishing the charging surface, and increasing the 
power of the machine. For, although the condenser really in- 
creases the charging surface, still the electric density is increased 
ina greater ratio. The spectrum of the brush discharge consists 
of a number of fine lines in the blue and violet end of the spec- 
trum, which are difficult to measure on account of the faint 
light. The spectrum differs greatly from that of the spark 
é ie use of condensers. The spectrum of the latter 
18 @ continuous one crossed by bright lines. I have measured 
the wave-lengths of the most prominent ones, and find their 
value as follows: 
ere Atmospheric lines, 
%  STbT 35 5767 N. Plicker 
5687 5686 N, " 
5620 
6019 
5185 5184 Thalen 
500 5003 N. Huggins 
The lines due to nitrogen are thus seen to be the most prom- 
ment. There were many fine lines, especially in the violet, 
which were partially obscured by the light of the continuous 
rpectrum ; and therefore did not admit of exact measurement. 
