68 Trans. Acad. Sci. of St. Louis. 
is a camera photograph of the discharge. The slightest 
change in the length of the spark-gaps a and a’, produces 
marked changes in the form and character of the dis- 
charge through the long gap. Such changes are attended 
by variation in the pitch of musical tones which accom- 
pany the discharge. Oscillations certainly play an im- 
portant part in the phenomena. These results seem to 
justify the suggestion made in the former paper that the 
striations in the vacuum tube are produced in somewhat 
the same manner as the waves in an organ pipe. 
The discharge at a! shown in Fig. D of Plate XXV, be- 
fore referred to, is one of great interest. In this figure 
the position and size of the knobs is indicated by the 
circular ares drawn in ink. This discharge shows the 
Faraday dark space, which is a region of convection of 
air molecules, which have been overloaded with Frank- 
lin’s fluid, in the region of the negative glow. The nega- 
tive glow is also shown in the figure. If the large knob 
be moved nearer to the small positive terminal, the dark 
space is made shorter. The negative convection appar- 
ently penetrates the positive luminous column. The end 
of the positive column begins to fray out into streamers. 
When the large knob reaches the end of the positive col- 
umn, disruptive discharges begin. They are joined to- 
gether at the positive terminal. If the gap is reduced to 
the length of the dark-space, the luminous positive dis- 
charge streamers are meanwhile separated from each 
other from knob to knob. Apparently negative convec- 
tion and negative conduction by transfer from molecule 
to molecule (positive discharge) are taking place side by 
side. Dark spaces and positive columns exist side by 
side. They jostle each other in a somewhat tumultuous 
way. This is the critical spark length. 
If the conditions represented in Fig. D exist, the dis- 
charge not being disruptive, it will become so if an insu- 
lated copper plate be placed between the knobs, at the 
end of the positive column. Moving the copper plate a 
fraction of a mm. towards the positive terminal, wholly 
cuts off the discharge. Such a minute change in position 
