Pressurized nitrogen tank capacitor for big VTTC?

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Original poster: "Scott Hanson" <huil888-at-surfside-dot-net> 

One of my dad's old industrial electronics books (Electronics for Industry,
W.I. Bendz, 1947) has several chapters covering high power vacuum-tube
oscillators for induction-heating applications.

Shown in a photograph on page 247 is a 10 KW vacuum-tube oscillator
featuring a "tank capacitor (pressurized nitrogen type with high voltage
terminal at bottom)".

This appears to be a metal cylinder about 10 inches in diameter by 12 inches
long, with flat end plates, and a conventional 2" diameter pressure gage
sticking out the top. Some sort of smaller cylindrical can is projecting out
the bottom, that may be the high voltage terminal referenced in the text.

The rest of the photo shows what might pass for the makings of a very large
VTTC, with a giant air-cooled glass/metal triode, very large open frame tank
coil, etc.

But, what is a "pressurized nitrogen capacitor"?

Does anyone have any insight into the internal construction, and the
operating pressure?

Web searches for "pressurized nitrogen capacitor" have yielded nothing.

Regards,
Scott Hanson