PAPERS ON PHYSICS 285 
may be thus supported is common observation when working 
with mercury vapor lamps. To make this easy of demonstra- 
tion the writer has designed a tube to show the above, to- 
gether with the familiar mercury hammer, and glow by fric- 
tion phenomena—all in one. 
The tube should be about 35 cm. long by 1% cm. in diameter, 
and have the usual bulb at each end that obtains for the mer- 
cury hammer. A stricture reducing the diameter to ¥% cm. 
is placed near one end. A small quantity of mercury (about 
5 grams) is put in the tube, and also a spherical pith ball about 
34 cm. in diameter is placed in the bulb farthest removed from 
the stricture. The tube is pumped out carefully and sealed off 
(the sealing off nipple should be attached to the stem and not 
to one of the bulbs). The tube is now ready for the exhibition 
of the three phenomena referred to above. To show the pres- 
sure of the mercury vapor it is only necessary to hold the tube 
by the upper bulb (the one farthest from the stricture) over 
a bunsen burner and allow it to heat gently. Soon condensed 
mercury vapor appears on the walls of the lower bulb and its 
progress up the tube is readily followed. The bombardment 
of the mercury vapor lifts the pith ball which, oscillating up 
and down, is forced into the upper bulb where it is violently 
agitated by the expanding mercury vapor stream. Removing 
the apparatus from the heat allows the oscillating pith ball 
to descend the tube until it again rests upon the stricture. At 
this moment if the bulb is shaken slightly the ball is again shot 
up momentarily, There is no danger of cracking the tube if 
it is blown out of pyrex glass. 
