284 ILLINOIS ACADEMY OF SCIENCE 
no deflecting effect on the heavy retrograde rays (ions nega- 
tively charged) streaming to the right in the bulb N, and of 
course none on the positively charged ions traveling to the 
left in M. 
As pointed out, in the article referred to above, the positive 
rays have their origin at the edge of the Crookes dark space in 
front of the cathode, indicated by the broken line mm. To 
show the color of the retrograde rays to the best advantage 
the air must be all removed from the tube and the quantity of 
helium just right, also the bulbs should be of lead glass to 
avoid excessive fluorescence, and the cathode openings should 
not be too large. 
These retrograde rays were overlooked for some time, 
though it was well known from their photographic effect that 
they were present in a tube of this construction. When the 
above conditions were complied with, we have the rather un- 
usual spectacle of three different colors, all visible to the naked 
eye, in the region immediately in front of the cathode—a purple 
color due to retrograde rays; orange, due to positive rays; 
and apple green, due to cathode rays. 
A SIMPLE DEMONSTRATION TUBE FOR EXHIBIT- 
ING THE MERCURY HAMMER, GLOW BY MER- 
CURY FRICTION, AND THE VAPORIZA- 
TION OF MERCURY AT REDUCED 
PRESSURES 
ABSTRACT 
Cuas. T. Knipp, UNIVERSITY OF ILLINOIS 
When the pressure over mercury is reduced to that of mer- 
cury vapor only, vaporization with heat takes place at sur- 
prisingly low temperatures, and the resulting mechanical pres- 
sure exerted by the issuing vapor from the mercury surface is 
even more surprising. The magnitude of this pressure over a 
surface confined in a large bulb, so that the vapor stream is not 
concentrated, is sufficient even at temperatures as low as 130° 
C, to freely support bits of cork. That small drops of mercury 
