46 
PHYSICS: A. J. DEMPSTER 
Proc. N. A. S. 
is theoretically a linear increase to a maximum and then a linear decrease. 
25 
The width half way to the maximum is given by m. — where m is the 
a 
atomic weight, 5 the slit width and d the diameter of the circle in which the 
rays travel. Under good vacuum conditions this theoretical sharpness 
is practically obtained. For 1 mm. slits this width of the curves should 
thus be one-half a unit on the atomic weight scale. The former measure- 
ment with the apparatus and the magnetic field determinations sufficed 
to locate elements between 20 and 30 within one unit, and identified the 
strong nitrogen rays (possible carbon monoxide) of molecular weight 28 
which are given off when the metal is first heated. 
One series of experiments was as follows : After heating the magnesium 
slightly and pumping, till a MacLeod gauge gave no pressure indication, 
the nitrogen molecule was the only particle present. The heating current 
was then increased by steps to vaporize the magnesium. With 0.7 
ampere, 28 alone was present, with 0.75 ampere an arc apparently 
struck as the cathode-anode current jumped suddenly to five times its 
value. The electron current was decreased to its former value by cooling 
the cathode and the rays were measured. It was found that three strong 
new lines had appeared. The new lines which are undoubtedly due to 
magnesium were compared with the nitrogen rays which were still faintly 
present and found to have atomic weights 24, 25 and 26. The observa- 
tions are illustrated in figure 1, which gives the current or number of 
Atotnfc WeiijKt. 
FIG. 2 
particles for different atomic weights. The nitrogen line had its maxi- 
mum at 817 volts, and the atomic weight abscissae are 28 X 817 divided 
by the volts applied. The ordinates of the 28 line are multiplied by 10 
in plotting to make them comparable with the other three lines. The 
dotted continuation to the axis indicates the slight overlapping of the 
lines. We conclude that magnesium consists of three isotopes of atomic 
weights 24, 25 and 26. 
Later curves made with steadier discharge conditions are more suitable 
