712 
CHEMISTRY: HARKINS AND ARONBERG 
Zeeman effect. A quartz spectrograph with a dispersion of about 25 
angstroms per millimeter gave a similar result when used with uranio- 
lead of atomic weight 206.34, but when the Michelson 10-inch plane 
grating was used in the sixth order, a difference of about 0.0043 A. U. 
was found, and as might have been predicted theoretically, the uranio- 
lead has the longer wave length and therefore the lower frequency. 
This difference was obtained for the line X = 4058, the strongest line 
in the spectrum of lead. Unfortunately the other lines of lead are too 
weak to give a photograph with this grating as used in the higher orders, 
in any reasonable time of exposure. 
The structure of the line X = 4058 in ordinary lead was studied by 
Jonicke^^ and by Wali-Mohamed,^^ who used a vacuum lead arc as a 
source of light. By the use of an echelon they found two satellites at 
+0.032 and —0.041. In the present investigation it was found, how- 
ever, that both the ordinary and the uranio-lead give only one satellite, 
at — 0.0480 and —0.0501 for the uranio and ordinary lead respectively, 
the difference being within the e^xj^etiment^l error of the measurement. 
The difficulty is due to the lack of definition of the satellite. 
As a source of light a sHghtly modified form of the oxycathode arc in . 
vacuo as employed by Wali-Mohamed, was used in the form shown in 
figure 1. This gives a bright source and at the same time very sharp 
and narrow lines, as is essential for work of such delicacy. Such an 
arc had the advantage too, that the extremely rare uranio-lead 3 was 
not wasted. It was first used with a Hilger quartz spectrograph, in 
which case the interesting observation was made that the lines X = 
2823 and 2833 had different intensities in the spectrum of the uranio- 
lead from what they had in the ordinary lead. This was similar to the 
result obtained by Soddy, but it was found to be due to the fact that 
after the arcs started the pressure in the lamp containing the uranio- 
lead rose from 0.05 mm. to 0.25 mm., and it was the small difference in 
pressure which caused the change in intensity. For this reason, when 
the grating was used, the two lamps were connected in parallel to the 
vacuum pump in order to keep the pressure in both the same, about 
0.04 mm. In order to avoid mechanical shifts both exposures were 
made at the same time. The light from one lamp went through a right 
angle prism attached to the slit, thus forming a spectrum, the middle 
of which belongs to one kind of lead while the lines both above and 
below belong to the other. The voltage, 40 volts, was kept the same in 
both within one volt, and the amperage, 1.1 amperes, was kept within 
0.05 ampere. In this way seven exposures were made in one position, 
