CHEMISTRY: RICHARDS AND HALL 
339 
must be high and low. If C is at G' the whole of each component 
beam may be caught and passed through the respective shanks of the 
U-tube. The fringes are strong, easily found and large, so that the 
center of ellipses is not far outside of the field of the telescope. 
Finally if the connecting tube p is nearly horizontal when in place, 
the fringes are usually found at about the same position of the microm- 
eter (at M') after the Hquid is introduced into the U-tube. 
Experiments were also made with this apparatus. Displacement in- 
terferometers in which the rays do not retrace their respective paths have 
an important special property which I wish to accentuate in conclusion. 
If either opaque mirror is displaced on its micrometer normal to itself 
or if a plate compensator rotates in one beam on a vertical axis, the 
center of elHpses moves parallel to the length of the spectrum. If how- 
ever the plate compensator rotates on a horizontal axis, the center of 
ellipses moves nearly transversely to the length of the spectrum. The 
phenomenon is quite sensitive. To this result I shall return in a suc- 
ceeding note, in connection with the development of the Jamin design 
for displacement interferometry. 
The present note will be presented in more extended form in a report 
to the Carnegie Institution of Washington. 
ATTEMPT TO SEPARATE THE ISOTOPIC FORMS OF LEAD BY 
FRACTIONAL CRYSTALLIZATION 
By Theodore W. Richards and Norris F. Hall 
WOLCOTT GIBBS MEMORIAL LABORATORY. HARVARD UNIVERSITY 
Communicated, March 10, 1917 
Although the complete inseparabihty of isotopes by chemical means 
has been frequently asserted, the evidence on which this assertion is 
based has always seemed insuflSicient. The methods used have been 
fractional crystallization and precipitation, but these processes have 
seldom been carried out more than ten times in a particular case, and 
frequently six or seven crystalHzations have been thought a sufficiently 
thorough test of inseparabihty. A search of the hterature revealed 
only one investigation, that of Radiothorium and Thorium by McCoy 
and Ross,i where as many as one hundred repetitions of a given process 
had been made. 
It seemed worth while, therefore, to apply to the important generali- 
zation of Fajans, Russell, Fleck and Soddy a more searching test carry- 
ing the fractionation further, and using as a criterion of success not only 
the measurement of radioactivity but also the determination of atomic 
weight. 
