698 
CHEMISTRY: RICHARDS AND WADSWORTH 
G (the supposed-end product of the decomposition, the present form 
of iso topic lead), the rate of fall in the gold leaf electroscope for the 
lead from cleveite would be the greatest, and that from the American 
carnotite the least; but our results were precisely the other way about. 
There can be little question, then, that radioactivity is not due to the 
isotope which gives the Ioav atomic weight. Probably it is due to radium 
E, since in all of our samples the half -value of the radioactivity was 
obtained in about five days, the half life of radium E. The maximum 
value, approached asymtotically, is nearly reached in a month. 
The spectrum of isotopic lead as thus far studied by Honigschmid, 
by Merton and by Harkins, as well as by Baxter (who kindly photo- 
graphed the ultraviolet spectrum of one of the samples of lead pre- 
pared by one of us with the help of Dr. Lembert) has always been found 
to be essentially like that of ordinary lead. In the present research 
we thought it worth while once more to test this question and especially 
to extend the inquiry to the visible portion of the spectrum — most of 
the tests in the past having been made in the ultraviolet region. We 
found difficulty in eliminating traces of copper, silver and calcium too 
small to be detected by ordinary analysis. Although these were far too 
diminutive to affect the atomic weight, we continued the purification, 
and after eight recrystallizations as nitrate and two as chloride, the 
product gave in every respect precisely the same spectrum as the purest 
ordinary lead prepared by Baxter and Grover, except for a vanishingly 
small trace of two of the most prominent silver lines. 
The photographs of the ultraviolet region in the Eery spectrometer 
were very kindly made by Professor Baxter. Photographs of the 
visible portion of the spectrum were made by us in the Gibbs Labora- 
tory on specially prepared plates sensitive as far as wavelength 7800. 
Ail the work of loading and developing had to be done in complete 
darkness. To make assurance doubly sure, a further study of the vis- 
ible spectrum was made, in collaboration with Mr. Norris F. Hall, 
with the help of the Hilger wave-length spectrometer, comparing visu- 
ally in the same field of view the spectra of pure ordinary lead and the 
best purified specimen from Australian carnotite. Every line was 
scrutinized between the range 4000 and 7600, especial pains being 
taken in the red and yellow portions, the least satisfactory from a pho- 
tographic point of view. No discernible differences between the two 
spectra were observed. 
Because no lines were detected between wave-lengths 7800 and 
2200 in any of the samples which were not due either to ordinary lead 
or to unimportant traces of well-known impurities, one of the alterna- 
