* 
a 
20 RADIO-ACTIVE SUBSTANCES [cH. 
and 300°C. In this way polonium of activity 700 times that of 
uranium was obtained. 
(2) Precipitation of nitric acid solutions by water. The 
precipitated sub-nitrate is much more active than the part that 
remains in solution. 
(3) Precipitation by sulphuretted hydrogen in a very acid 
hydrochloric acid solution. The precipitated sulphides are much 
more active than the salt which remains in solution. 
For concentration of the active substance Mme Curie? has made 
use of method (2). The process is, however, very slow and tedious, 
and is made still more complicated by the tendency to form 
precipitates imsoluble either im strong or weak acids. After a 
large number of fractionations, a small quantity of matter was 
obtained, enormously active compared with uranium. On exami- 
nation of the substance spectroscopically, only the bismuth lines 
were observed. A spectroscopic examination of the active bismuth 
by Demargay and by Runge and Exner has led to the discovery 
of no new lines. On the other hand Sir William Crookes’ states that 
he found one new line in the ultra-violet, while Berndt?, working 
with polonium of activity 300, observed a large number of new 
lines in the ultra-violet. These results await further confirmation. 
The polonium prepared by Mme Curie differs from the other 
radio-active bodies in several particulars. In the first place the 
radiations include only very easily absorbable rays. The two 
penetrating types of radiation given out by uranium, thorium, 
and radium are absent. In the second place the activity does: 
not remain constant, but diminishes continuously with the time. 
Mme Curie found that the polonium lost half its origmal activity 
in the course of eleven months. 
18. The decay of the activity of polonium with time has led 
to the view that polonium is not a new active substance, but 
merely active bismuth, ze. bismuth which in some way had been 
made active by admixture with radio-active bodies. 
The activity of any product is not necessarily a proof that 
a radio-element is present, for it has been shown that many 
inactive elements become active by association with active matter. 
1 Théses, Paris, 1903. 2 Proc. Roy. Soc. May, 1900. 
° Phys. Zeit. 2, p. 180, 1900. 
