16 RADIO-ACTIVE SUBSTANCES [cH. 
and submitted them for examination of their spectrum to 
Demargay, an authority on that subject. The first specimen of 
radium chloride examined by Demarcay! was not very active, but 
showed, besides the lines due to barium, a very strong new line in 
the ultra-violet. In another sample of greater activity, the le 
was still stronger and others also appeared, while the intensity of 
the new lines was comparable with those present due to barium. 
With a still more active specimen which was probably nearly pure, 
only three strong lines of barium appeared, while the new spectrum 
was very bright. The followmg table shows the wave-length of 
the new lines observed for radium. The wave lengths are expressed 
in Angstrém units and the intensity of each ray is denoted by a 
number, the ray of maximum intensity being 16. 
Wave length Intensity Wave length Intensity 
4826°3 10 4600°3 3 
4726°9 5 4533°5 9 
4699°6 3 44361 6 
4692-1 7 4340°6 12 
46830 14 3814°7 16 
4641°9 4 3649°6 12 
The lines are all sharply defined, and three or four of them 
have an intensity comparable with any known lines of other 
substances. There are also present in the spectrum two strong 
nebulous bands. In the visible part of the spectrum, which has 
not been photographed, the only noticeable ray has a wave 
length 5665, which is, however, very feeble compared with that of — 
wave length 4826°3. The general aspect of the spectrum is similar 
to that of the alkaline earths; it is known that these metals have 
strong lines accompanied by nebulous bands. 
The principal line due to radium can be distinguished in 
impure radium of activity 50 times that of uranium. By the 
electrical method it 1s easy to distinguish the presence of radium 
in a body which has an activity only 1/100 of uranium. With a 
more sensitive electrometer 1/10000 of the activity of uranium 
could be observed. For the detection of radium, the examination 
of the radio-activity is thus a process nearly a million times more 
sensitive than spectrum analysis. 
1 C. R. 127, p. 1218, 1898; 129, p. 716, 1899; 131, p. 258, 1900. 
