
26 RADIO-ACTIVE SUBSTANCES [CH. 
Hofmann and Strauss’ found that lead sulphate obtamed from 
pitchblende was active. This was not due to admixture with either 
uranium or radium or polonium. They gave the name of radio- 
active lead to the substance. This radio-active lead, in most of 
its reactions, resembled ordinary lead, but showed differences in 
the behaviour of the sulphide and the sulphate. The sulphate was 
very strongly phosphorescent. This sulphate apparently lost its 
activity with time, but recovered it in a few minutes after exposure 
to cathode rays in a vacuum tube. 
Giesel? also was able to obtain radio-active lead, but found that 
the activity diminished with time, while Hofmann states that his 
preparations preserve their activity. It thus appears probable 
that radio-active lead is either one of the numerous examples of 
substances made active for the time by solution with radio- 
elements, or lead with a shght admixture of a radio-element. The 
peculiar action of the cathode rays in causing an increase of the 
photographic and electric action of radio-lead sulphate has ap- 
parently nothing whatever to do with the activity proper of the 
substance, but seems to be an additional effect due to the strong 
phosphorescence set up. The sulphide does not show any such 
action. The phosphorescent light probably includes some short 
ultra-violet light waves which are capable of ionizing the gas. 
24. If elements heavier than uranium exist, it is probable that 
they will be radio-active. The extreme delicacy of radio-activity 
as a means of chemical analysis would enable such elements to 
be recognized even if present in infinitesimal quantities. It is 
probable that considerably more than the three or four radio- 
elements at present recognized exist in minute quantity, and that 
the number at present known will be augmented in the future. 
In the first stage of the search, a purely chemical examination is 
of little value, for it is not probable that the new element should 
exist in sufficient quantity to be detected by chemical or spectro- 
scopic analysis. The main criteria of importance are the existence 
or absence of distinctive radiations or emanations, and the perman- 
ence of the radio-activity. The presence of a radio-active emanation 
1 Ber. deutsch. chem. Ges. p. 3035, 1901. 
> Ber. deutsch. chem. Ges. p. 3775, 1901. 
