16 Messrs Campbell and Wood, 
period the air above the solution was drawn into a testing vessel. 
No increase in the ionisation was observed. To indicate the 
sensitiveness of the test, it may be remarked that one-thirtieth 
part of the radium emanation in a volume of Cambridge water 
equal to that of the solution would have been detected with 
certainty. The presence of radium, thorium or actinium is 
negatived decisively by this test. 
(2) Another sample of potassium sulphate was tried. The 
first sample came through a local dealer from Messrs Harrington : 
the second was the purest sample which Dr. Kahlbaum could 
supply. No difference in the activity of the two samples amount- 
ing to 5 0 / o of the whole could be detected. 
(3) Other salts of potassium were tried. The following 
table shows the results obtained and the values of the activity 
of potassium calculated on the assumption that the activity is 
an atomic property and that none of the other elements contained 
in the salts have an appreciable activity. 
Table 1. 
Salt 
°/o of K 
in salt 
Activity 
of salt 
Activity of K 
Weight 
of salt 
K 2 S0 4 — Sample 1 
44-7 
489 
1090 
220 
K. 2 S0 4 — Sample 2 
44-7 
471 
1050 
215 
KC1 — Sample 1 
52-1 
495 
951 
135 
KCI— Sample 2 
52-1 
495 
951 
139 
KI 
23-5 
276 
1180 
232 
kno 3 
38-6 
388 
1005 
Mean 1038 
199 
The discrepancy of the values for potassium given in column 4 
are not greater than might be expected. The layers of the salts 
used were only T5 cm. thick. This thickness may not be sufficient 
to absorb all the rays from the lowest stratum and hence the 
activity of the layer may depend somewhat on the density of the 
salt used. In the 5th column are given the weights of the layers 
of the respective salts : it will be seen that the smallest values 
of the activity of potassium correspond to the least dense layers. 
(4) All the potassium salts used in commerce are prepared 
from the same deposits of sylvine and kainite at Stassfurt. It 
is not impossible, therefore, that all potassium salts from all 
dealers should contain a common impurity. Accordingly steps 
were taken to procure potassium salts which were known to be 
derived from some other source. A considerable quantity of 
