with special reference to solutions of its salts. 567 
of the values of the ratio of the density of the substance to the 
absorption coefficient of the rays emitted by it. In the case of 
solutions, at least, the variations in the value of this ratio are 
surprisingly large and irregular. The ratio is not necessarily 
greater for the solution of greater density or greater concen- 
tration. 
Note. 
As this paper is on the point of publication, too late for any 
further observations to be made, a letter has appeared in Nature 
in which Prof. McLennan describes experiments by himself and 
Mr Kennedy leading to conclusions exactly contradictory to those 
which have been urged above. He states that the activity of 
potassium compounds is not even approximately constant, but 
extremely capricious : large variations may be noted in different 
samples of the same salt. 
Enough details are not given to admit of a thorough com- 
parison between the methods which have led to such different 
results, but it may be noted that Prof. McLennan and Mr Kennedy 
place their active material inside the ionisation chamber, while 
my material was separated from the chamber by thin aluminium 
leaf or, as in the former experiments, by wire gauze, the potential 
of which differed by equal amounts from the electrode and that of 
the active material. If, beside the penetrating rays from potassium, 
there are also very soft rays emitted, or even ions with no velocity 
of their own, the discrepancy of our results might be explained. 
The observations which have been made on absorption indicate 
that the apparent activity of an active substance giving very soft 
rays might exhibit great complexity. There are also considerable 
dangers in placing substances which are hygroscopic or produce 
vapours, such as the cyanide, directly in contact with the air of 
which the ionisation is measured. 
(Added June 16.) 
In view of Prof. McLennan’s results, the activity of samples of 
potassium sulphate prepared from the cyanide and acetate men- 
tioned in Table I has been examined. No difference can be 
detected between these samples and those given in the table. 
I have attempted to repeat Prof. McLennan’s work by placing 
trays containing the potassium preparations inside the ionisation 
vessel. However it was found that the change in the ‘ natural 
ionisation ’ of the vessel caused by the admission of fresh air, 
when the vessel was opened to insert the tray, was so large and 
irregular that it was impossible to obtain consistent results for the 
activity of a single sample. 
