In the case of inactive mud the leakage / ,,-\ was 13.9. With the active 



37 215 



in water, and mixed the sohition with 31 gram of dry mud powder which I had 



previously proved to be quite inactive. After desiccation and pulverisation the 



mud, whicli has now become active on account of the uranium contained in it, is 

 again examined. 



mud the leakage was 153. Hence the increase in leakage was 139. Under ordinary 

 conditions I could perceive an increase in the leakage corresponding to , of the 

 natural leakage, or 1.4 of the arbitrary units before described. Hence I conclude 

 lliat, by means of this apparatus, it is possible to detect 0.00201 gram of nitrate of 

 uranyl contained in the mud sample, or, as I usually in every experiment examined 

 about 30 grams of the desiccated mud, I was thus able to delect radioactive sub- 

 stances in the sample, provided they produced the same ionising effect per gram 

 of the examined matter as 0.00067 gram of the nitrate of uranyl. 



The nitrate of uranyl contains 47.6 "o uranium. Therefore 31.9 x 10^ gram 

 of uranium per gram of the mud is the smallest amount of uranium to be detected 

 by this method. 



According to Mc Coy ' 1 gram of radium, in equilibrium with its disintegration 

 products, is equally as active as 3.8 x 10*" gram of uranium, while Soddy and 

 Mackenzie- found 1 gram of radium as active as 14.6x10" gram of uranium. 

 Taking the average of these measurements, and estimating the activity of radium 

 at 9 X 10'' times the activity of uranium, I must have been able to detect, by 

 means of the above described apparatus, 3.5 x 10~" gram of radium contained in 

 1 gram of dry mud. 



According to Strutt \ and Eve and McIntosh ^ the rocks on an average contain 

 1.4 X 10 '2 gram of radium per gram of the mineral, i. e. ^ of what I was able 

 to detect. 



At each of the hot spring groups investigated by us, I examined for radio- 

 activity 5 — 10 different samples of mud, silicious sinter and other deposits from 

 the hot springs, but without detecting any trace of radioactive substances. On 

 this account I must conclude that the amount of radium contained in the sedi- 

 ments, mud etc. of the hot springs examined, does not exceed 25 times the amount 

 of radium contained in common rocks. 



2. The collection of spring gases. 



The spring gases were preferably collected from springs where the gas 

 evolved bubbled through water or mud, because we judged thai the gas collected 

 here was less likely to be mixed with atmosplieric air than that collected from 



■■ Phil. Mag. 11, 183, 1906. 

 - Phil. Mag. 14, 272, 1907. 

 ä Proc. Roy. Soc. 77, 472, 1906. 

 * Phil. Mag. U, 231, 1907. 



