380 Radioactive Matter in the Earth and Atmosphere, 



charged at constant potential of about — 500 v. by means of 

 a battery of small storage-cells, was repeatedly exposed to 

 the open air for periods lasting three days, after which the 

 activity of the wire, previously wound on a frame, was tested 

 in the usual way, by means of a gold-leaf electroscope. 



The results were always similar. After an initial rapid 

 decay, the activity went decreasing according to an exponential 

 law, the constant of decay being, within the limits of experi- 

 mental error, identical with that of thorium A. 



It was easy, by extrapolation, to calculate the effect due to 

 the excited activity of the radiothorium type at the moment 

 in which the activation process had come to an end. Now 

 the important fact is that the excited activity of the radio- 

 thorium type represents generally a large part of the total 

 amount exhibited by the wire, and precisely from 50 to 

 70 per cent. 



Measurements were made on the terrace of the Physical 

 Institute of Rome, and, to make sure that the effect was not 

 due to the presence of thorium salts in the laboratory, also in 

 a garden at a certain distance from the town. In both cases, 

 results were similar. 



A series of experiments was also carried out by activating 

 a wire inside the Catacombs of, Sanf Agnese, near Rome, 

 which are dug in the volcanic formation called pozzolana. Here 

 also a very strong proportion of radiothorium excited activity 

 was observed, for exposures of the wire lasting three days. 



For short exposures of the wire, not exceeding three or 

 four hours, the law of decay was always very similar to that 

 observed by Rutherford and Allan in the case of a wire 

 activated in Montreal ; the characteristic time of reduction 

 of the activity to half value varying between 50 and 

 (\0 minutes. 



It is evident that the large amount of radiothorium excited 

 activity obtained on a wire exposed to the open air at a certain 

 distance from the soil, cannot possibly be due to direct contact 

 with the emanation. The disintegration products thorium A 

 and thorium B, probably fixed on atmospheric dust particles, 

 are evidently carried by the air, and, owing to their slow rate 

 of decay, are able to cause the observed effects. It is probable, 

 on the other hand, that no products such as thorium A or 

 thorium B could, in any appreciable quantity, escape through 

 the capillary conduits of the soil into the atmosphere, owing 

 to the property these products have of remaining fixed on 

 solids with which they come into contact. 



Experiments are actually being made with the object of 

 determining as exactly as possible the amount of radiothorium 

 emanation which, by diffusion, escapes from a given area of 



