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XIIl. On Radioactivity of Mineral Springs. By G. A. Blanc, 

 Dr. in Physics of the University of Rome*. 



THE researches made by Sella, J. J. Thomson, Himstedt, 

 and other physicists have shown that ordinary tap- 

 water generally contains a radioactive gas or emanation that 

 can be easily obtained by boiling the water or by drawing- 

 air through it. 



Mineral waters, and the deposits which they form, have 

 hitherto generally shown considerable activity, the subject 

 having been studied by Elster and Geitel, Strutt, Lord 

 Blythswood and Allen, Curie and Laborde, and others. 



1 shall give here the results o£ an investigation concerning 

 the radioactive properties of the materials deposited by a 

 certain number of mineral and thermal waters belonging to 

 the Alpine region, which I have had the opportunity of 

 examining during the summer. 



To measure the activity of the various sediments I adopted 

 the apparatus shown in fig. 1, which has been used by 

 Sella in similar researches. It consists of a glass vessel 



V with an opening at the top through which a metal rod R 

 is fixed by means of a sulphur insulator. To the lower end of 

 this rod a wire-gauze disk can be fixed horizontally, while 

 the upper part carries an aluminium leaf and is covered by 

 an electroscope case. The vessel stands on a metallic plate 

 provided with an opening in its centre, under which a dish 

 D may be placed containing the substance to be tested. The 

 charge is given to tbe apparatus by a movable contact G of 

 the usual type. 



* Communicated by Lord Kelvin. 



