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LXYTII. The Radioactivity of Terrestrial Surface Materials. 

 By J. Joly, F.R.S* 



A SOMEWHAT considerable experience of the electric 

 furnace as a means of determining the radium content 

 of rocks f has led me to the conclusion that a revision of the 

 subject of terrestrial radioactivity by its means is desirable. 

 This view is founded upon two circumstances : (1) the method 

 by fusion possesses features rendering it more reliable than 

 any other method, and (2) in its results it differs considerably 

 from the method by solution. 



As regards the first point, the reliability and searching- 

 character of the method, I have dealt with the subject else- 

 where (loc. cit.). It may, however, be well to recall here 

 that in this method the emanation is taken directly from the 

 pulverized rock while this is being decomposed by the alka- 

 line carbonates at temperatures rising to 1100° C. or more. 

 In this process the melt possesses but small bulk and the 

 exposed surface is very large. In fact the melt makes but a 

 shallow film in the platinum boat in which it is being heated. 

 (I re'er now to the use of the tube furnace.) There is complete 

 decomposition and intense ebullition lasting for about half 

 an hour. The gases evolved are led through a soda-lime 

 tube ; the unabsorbed gas (for the most part air driven by 

 thermal expansion from the furnace) is stored in a strong 

 rubber bag, and finally carried again through tlie soda-lime 

 tube into the electroscope ; a final current of air washes out 

 both furnace and tube, and ensures that practically the 

 whole of the emanation is brought into the electroscope. 

 The completeness of the wash-out has been tested by filling 

 a second electroscope through the entire system : when little, 

 if any, residual emanation has been detected. It may, there- 

 fore, be assumed that, sensibly, all the emanation evolved in 

 the furnace reaches the electroscope. There is no reason (o 

 assume that any part of it leaks through the platinum. Such 

 an occurrence, in view of the high atomic weight of emana- 

 tion, would be contrary to expectation : moreover, tests 

 carried out on varying quantities of material have been quite 

 satisfactory. 



Hardly less important is the protection against contami- 

 nation, which naturally arises in the simple nature of the 

 procedure involved. The rock is pulverized till it passes a 

 sieve of about 25 mesh to the centimetre. From seven to ten 



* Communicated bv the Author. 

 t Joly, Phil Mag-.Jiily 1911. 



