1904] 



Radio-activity of certain Minerals, etc. 



191 



" A Study of the Eadio-activity of certain Minerals and Mineral 

 Waters." By Hon. E. J. Stkutt, Fellow of Trinity College, 

 Cambridge. Communicated by Lord Eayleigh, O.M., 

 F.E.S. Eeceived February 29 — Eeacl March 10, 1904. 



Part I. 



A considerable number of minerals are known in varying degrees 

 to be radio-active. Lists have been given by M. and Madame 

 Curie,* and by Sir W. Crookes.f Except in the case of pitchblende, 

 little has been done to determine the nature of the radio-active 

 constituents ; or to decide whether any hitherto unknown radio-active 

 body is present. 



To obtain complete information on these subjects, the only method 

 available would be to completely analyse the mineral, and examine 

 every precipitate and filtrate for radio-activity. This process is of 

 course very tedious, and the results have to be interpreted with care, 

 since traces of radio-active elements may often be carried down in the 

 groups to which they do not properly belong, and thus cause con- 

 fusion. A much easier method is to heat the crude mineral, and to 

 examine the rate of decay of the emanation which it gives off. Each 

 emanation has a characteristic time-constant of decay, and by deter- 

 mining this we can identify it. 



The method is of course useless for testing the presence or absence 

 of radio-active elements such as uranium,^ which do not give off a 

 characteristic emanation. But the great facility with which it may be 

 applied to a small quantity of material, and the definiteness of the 

 results, are great merits. 



In any case, when a material suspected to contain radium is obtainable 

 in abundance, it is better to test for the presence of emanation than to 

 look for activity in the solid. For but little of the solid material can 

 be advantageously used in the test. Thick layers give no larger effect 

 than thin ones, since the upper layers absorb the radiation from the 

 lower. But the emanation can be extracted from any desired bulk of 

 material, and the effect proportionately increased. If carbonic acid, 

 or any other gas, is evolved at the same time in inconvenient quantities, 

 it can be absorbed with a suitable reagent, and the emanation contained 

 in it thereby concentrated. 



* ' These presentee a la Faculte des Sciences,' Paris, p. 19. 

 f ' Eoy. Soc. Proc.,' vol. 66, p. 411. 



X I hare found a distinct, though feeble, emanation from re-crystallised 

 uranium nitrate, having a rate of decaj equal to that of the radium emanation. 

 Whether this is really due to uranium, or to traces of radium, which the uranium 

 still contains, must be left for the present an open question. 



