530 SCIENCE PROGRESS 



to give a constant supply of radiation. In this way any variations 

 in the instrument are corrected. 



The photographic action of the radiations has been largely 

 used in certain branches of the subject, but does not give good 

 quantitative results. The method employed is the exposure 

 of the photographic plate wrapped in black paper — to cut off 

 phosphorescent light — to the effect of the radiations. The time 

 of exposure varies from a few minutes, with almost pure radium 

 preparations, to perhaps seventy-two hours, with very weak 

 radioactive preparations. With long exposures the results are 

 especially untrustworthy on account of the difficulties experi- 

 enced through the plates fogging. 



In order to show how the instruments are actually employed 

 and deductions made, a brief account will be given of the 

 results obtained to the present time from the study of thorium 

 compounds. The greater part is taken from Rutherford's 

 Radioactivity, second edition. 



Schmidt and Madame Curie independently discovered the 

 radioactivity of thorium compounds in 1898. Four years later 

 Rutherford and Soddy found that the thorium hydroxide 

 precipitated by ammonia from a solution of the nitrate was 

 less than half as active as the original solid salt; the filtrate 

 on evaporation gave a residue which, weight for weight, 

 was several thousand times more active. They named this 

 active residue thorium X (ex-thorio). It consisted largely of 

 impurities existing in the original nitrate ; the true thorium X 

 was present only in minute quantity, and could not be sepa- 

 rated. It became inactive at the end of a month ; in the same 

 time the thorium completely regained its activity. They 

 concluded therefore that thorium X was actually produced 

 from thorium. They proceeded to study the time-rates of 

 these processes, making electrical measurements at intervals 

 over a long time. Their results are shown in fig. 2, a (Radio- 

 activity, p. 221); the curves are typical of most of those 

 illustrating the decay and production of radioactive matter. 

 The initial activity of thorium X was arbitrarily taken as 100 ; 

 similarly that figure was taken for the final activity of the 

 thorium compound. The activities were expressed along the 

 ordinates, the times from the initial measurement along 

 the abscissae. The initial irregularities in the curves were 

 due to the presence in the original thorium compound of further 



