July 12, 1901.] 



SCIENCE. 



53 



Andrew S. Draper. 

 Charles W. Eliot. 

 William H. Maxwell. 

 j. g. schurman. 

 F. Louis Soldan. 



Note. — Mr. E. J. James was unable to attend 

 the meetings of the Committee and declines to sign 

 the report. 



May 24, 1901. 



BADIO-AQTIVE SUBSTANCES AND THEIR 

 RADIATIONS. 



During the past five years many physi- 

 cists, attracted by the freshness of the field 

 and the promise of important discoveries, 

 have turned their attention to the study of 

 the newly discovered radio-active sub- 

 stances. The result has been a rapid in- 

 crease in knowledge of and interest in the 

 phenomena, until now the main facts are 

 known to all scientists, but, since the 

 knowledge of the subject is increasing so 

 fast, a short review is now and then accept- 

 able and necessary, especially to those 

 whose chief interests lie along other lines. 

 In this article an attempt is made to point 

 out the more interesting features of the 

 subject. 



The real discovery of the persistent radi- 

 ations from the uranium compounds was 

 made by M. Henri Becquerel in 1896. It 

 had been stated by M. ISTiewenglowski that 

 under the action of sunlight certain phos- 

 phorescent salts emit radiations which can 

 penetrate black paper. In testing whether 

 this applies to uranium salts, M. Becquerel 

 discovered to his surprise that with uranium 

 salts exposure to sunlight is unnecessary ; 

 uranium compounds are all the time giving 

 off radiations which can pass through 

 opaque bodies and affect a photographic 

 plate on the other side. It was soon found 

 that the uranium radiations discharge elec- 

 trified bodies in the neighborhood by ioniz- 

 ing the surrounding air after the manner 

 of kathode and X-rays. Naturally, about 



the first hypothesis was that the radiations 

 are ether vibrations, perhaps of very short 

 wave-length, and many attempts were made 

 to find evidence of reflection, refraction or 

 polarization, with the result that none of 

 these properties nor any of the properties 

 peculiar to wave motion has yet been 

 shown to belong to these radiations. 



A few months after the discovery of the 

 uranium radiations Professor Schmidt and 

 Mme. Curie, a Polish physicist working in 

 Paris, independently discovered the radio- 

 activity of thorium compounds. An elab- 

 orate study of thorium radiations has since 

 been made at McGill University by Pro- 

 fessors Butherfocd and Owens. 



A greater discovery, however, was in 

 store for Mme. Curie ; for observing that 

 many specimens of pitchblende, the princi- 

 pal ore of uranium, were more strongly 

 radio-active than the pure uranium salts, 

 she and her husband attempted a chemical 

 separation of the suspected more active 

 element. The result is well known ; they 

 succeeded in isolating two substances hav- 

 ing at least 100,000 times the radio-activity 

 of uranium. The first of these substances, 

 which they named polonium, follows the 

 bismuth in the separation from pitchblende. 

 The separation from the bismuth is effected 

 by taking advantage of the fact that polo- 

 nium sulphide is more volatile than bis- 

 muth sulphide. The second substance they 

 named radium. It follows the barium in 

 its chemical reactions, but its chloride is 

 less soluble in water than barium chloride, 

 which affords a means of separation from 

 the barium. Another very active sub- 

 stance has been obtained from pitchblende 

 by M. Debierne. He has named it actinium. 

 Chemically it is closely allied to titanium. 



ISTo one of these three substances has 

 been obtained free from impurity, and the 

 amounts obtained are exceedingly small, 

 only a few centigrams from a thousand ki- 

 lograms of pitchblende. 



