JlLY 10, 1903.] 



SCIENCE. 



41 



REVEST DEVELOPMENTS IX THE STUDY 

 OF RADIOACTIVE SUBSTASCES.* 



Barely eight years have elapsed since 

 the diseo^try of Becquerel rays. Yet dur- 

 ing that time the subject of radioactivity 

 has developed so rapidly that it has now 

 become an important branch of physics and 

 chemistry. The phenomena are interest- 

 ing in themselves, in some cases almost 

 startling. But even more important is the 

 bearing of the results upon some of the 

 conceptions that lie at the very founda- 

 tion of physical science. The study of 

 radioactivity seems destined to exert a pro- 

 found influence upon physical and chemical 

 theories. 



Without entering into the history of the 

 siibject, I shall first call attention to the 

 results that are now best established, put- 

 ting the facts into as systematic form as 

 possible. The contradictory character of 

 the early work, and the great complexity 

 of the phenomena themselves, make this as 

 difficult as it is desirable. 



A radioactive substance may be defined 

 as a substance which sends out Becquerel 

 rays; i. e., rays that are capable of pene- 

 trating bodies usually regarded as opaque, 

 and which produce certain characteristic 

 photographic and electric effects. In their 

 general behavior such rays show a close 

 resemblance to Roentgen rays; the differ- 

 ences will be referred to later. In the table 

 below is given a list of the radioactive sub- 

 stances now known. 



Radioacti\t: Substances. 

 Permanently Active. 



Uranium (238) 



Thorium (232) 



Radium ( 225t ) 



* Address delivered before the Cornell Section 

 of the American Chemical Society on May 18, 

 1903. 



t A study of the spectrum of radium has led 

 Eunge and Prccht to assign to this new element 

 the atomic weight 25.8 instead of the value 22.5 



Polonium (radioactive bismuth) 



Actinium. 

 Radioactive lead. 



Temporarily Active. 

 Ur-X, Th-X, excited activity obtained from air, 

 from freslily fallen rain or snow, or from per- 

 manently active bodies. 



It will be noticed that the list is divided 

 into three groups. The first, containing 

 uranium, thorium and radium, consists of 

 elements whose separate existence is well 

 established. This statement may now be 

 made in the case of radium as well as in 

 the case of the other two, since this sub- 

 stance has recently been so completely iso- 

 lated as to make possible the determina- 

 tion of its atomic weight, while it has been 

 shown by several observers to possess a 

 characteristic spectrum. It will be noticed 

 that the elements in this radioactive group 

 possess atomic weights greater than any 

 other known elements. 



The second group is made up of sus- 

 pected new elements. These elements have 

 not been isolated and have not yet been 

 found to give characteristic spectra. It is 

 thought by some that the radioactivity in 

 these cases is due to the presence of a trace 

 of radium— so small as not to be detected 

 by an ordinary test. It appears to me that 

 the arguments against this view are strong. 

 But the question can only be settled by 

 more extended experimental study. 



In the case of the substances of the first 

 two groups, with the possible exception of 

 polonium, the radioactivity is permanent 

 so far as our present knowledge goes. In 

 other, words, these substances continue to 

 give out Becquerel rays without special 

 stimulation, such as is required for ordi- 

 nary phosphorescence, and with no diminu- 

 tion in intensity that has thus far been 

 detected. The question as to whether any 



obtained by Madame Curie by chemical methods. 

 Some uncertainty therefore still exists. 



