SYMPOSIUM OX RADIOACTIVITY. 45 



Passing now to some of the properties of radium other than 

 those which are employed to define it, let us consider first its 

 spectrum. 



SPECTRUM OF RADIUM. 



It was on the day after Christmas, 1898. that Demarcay* 

 found in the spectrum of a few milligrams of very pure radium 

 '^hloride. submitted to him by the Curies, a strange line, of wave 

 length 3814.8 Angstrom units, a line which he was unable to 

 find in the spectrum of any other substance. Later measures by 

 Demarcay disclosed some ten or fifteen lines which he considered 

 characteristic of radium. 



But it was not until a year later that Runge, working with 

 larger dispersion and higher accuracy, banished the last doubt 

 as to the elemental character of radium. 



A few weeks ago, at Minneapolis, Professor Minot defined 

 science as "knowledge which has acquired impersonal validity." 

 Now while everyone entertained the highest regard for Demar- 

 cay's work, yet I believe all felt that this independent determina- 

 tion of Runge^ was just what was needed to give the result 

 "impersonal validity." 



Still later Runge and Precht- mapped no less than -10 lines 

 in the spark spectrum of radium-bromide, and showed that it is 

 a strict analogue of the spectrum of the alkaline earths, mag- 

 nesium, calcium, strontium and barium. 



The strong blue radium hne at -1826. is, for instance, the 

 analogue of one of the best known of all spectral lines, Fraun- 

 hofer's g, Ca 4236.; while the radium pair {^[ constitute the 

 analogue of Fraunhofer's H and K, 



So strongly did chemists feel that the proper test for an ele- 

 ment is the discovery of a characteristic spectrum that it was 

 decided at the Paris Congress of Chemists in 1900 that no element 

 should receive a name until its spectrum had been mapped. 



SELF-LUMIXOSITY OF RADIUM. 



But radium emits other light than that of its spark spectrum. 

 Even in its purest state, a radium salt is self-luminous in the dark. 

 By placing a small particle of radium at the slit of a quartz 

 spectograph and leaving it there for several days. Sir William 

 and Lady Huggins^ succeeded in obtaining the photograph of the 



•Demarcay. C. R. 127, 1218 (1898). 



>Runge; Ann. der Physik., 2, 742 (1900). 



2Runge and Precht; Ann. der Physik, 12, 407 (1903). 



»Huggins, Astrop. Tour. 18, 395 (1903), and 23, 153 (1906). 



