146 CHEMICAL DISCOVERY AND INVENTION 



and on, since 1905, and has been tested on a sensitive photo- 

 graphic film at intervals of a year or more. No appreciable 

 difference in its radio-activity can be detected from that which 

 it possessed when first removed from the radium bromide in 

 September, 1905. Examined at the present time, nine years 

 after its removal from the bottle of radium bromide, it is 

 luminous in the dark, it rapidly discharges a sensitive electro- 

 scope when held near it, and produces scintillations on a zinc 

 sulphide screen as if it were a radium compound." 



Another very remarkable fact about radium (discovered by 

 P. Curie and Laborde in 1903) is that a mass of the salt is always 

 at a temperature several degrees above that of the surrounding 

 atmosphere. Obviously the exact difference will depend upon 

 circumstances, but this spontaneous liberation of energy was 

 made the subject of many later experiments, and among other 

 facts it was found that the rate of emission depended on the age 

 of the specimen. The quantitative estimation of the heating 

 effect by Rutherford and Barnes in 1904 led to the result that 

 one gram of radium bromide gives out 110 gram calories per hour. 



The element radium was obtained in the metallic state in 1910 

 by Madame Curie and M. Debierne. It is a white metal which 

 melts at about 700 C., and which dissolves in water, decom- 

 posing it and forming the alkaline hydroxide, hydrogen gas 

 being given off. The salts of radium are very similar to those 

 of barium, and it agrees in general properties, valency, etc., 

 with the metals of the alkaline earths, and is consequently 

 placed in the periodic table below barium. But while the other 

 members of the same series are destitute of radio-active properties 

 the activity of radium as measured by the electroscope is about 

 2,000,000 times that of uranium. 



The atomic weight of radium has been the subject of much 

 careful experiment. The conditions necessary to ensure the 

 utmost possible accuracy in such determinations, and the chief 

 considerations involved have been so admirably exposed in the 

 paper by Sir William Ramsay and Dr. R. Whytlaw Gray on 

 " The Atomic Weight of Radium," in the Proceedings of the 

 Royal Society for 1912, p. 270, that we cannot do better than 

 quote the greater part of the introductory portions of this paper. 

 It not only gives the history of the important question as to the 

 atomic weight of radium, but it affords very instructive informa- 

 tion as to the procedure in work of this kind in genera], 



