THE ADVANCE OF SCIENCE 75 



By a long series of fusions, solutions, and crystallizations 

 the Curies succeeded in * hunting down,' as it were, the 

 radio-active element. The first step gave them a powder 

 mixed with barium chloride, and having 2,000 times the 

 activity of the uranium in which Becquerel first proved 

 the existence of the new property radio-activity. Then 

 step by step they purified it to a condition 10,000 times, 

 then to 100,000 times, and finally to the condition of a 

 crystalline salt having 1,800,000 times the activity of 

 Becquerel's sample of uranium. The purification could 

 go no further, but the extraordinary minuteness of the 

 quantity of the pure radio-active substance obtained 

 and the amount of labour and time expended in pre- 

 paring it may be judged of from the fact that of one 

 ton of the pitch-blende ore submitted to the process of 

 purification only the hundredth of a gram the one- 

 seventh of a grain remained. 



The amount of radium in pitch-blende is one ten- 

 millionth per cent.; rarer than gold in sea- water. The 

 marvel of this story and of all that follows consists 

 largely in the skill and accuracy with which our chemists 

 and physicists have learnt to deal with such infinitesimal 

 quantities, and the gigantic theoretical results which are 

 securely posed on this pin-point of substantial matter. 



The Curies at once determined that the minute 

 quantity of colourless crystals they had obtained was 

 the chloride of a new metallic element with the atomic 

 weight 225, to which they gave the name radium. The 

 proof that radium is an element is given by its * sign- 

 manual ' the spectrum which it shows to the observer 

 when in the incandescent state. It consists of six bright 

 lines and three fainter lines in the visible part of the 

 spectrum, and of three very intense lines in the ultra- 

 violet (invisible) part (fig. 10). A very minute quantity is 



