t] RADIO-ACTIVE SUBSTANCES 15 
the radium salt has been kept in the dry state for about a month. 
For control experiments in purification, it 1s advisable to measure 
the initial rather than the final activity. 
Mme Curie has utilized the coloration of the crystals of radi- 
ferous barium as a means of controlling the final process of puri- 
fication. The crystals of salts of radium and barium deposited from 
acid solutions are indistinguishable. The crystals of radiferous 
barium are at first colourless, but, in the course of a few hours, 
become yellow, passing to orange and sometimes to a beautiful rose 
colour. The rapidity of this coloration depends on the amount of 
barium present. Pure radium crystals do not colour, or at any rate 
not as rapidly as those containing barium. The coloration is a 
maximum for a definite proportion of radium, and this fact can be 
utilized as a means of testing the amount of barium present. When 
the crystals are dissolved in water the coloration disappears. 
Giesel* has observed that pure radium bromide gives a beautiful 
carmine colour to the Bunsen flame. If barium is present in any 
quantity, only the green colour due to barium is observed and a 
spectroscopic examination shows only the barium lines. This 
carmine coloration of the Bunsen flame is a good indication of the 
purity of the radium. 
Since the preliminary announcement of the discovery of 
radium, Giesel? has devoted a great deal of attention to the 
separation of radium, polonium and other active bodies from pitch- 
blende. He was indebted for his working material to the firm 
of P. de Haen of Hanover, who presented him with a ton of pitch- 
blende residues. Using the method of fractional crystallization of 
the bromide instead of the chloride, he has been able to prepare 
considerable quantities of pure radium. By this means the labour 
of final purification of radium has been much reduced. He states 
that six or eight crystallizations with the bromide are sufficient to 
almost completely free the radium from the barium. 
13. Spectrum of radium. It was of great importance to 
settle as soon as possible whether radium was in reality modified 
barium or a new element with a definite spectrum. For this 
purpose the Curies prepared some specimens of radium chloride, 
1 Phys. Zeit. 3, No. 24, p. 578, 1902. 
2 Wied. Annal. 69, p. 91, 1890. Berichte d. d. chem. Ges. p. 3608, 1902. 
