"Mme Curie has utilized the coloration of the crystals of radiferous barium as a means of controlling the final process of purification. The crystals of salts of radium and barium deposited from acid solutions are indistinguishable. The crystals of radiferous barium are at first colorless, but in the course of a few hours, become yellow, passing to orange and sometimes to a beautiful rose color. The rapidity of this coloration depends on the amount of barium present. Pure radium crystals do not color, 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."