could be observed. For the detection of radium, the examination of the radio-activity is thus a process nearly a million times more sensitive than spectrum analysis.
Later observations on the spectrum of radium have been made by Runge[1], Exner and Haschek[2], with specimens of radium prepared by Giesel. Crookes[3] has photographed the spectrum of radium in the ultra-violet, while Runge and Precht[4], using a highly purified sample of radium, observed a number of new lines in the spark spectrum. It has been mentioned already that the bromide of radium gives a characteristic pure carmine-red coloration to the Bunsen flame. The flame spectrum shows two broad bright bands in the orange-red, not observed in Demarçay's spectrum. In addition there is a line in the blue-green and two feeble lines in the violet.
14. Atomic weight of radium. Mme Curie has made
successive determinations of the atomic weight of the new element
with specimens of steadily increasing purity. In the first observation
the radium was largely mixed with barium, and the atomic
weight obtained was the same as that of barium, 137·5. In
successive observations with specimens of increasing purity the
atomic weights of the mixture were 146 and 175. The final value
obtained recently was 225, which may be taken as the atomic
weight of radium on the assumption that it is divalent.
In these experiments about 0·1 gram of pure radium chloride was obtained by successive fractionations. The difficulty involved in preparing a quantity of pure radium chloride large enough to test the atomic weight may be gauged from the fact that only a few centigrams of fairly pure radium, or a few decigrams of less concentrated material, are obtained from the treatment of about 2 tons of the mineral from which it is derived.
Runge and Precht[5] have examined the spectrum of radium in a magnetic field, and have shown the existence of series analogous to those observed for calcium, barium, and strontium. These series
