| A. | M. | |
| 3500 | 140 | Spectrum of radium faint. |
| 4700 | 141 | |
| 7500 | 145·8 | Spectrum of radium strong, but that of barium predominating. |
| Order of magnitude, | 173·8 225 |
The two spectra of almost equal intensity. Only a trace of barium present. |
A represents the activity of the chloride, that of uranium being unity; M the atomic weight found.
At the termination of the processes described above, I obtained, in March, 1902, 0·12 grm. of radium chloride, of which Demarçay made the spectral analysis. This radium chloride, in the opinion of Demarçay, was fairly pure; its spectrum, however, showed the three principal barium lines with considerable intensity. I made four successive estimations of the chloride, the results of which are as follows:—
| Anhydrous radium chloride. |
Silver chloride. |
M. | |
| I.......... | 0·1150 | 0·1130 | 220·7 |
| II.......... | 0·1140 | 0·1119 | 223·0 |
| III.......... | 0·11135 | 0·1086 | 222·8 |
| IV.......... | 0·10925 | 0·10645 | 223·1 |
I then re-purified this chloride, and obtained a much purer substance, in the spectrum of which the two strongest barium lines were very faint. Given the sensitiveness of the spectrum reaction of barium, Demarçay estimated that the purified chloride contained only the merest traces of barium, incapable of influencing the atomic weight to an appreciable extent. I made three determinations with this perfectly pure radium chloride. The results were as follows:—
| Anhydrous radium chloride. |
Silver chloride. |
M. | |
| I.......... | 0·09192 | 0·08890 | 225·3 |
| II.......... | 0·08936 | 0·08627 | 225·8 |
| III.......... | 0·08839 | 0·08589 | 224·0 |
The mean of these numbers is 225. They were calculated in the same way as the preceding ones by considering radium as a bivalent element, the chloride having the formula RaCl2, and taking for silver and chlorine the values Ag=107·8, Cl=35·4.
Hence the atomic weight of radium is Ra = 225.
The weighings were made with a Curie aperiodic balance, perfectly regulated, accurate to the twentieth of a
