Radium is, so far, the only member of the new radio-active substances that has been isolated as the pure salt.
Spectrum of Radium.
It was of the first importance to check, by all possible means, the hypothesis, underlying this work, of new radioactive elements. In the case of radium, spectrum analysis was the means of confirming this hypothesis.
M. Demarçay undertook the examination of the new radioactive bodies by the searching methods which he employs in the study of photographic spark spectra.
The assistance of so competent a scientist was of the greatest value to us, and we are deeply grateful to him for having consented to take up this work. The results of the spectrum analysis brought conviction to us when we were still in doubt as to the interpretation of the results of our research.
The first specimens of fairly active barium chloride containing radium, examined by M. Demarçay, exhibited together with the barium lines a new line of considerable intensity and of wave-length λ = 381·47 μμ in the ultra-violet. With the more active products prepared subsequently, Demarçay saw the line 381·47 μμ more distinctly; at the same time other new lines appeared, and the intensity of the new lines was comparable with that of the barium lines. A further concentration furnished a product for which the new spectrum predominated, and the three strongest barium lines, alone visible, merely indicated the presence of this metal as an impurity. This product may be looked upon as nearly pure radium chloride. Finally, by further purification, I obtained an exceedingly pure chloride, in the spectrum of which the two chief barium lines were scarcely visible.
The following is a list, according to Demarçay, of the principal radium lines for the portion of the spectrum included between λ = 500·0 and λ = 350·0 μμ. The intensity of each line is represented by a figure, the strongest being marked 16:—
| λ. | Intensity. | λ. | Intensity. |
| 482·63 | 10 | 460·03 | 3 |
| 472·69 | 5 | 453·35 | 9 |
| 469·98 | 3 | 443·61 | 8 |
| 469·21 | 7 | 434·06 | 12 |
| 468·30 | 14 | 381·47 | 16 |
| 464·19 | 4 | 364·96 | 12 |
All the lines are clear and narrow, the three lines 381·47,
