It is thus difficult to control the final stages of the purification of radium by measurements of its activity alone. Moreover the activity of radium immediately after its preparation is only about one-fourth of its final value; it gradually rises to a maximum after the radium salt has been kept in the dry state for about a month. For control experiments in purification, it is advisable to measure the initial rather than the final activity.
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 by the eye. 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[[18]] has observed that pure radium bromide gives a beautiful carmine colour to the Bunsen flame. If barium be 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[[19]] has devoted a great deal of attention to the separation of radium, polonium and other active bodies from pitchblende. He was indebted for his working material to the firm of P. de Haen, of Hanover, who presented him with a ton of pitchblende 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 free the radium almost completely 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, and submitted them for examination of their spectrum to Demarçay, an authority on that subject. The first specimen of radium chloride examined by Demarçay[[20]] was not very active, but showed, besides the lines due to barium, a very strong new line in the ultra-violet. In another sample of greater activity, the line was still stronger and others also appeared, while the intensity of the new lines was comparable with those present due to barium. With a still more active specimen which was probably nearly pure, only three strong lines of barium appeared, while the new spectrum was very bright. The following table shows the wave-length of the new lines observed for radium. The wave lengths are expressed in Ångström units and the intensity of each ray is denoted by a number, the ray of maximum intensity being 16.
| Wave length | Intensity | Wave length | Intensity |
|---|---|---|---|
| 4826·3 | 10 | 4600·3 | 3 |
| 4726·9 | 5 | 4533·5 | 9 |
| 4699·6 | 3 | 4436·1 | 6 |
| 4692·1 | 7 | 4340·6 | 12 |
| 4683·0 | 14 | 3814·7 | 16 |
| 4641·9 | 4 | 3649·6 | 12 |
The lines are all sharply defined, and three or four of them have an intensity comparable with any known lines of other substances. There are also present in the spectrum two strong nebulous bands. In the visible part of the spectrum, which has not been photographed, the only noticeable ray has a wave length 5665, which is, however, very feeble compared with that of wave length 4826·3. The general aspect of the spectrum is similar to that of the alkaline earths; it is known that these metals have strong lines accompanied by nebulous bands.
The principal line due to radium can be distinguished in impure radium of activity 50 times that of uranium. By the electrical method it is easy to distinguish the presence of radium in a body which has an activity only ¹⁄₁₀₀ of uranium. With a more sensitive electrometer ¹⁄₁₀₀₀₀ of the activity of uranium 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[[21]], Exner and Haschek[[22]], with specimens of radium prepared by Giesel. Crookes[[23]] has photographed the spectrum of radium in the ultra-violet, while Runge and Precht[[24]], 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.