This question has been experimentally attacked by Boltwood[[352]], McCoy[[353]] and Strutt[[354]]. McCoy measured the relative activities of different minerals in the form of powder by means of an electroscope, and determined the amount of uranium present by chemical analysis. His results indicated that the activity observed in the minerals was very approximately proportional to their content of uranium. Since actinium is present as well as uranium and its products, this would indicate that the amount of radium and actinium taken together is proportional to the amount of uranium. This problem has been attacked more directly by Boltwood and Strutt by measuring the relative amount of the radium emanation evolved by different minerals. By dissolving the mineral and then setting it aside in a closed vessel, the amount of emanation present reaches a maximum value after about a month’s interval. The emanation is then introduced into a closed vessel containing a gold-leaf electroscope similar to that shown in [Fig. 12]. The rate of movement of the gold-leaf is proportional to the amount of emanation from the solution, and this in turn is proportional to the amount of radium. Boltwood has made in this way a very complete and accurate comparison of the radium content of different varieties of pitchblende and other ores containing radium. It was found that many of the minerals in the solid state allowed a considerable fraction of the emanation to escape into the air. The percentage fraction of the total amount of emanation lost in this way is shown in Column II of the following table. Column I gives the maximum amount of emanation present in 1 gram of the mineral in arbitrary units when none of the emanation escapes; Column III the weight in grams of uranium contained in 1 gram; and Column IV the ratio obtained by dividing the quantity of emanation by the quantity of uranium. The numbers in Column IV should be constant, if the amount of radium is proportional to the amount of uranium.
| Substance | Locality | I | II | III | IV |
|---|---|---|---|---|---|
| Uraninite | North Carolina | 170·0 | 11·3 | 0·7465 | 228 |
| Uraninite | Colorado | 155·1 | 5·2 | 0·6961 | 223 |
| Gummite | North Carolina | 147·0 | 13·7 | 0·6538 | 225 |
| Uraninite | Joachimsthal | 139·6 | 5·6 | 0·6174 | 226 |
| Uranophane | North Carolina | 117·7 | 8·2 | 0·5168 | 228 |
| Uraninite | Saxony | 115·6 | 2·7 | 0·5064 | 228 |
| Uranophane | North Carolina | 113·5 | 22·8 | 0·4984 | 228 |
| Thorogummite | North Carolina | 72·9 | 16·2 | 0·3317 | 220 |
| Carnotite | Colorado | 49·7 | 16·3 | 0·2261 | 220 |
| Uranothorite | Norway | 25·2 | 1·3 | 0·1138 | 221 |
| Samarskite | North Carolina | 23·4 | 0·7 | 0·1044 | 224 |
| Orangite | Norway | 23·1 | 1·1 | 0·1034 | 223 |
| Euxinite | Norway | 19·9 | 0·5 | 0·0871 | 228 |
| Thorite | Norway | 16·6 | 6·2 | 0·0754 | 220 |
| Fergusonite | Norway | 12·0 | 0·5 | 0·0557 | 215 |
| Aeschynite | Norway | 10·0 | 0·2 | 0·0452 | 221 |
| Xenotine | Norway | 1·54 | 26·0 | 0·0070 | 220 |
| Monazite (sand) | North Carolina | 0·88 | 0·0043 | 205 | |
| Monazite (crys.) | Norway | 0·84 | 1·2 | 0·0041 | 207 |
| Monazite (sand) | Brazil | 0·76 | 0·0031 | 245 | |
| Monazite (massive) | Conn. | 0·63 | 0·0030 | 210 |
With the exception of some of the monazites, the numbers show a surprisingly good agreement, and, taking into consideration the great variation of the content of uranium in the different minerals, and the wide range of locality from which they were obtained, the results afford a direct and satisfactory proof that the amount of radium in the minerals is directly proportional to the amount of uranium.
In this connection, it is of interest to note that Boltwood found that a considerable quantity of radium existed in various varieties of monazite, although most of the previous analyses agreed in stating that no uranium was present. A careful examination was in consequence made to test this point, and it was found by special methods that uranium was present, and in about the amount to be expected from the theory. The ordinary methods of analysis failed to give correct results on account of the presence of phosphates. Results of a similar character have recently been given by Strutt[[355]].
The weight of radium in a mineral per gram of uranium is thus a definite constant of considerable practical importance. Its value was recently determined by Boltwood by comparison of the emanation, liberated from a known weight of uraninite, with that liberated from a known quantity of pure radium bromide, supplied for the purpose by the writer. A measured weight of radium bromide was taken from a stock which gave out heat at a rate of slightly over 100 gram calories per hour per gram, and was thus probably pure. This was dissolved in water, and, by successive dilutions, a standard solution was made up containing 10⁻⁷ gram of radium bromide per c.c. Taking the constitution of radium bromide as RaBr2, it was deduced that the weight of radium per gram of uranium in any mineral was 8·0 × 10⁻⁷ gram. The amount of radium in a mineral per ton of uranium is thus 0·72 gram.
Strutt (loc. cit.) obtained a value nearly twice as great, but he had no means of ascertaining the purity of his radium bromide.
This amount of radium per gram of uranium is of the right order of magnitude to be expected on the disintegration theory, if uranium is the parent of radium. The activity of pure radium, compared with uranium, is not known with sufficient accuracy to determine with accuracy the theoretical proportion of radium to uranium.
The production of radium from uranium, while very strongly supported by these experiments, cannot be considered definitely established until direct experimental evidence is obtained of the growth of radium in uranium. The rate of production of radium to be expected on the disintegration theory can readily be estimated. The fraction of uranium breaking up per year has been calculated ([section 261]) and shown to be about 10-9 per year. This number represents the weight of radium produced per year from 1 gram of uranium. The emanation, released from the amount of radium produced in one year from 1 gram of uranium, would cause an ordinary gold-leaf electroscope to be discharged in about half-an-hour. If a kilogram of uranium is used, the amount of radium produced in a single day should be easily detectable.
Experiments to detect the growth of radium in uranium have been made by several observers. Soddy[[356]] examined the amount of emanation given off at different times from one kilogram of uranium nitrate in solution, which was originally freed from the small trace of radium present by a suitable chemical process. The solution was kept stored in a closed vessel, and the amount of emanation which collected in the solution was measured at regular intervals.
Preliminary experiments showed that the actual rate of production of radium was far less than the amount to be expected theoretically, and at first very little indication was obtained that radium was produced at all. After allowing the uranium to stand for eighteen months, Soddy states that the amount of emanation was distinctly greater than at first. The solution after this interval contained about 1·5 × 10-9 gram of radium. This gives the value of about 2 × 10-12 for the fraction of uranium changing per year, while the theoretical value is about 10-9.