In order to obtain a large amount of the active substance, 2000 kilos. of pitchblende were worked up. This yielded 6 kilos. of bismuth oxychloride, and from this was separated 1·5 grams of radio-tellurium. The tellurium present was precipitated from a hydrochloric acid solution by hydrazine hydrochloride. The precipitated tellurium still showed some activity, but this was removed by repeating the process. The active matter then remained in the filtrate, and, after evaporation, the addition of a few drops of stannous chloride caused a small quantity of a dark precipitate which was intensely active. This was collected on a filter and weighed only 4 milligrams.
When plates of copper, tin or bismuth were dipped into an hydrochloric acid solution of this active substance, the plates were found to be covered with a very finely divided deposit. These plates were intensely active, and produced marked photographic and phosphorescent action. As an illustration of the enormous activity of this deposit, Marckwald stated that a precipitate of ¹⁄₁₀₀ milligram on a copper plate, 4 square centimetres in area, illuminated a zinc sulphide screen so brightly that it could be seen by an audience of several hundred people.
The active substance of Marckwald is very closely allied in chemical and radio-active properties to the polonium of Mme Curie. Both active substances are separated with bismuth and both give out only easily absorbed rays. The penetrating rays, such as are given out by uranium, radium or thorium, are completely absent.
There has been a considerable amount of discussion as to whether the active substance obtained by Marckwald is identical with that present in the polonium of Mme Curie. Marckwald stated that his active substance did not sensibly diminish in activity in the course of six months, but it is doubtful whether the method of measurement used was sufficiently precise.
The writer has found that radio-tellurium of moderate activity, prepared after Marckwald’s method and sold by Dr Sthamer