He told me a little about Mme. Curie, who, it appears, was a Polish student in the Latin Quarter, very poor, but possessed of rare talents. They say that her marriage with M. Curie was just such a union, as must have produced some fine result. Without his scientific learning and vivid imagination it is doubtful if radium would ever have been dreamed of, and without her determination and patience against detail it is likely the dream would never have been realized.
The next day I found M. Curie in one of the rambling sheds of the École de Physique bending over a small porcelain dish, where a colourless liquid was simmering, perhaps half a teacupful, and he was watching it with concern, always fearful of some accident. He had lost nearly a decigramme (1·5 grains troy) of radium, he said, only a few weeks before in a curious way. He had placed some radium salts in a small tube, and this inside another tube, in which he created a vacuum. Then he began to heat both tubes over an electric furnace, when, suddenly, at about 2,000 degrees F., there came an explosion which shattered the tubes and scattered their precious contents. There was absolutely no explanation of this explosion; it was one of the tricks that radium is apt to play on you. Here his face lightened with quite a boyish smile.
M. Curie proceeded to explain what he was doing with the little dish; he was refining some radium dissolved in it—that is, freeing it from contaminating barium by repeated crystallization, this being the last and most delicate part of the process of obtaining the pure metal.
"We have our radium works outside Paris," he said, "where the crude ore goes through its early stages of separation and where the radium is brought to an intensity of 2,000, as we express it. After that the process requires such care and involves so much risk of waste that we keep the precious stuff in our own hands and treat it ourselves, my wife and I, as I am doing now, to bring it to the higher intensities, 50,000, 200,000, 500,000, and, finally, 1,500,000. What you see here is about 100,000. It will take many more crystallizations to bring it to the maximum."
"That is, to the state of pure radium?"
"To the state of pure chloride of radium. You know the metal exists only as a chloride or bromide. It has never yet been isolated, although it easily might be."
"Why has it never been isolated?"
"Because it would not be stable; it would immediately be oxidized by the air and destroyed, as happens with sodium, whereas it remains permanent as a bromide or chloride and suffers no change."
"Does radium change in appearance as it increases in intensity?" I asked.
"No; it keeps the form of small white crystals, which may be crushed into a white powder, and which look like ordinary salt. See, here are some."