Very rarely have a man and woman worked together so perfectly as Professor Curie and his wife. Madame Curie was a Polish girl; she came to Paris to study, very poor, but possessed of rare talents. Her marriage with M. Curie was 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 realised. One of the chief problems to be met in finding the secrets of radium is the great difficulty and expense, in the first place, of getting any of the substance to experiment with. The Curies have had to manufacture all they themselves have used. In the first place, pitch-blende, which closely resembles iron in appearance, is not plentiful. The best of it comes from Bohemia, but it is also found in Saxony, Norway, Egypt, and in North Carolina, Colorado, and Utah. It appears in small lumps in veins of gold, silver, and mica, and sometimes in granite. Comparatively speaking, it is easy to get uranium from pitch-blende. But to get the radium from the residues is a much more complicated task. According to Professor Curie, it is necessary to refine about 5,000 tons of uranium residues to get a kilogramme--or about 2.2 pounds--of radium. It is hardly surprising, therefore, considering the enormous amount of raw material which must be handled, that the cost of this rare mineral should be high. It has been said that there is more gold in sea-water than radium in the earth. Professor Curie has an extensive plant at Ivry, near Paris, where the refuse dust brought from the uranium mines is treated by complicated processes, which finally yield a powder or crystals containing a small amount of radium. These crystals are sent to the laboratory of the Curies where the final delicate processes of extraction are carried on by the professor and his wife. And, after all, pure metallic radium is not obtained. It could be obtained, and Professor Curie has actually made a very small quantity of it, but it is unstable, immediately oxidised by the air and destroyed. So it is manufactured only in the form of chloride and bromide of radium. The "strength" of radium is measured in radio-activity, in the power of emitting rays. So we hear of radium of an intensity of 45 or 7,000 or 300,000. This method of measurement is thus explained. Taking the radio-activity of uranium as the unit, as one, then a certain specimen of radium is said to be 45 or 7,000 or 300,000 times as intense, to have so many times as much radio-activity. The radium of highest intensity in this country now is 300,000, but the Curies have succeeded in producing a specimen of 1,500,000 intensity. This is so powerful and dangerous that it must be kept wrapped in lead, which has the effect of stopping some of the rays. Rock-salt is another substance which hinders the passage of the rays.