Over the next few weeks quite a few things happened to the boys and their paintings. Three of them were discarded right away because they were immediately recognized as being copies of no value. Two were relatively modern paintings with the signature Alfred Sisley; if genuine, they were less than 100 years old. The remaining four appeared to be very old paintings. Two of them seemed to correspond to paintings that disappeared during the Second World War. Photographs and X rays were taken and sent to the museum in Holland, which had owned the missing pictures, so that they could make a preliminary examination. [Illustration: uncaptioned] Radioactivity of Lead-210 Lead-210 decaying with a half-life of 22 years. When no radium is present there is almost none left after 6 half-lives or 132 years. Radioactivity of Radium-226 Over the same period of time, a small amount of radium decays very little because its half-life is about 1600 years. Radioactivity of Radium 226 Radioactivity of Lead-210 But when lead-210 decays in the presence of radium-226, the radioactivity of the lead-210 only decreases until it is equal to the radioactivity of the radium. That left two that could have been old but whose origins were unknown. A series of simple chemical tests were begun on these and the boys watched experts take very small samples of paint for examination under the microscope. After several months a list of the pigments present in the paintings was prepared. All the pigments found were typical of old paintings and the ordinary examinations and tests couldn’t prove whether the works were old or not. Finally, it was decided that the only way to tell if these paintings were truly old was to apply the test that Dad had described to the boys. The boys watched a painting restorer remove samples of nearly white paint right at the edge of the paintings. He worked carefully, using a very sharp scalpel and a stereo-binocular microscope, through which objects appeared to be sixty times larger than they really were. The sample of paint weighed approximately twenty-thousandths of a gram. The boys and their father took the samples to a radiochemical laboratory where they watched a radiochemist do the required analysis for lead-210 and radium in the samples. First the chemist dissolved the paint in acetic acid. This removed the lead white from the oil and from the small amounts of other pigments in the paint. The solutions were then heated and stirred with a silver disc hanging in the liquid. After several hours the disc still looked clean, but the chemist said that a radioactive element, polonium-210, was now plated onto the silver. Polonium-210 is a member of the uranium series following the lead-210, and a measurement of its radioactivity would be an accurate measurement of the radioactivity of lead-210.