The silver discs prepared from the two samples were each placed in an instrument called an alpha-particle spectrometer. This instrument is extremely sensitive and can measure the very small amounts of polonium-210 prepared from the tiny sample of paint that they started with. While the instruments were making the measurements, which took a couple of days, the chemist turned to the remaining solutions and began the analyses for radium. [Illustration: _A painting being sampled under a stereo-binocular microscope._] [Illustration: _Lead white weighing twenty-thousandths of a gram (20 milligrams). This is the amount needed to measure lead-210 and radium-226 to determine if the lead white is old._] In a series of chemical steps, he purified the solutions, removing the lead and other materials so that finally he had a small amount of solution that contained little else but the original radium and a very small amount of barium (an element that he deliberately added and one which is very similar to radium in its chemical properties). By adding dilute sulfuric acid, he prepared an insoluble material, barium sulfate, which was barely visible suspended in the solution. [Illustration: _Polonium plating apparatus. A heated solution of lead white in acetic acid is stirred with silver discs for 4 to 8 hours._] [Illustration: _The disc above appears clean after removal, but on its surface it retains a minute amount of polonium which can be measured._] By forcing the solution through a special thin plastic filter having tiny holes, the particles of barium sulfate together with the radium that had been in the solution were caught on the surface of the filter. This was mounted on a solid disc so that it too could be placed in the alpha-particle spectrometer for the measurement of radioactivity from the radium. Two weeks later the results were ready. Dad, the boys, and one of the experts from the museum met with the chemist to discuss them. For one of the two paintings, the polonium-210 radioactivity was about ten times that of the radium activity. The boys were disappointed because this meant that the painting could not have been 300 or 400 years old as it first appeared to be. [Illustration: _An alpha-particle spectrometer is used to measure the radioactivity of the radium and polonium prepared from the lead white._] [Illustration: _A plastic disc on which is cemented a filter containing a nearly invisible deposit of barium sulfate (BaSO₄) that “carried” the radium._] But in the second painting the radioactivity from the polonium-210 and from the radium-226 were just about equal. That meant that this painting was at least 100 years old and, from its appearance, probably more. The boys were excited. “We have a really valuable painting!” said Martin. “Not so fast, boys,” cautioned Dad. “We don’t know who painted it and we don’t know exactly how old it is.”