[Illustration: _X-ray diffraction patterns from three different lead compounds that might occur in lead white. The middle one is the ideal lead white produced for over 2000 years. While some of the bottom compound may be found mixed with it, the compound shown at the top is only a 20th-century invention._] 4PbCO₃ · 2PB(OH)₂ · PbO 2PbCO₃ · PB(OH)₂ PbCO₃ “How do you measure lead isotopes?” asked Harley. “With an instrument called a mass spectrometer. This instrument is capable of separating the lead isotopes. First, the atoms of lead in the sample are electrically charged and ‘fired’ in a beam down the length of a tube between the poles of a strong magnet. There, the charged atoms (or ions) in the beam are deflected by different amounts according to how heavy they are. Thus the different isotopes are separated. This method is also still being studied and, although it shows great promise, it will be some time before it can solve problems of art identification. Also the study of the natural variation in isotopes of other elements, such as sulfur, is useful for identification of other pigments as well. [Illustration: _Diagram of a simple mass spectrometer. The ionized atoms of lead travel in a beam at the same speed. The heavier atoms bend less than the lighter ones when the beam passes the magnet. Thus two beams emerge instead of one. Actually there are four isotopes of lead so there will be four beams._] [Illustration: _“Agostina”, a genuine painting by Jean Baptiste Camille Corot._] “Another new method that shows great promise has been developed, but this one is not applicable to the paintings that you boys found in the box.” “Why not?” asked Bill. “Since the Second World War, the art forgery business has been growing rapidly. For example, it has been said that of the 2000 pictures that Corot, a 19th century Frenchman, is known to have painted, more than 5000 of them are in the United States. This may be only a humorous exaggeration, but a large number of forgeries have been produced in the last several years. These are usually supposed to be paintings that are less than 100 years old. Present-day forgers like to forge paintings that aren’t very old because it’s easier to get away with. Now this new method, which will detect such recent forgeries, is based upon the presence of carbon-14, a radioactive isotope of carbon, in our atmosphere and in all things that grow on our planet.