[Illustration: _The radioactive series that starts with uranium is like a series of lakes connected by waterfalls. As long as uranium, the big one on top, has water in it, the others will be full and the falls will keep flowing. But when the first waterfall is shut off, the small lakes below it will run dry._] “I get it,” said Bill. “That means that when you take a sample of old lead white paint, there shouldn’t be any radioactive lead-210 left.” “That’s right. But that would only be true if you removed all the radium. Actually, in the smelting process it’s more usual to remove only 90 or 95% of the radium. In that case, the lead-210 would decay only until the amount left would be equal to the small amount of radium that wasn’t removed. In effect, this would be like shutting off only part of the waterfall.” “So what do you find,” asked Harley, “if you measure the radioactivity in a sample of lead white paint?” “We find that if the paint is old, compared to the 22-year half-life of the lead, let’s say 100 years old or more, then the amount of radioactivity from the lead-210 in the sample of paint will be equal to the amount of radioactivity from the radium in the sample. But if the paint is modern, let’s say only 20 years old or so, then the amount of radioactivity from the lead-210 will be greater than the amount of radioactivity from the radium.” Martin, who had been quiet through all this explanation, finally spoke up. “Well, was it finally tried out? How did it work?” “Hundreds of samples were analyzed. These samples were taken from paintings of all ages, from some over 300 years old right up to others only a couple of years old. The old samples always showed equal amounts of radioactivity from lead-210 and radium while the modern ones always showed larger amounts of radioactivity from lead-210 than from radium. That meant that scientists had a way of definitely telling if a lead white paint was modern or not. “Eventually, the method was tried on a number of paintings believed to be by Van Meegeren. Sure enough, every one of them showed that the paint couldn’t possibly have been more than 30 or 40 years old and that Van Meegeren probably was telling the truth when he said that he had painted them. The paintings certainly were not genuine Vermeers from the 17th century.” “Okay, Dad,” said Martin, “can we use the method on any of the paintings we found? Are any of these paintings supposed to be old enough so that we can use this test?” “Not so fast. To find that out we have to do a lot of checking first.” “How do we go about it?” asked Bill. “Let’s see now. There are nine paintings in the box you found. The first thing we should do is take them down to a museum or gallery and let the art experts look at them. Since we have a few weeks of vacation time left, what do you say we take a trip down to Washington, D. C., and show them to some experts at the National Gallery of Art?”