“How did it come out?” asked Martin. [Illustration: _An X ray of part of the Van Meegeren forgery, “Christ and His Disciples at Emmaus”. In the white circle are traces of paint from the original painting that Van Meegeren scraped off to obtain the old canvas. When the painting was believed to be a genuine Vermeer, it was sold for about $300,000._] [Illustration: The complete painting.] [Illustration: _A Van Meegeren forgery of a Vermeer._] “How does it work?” asked Harley. “You mean paintings are radioactive?” exclaimed Bill. “Can we do it to the paintings we found?” asked all three together. How Old Is a Painting? “One question at a time. I’ll tell you how the method works and what it does if you’re really interested.” “We’re interested! We’re interested!” chorused the boys. “In the first place, this method works only in certain cases of suspected forgery. Over the last 50 or 100 years, a number of paintings have turned up that seemed, even to the best art experts, to be several hundred years old. Some of these were genuine, and some were painted by forgers who could not resist the high prices paid for works of art. The National Gallery of Art, in Washington, D. C., thinking that there might be a way of detecting these forgeries, gave its support to a group of scientists who developed a method for this purpose. “To understand how the method works, you need to know a little about how radioactive atoms disintegrate to form atoms of other elements. In this case we are interested in the natural radioactivity that occurs in certain rocks. As a matter of fact, in almost all rocks in the earth’s crust there is a certain small quantity of uranium.” “I thought uranium was rare,” interrupted Bill. “It is, but we’re talking about such small quantities that its difficult for scientists using the most sensitive equipment to detect it. The uranium in the rock decays to another radioactive element and that one decays to another, and another, and another, and so forth, in a series of elements that results in lead, which is not radioactive. In this series are two radioactive elements, radium and a radioactive isotope of lead, that help us to date paintings. To understand this, we must first understand how radioactive elements decay. “All radioactive elements have what is known as a ‘half-life’; that is, in a certain period of time, half of the element disintegrates to another form. In another equal period of time, half of what is left disintegrates, and then half again, and so on. In the case of the uranium, which starts the series I am describing, the half-life is over 4,000,000,000 years. Because of its long half-life there is plenty of uranium around and will be for a long, long time. On the other hand, radium, which I mentioned a moment ago, has a half-life of only 1600 years. In 1600 years, half of it would be gone, and in another 1600 years half of that would be gone, and so on.