The Gallery’s expert was happy too. He believed that the second picture was a genuine Dutch painting from the 17th century. It was a landscape and the artist might have been Aelbert Cuyp. [Illustration: _“The Maas at Dordrecht”, a genuine painting by Aelbert Cuyp._] “What do we do now?” asked Harley. “How can we prove that the painting was painted in Holland in the 17th century by Cuyp?” “There is a method now being developed,” said Dad, “that could give us that kind of information.” “How does it work?” Martin asked. Who Was the Artist? “Do you know how criminals are caught by using fingerprints?” asked Dad. “Sure we do,” said Martin. “Each person has a set of fingerprints that is different from anyone else’s.” Harley spoke up. “Did the artist leave his fingerprints on the paintings?” “Probably not,” said Dad. “Besides, they would have been wiped off long ago. Also, who knows what each artist’s fingerprints were like?” “Then what do you mean?” asked Bill. “What I mean is, there is another kind of ‘fingerprint’ that scientists are just now learning to use in all kinds of identification problems. It’s not really a fingerprint, but it’s just as distinctive as a real fingerprint. “You see, in every material, no matter how pure you try to make it, there are always other substances contained in it in very, very small quantities, which are there only by chance. Usually the person making or using that material doesn’t even know they are there, and the quantities are so small they don’t do any harm. During the last several years, scientists have developed extremely sensitive methods of analysis, which have been applied to all kinds of problems. “One such method is called neutron activation analysis. In this method these small amounts of impurities can be detected in tiny samples of material. This is quite important because only very small samples can be taken from a precious painting without damaging it. Normally, a scientist or an art restorer takes samples that are no bigger than the head of a pin.” “How can you do anything with a sample that small?” asked Bill. [Illustration: uncaptioned] “With neutron activation analysis you can do a great deal. To give you an example of how sensitive this method is, think of a bathtub containing 500 quarts of milk. Add 1 drop of an acid containing a speck of gold dissolved in it. After you mix the acid and milk thoroughly, you won’t be able to tell by looking at it that anything was added. But if you take a thimble full of liquid out of the bathtub, you can easily tell with neutron activation analysis that gold was added to the milk. “Scientists call low concentrations of accidental impurities ‘trace elements’, and the amounts that are present are measured in parts per million rather than percent. One part per million is one ten-thousandth of a percent.” Bill spoke up again. “So how does that make a fingerprint, Dad?”