PARIS (AP) — The “Mona Lisa” has given up another secret. Using X-rays to peer into the chemical structure of a tiny speck of the celebrated work of art, scientists have gained new insight into the techniques that Leonardo da Vinci used to paint his groundbreaking portrait of the woman with the exquisitely enigmatic smile. The research, published Wednesday in the Journal of the American Chemical Society, suggests that the famously curious, learned and inventive Italian Renaissance master may have been in a particularly experimental mood when he set to work on the “Mona Lisa” early in the 16th century.