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PHYSICISTS are helping historians to pinpoint the date of Galileo鈥檚 discovery of the law of free-fall motion, by analysing the ink with which the great man wrote.

Galileo Galilei is famous for formulating the law, which says that objects of different masses fall at the same rate under the Earth鈥檚 gravity. Historians of science have always wondered when and how he got the idea, which was the first break with Aristotelian theories of motion. The law is absent from Galileo鈥檚 error-ridden On Motion, written in 1590, but was all worked out by 1632 when Galileo published his Dialogue Concerning the Two Chief World Systems. This was the work on cosmology in which he concluded that the Earth revolves around the Sun, landing him in trouble with the Catholic church. But notes from the period between these works are largely undated. 鈥淚t鈥檚 a muddle,鈥� says Peter Machamer, a Galileo scholar at the University of Pittsburgh in Pennsylvania.

Now physicists at the National Institute of Nuclear Physics in Florence have bombarded pages of Galileo鈥檚 notes with a proton beam, creating X-rays with a spectrum that reveals the ratios of iron, copper, zinc and lead in the ink. Wallace Hooper, a historian of science, and statistician Steen Andersson, both at Indiana University in Bloomington, identified more than 20 different batches of ink. This revealed that the ink used in Galileo鈥檚 first formulation of the law was also used in financial records dated 1604. Their findings have been submitted to Nuncius, the quarterly journal of the Institute and Museum of History of Science in Florence.

By combining the ink data with what鈥檚 known about Galileo鈥檚 life, Hooper hopes to put the rest of the papers in chronological order, and get more clues to how he made his breakthrough.