is not one because it can be reduced to water. On the one hand, Van Helmont was a disciple of the mystic and alchemist, Paracelsus, though he scornfully repudiated the errors of most contemporary authorities, including Paracelsus. On the other hand, he engaged in the new learning based on experimentation that was producing men like William Harvey, Galileo Galilei and Francis Bacon. Van Helmont was a careful observer of nature; his analysis of data gathered in his experiments suggests that he had a concept of the conservation of mass. He was an early experimenter in seeking to determine how plants gain mass. The Willow tree experiment Helmont's experiment on a willow tree has been considered among the earliest quantitative studies on plant nutrition and growth and as a milestone in the history of biology. The experiment was only published posthumously in Ortus Medicinae (1648) and may have been inspired by Nicholas of Cusa who wrote on the same idea in De staticis experimentis (1450). Helmont grew a willow tree and measured the amount of soil, the weight of the tree and the water he added. After five years the plant had gained about 164 lbs (74 kg). Since the amount of soil was nearly the same as it had been when he started his experiment (it lost only 57 grams), he deduced that the tree's weight gain had come entirely from water. Religious and philosophical opinions Although a faithful Catholic, he incurred the suspicion of the Church by his tract De magnetica vulnerum curatione (1621), against Jean Roberti, since he could not explain the effects of his 'miraculous cream'. The Jesuits therefore argued that Helmont used 'magic' and convinced the inquisition to scrutinize his writings. It was the lack of scientific evidence that drove Roberti to this step. His works were collected and edited by his son Franciscus Mercurius van Helmont and published by Lodewijk Elzevir in Amsterdam as Ortus medicinae, vel opera et opuscula omnia ("The Origin of Medicine, or Complete Works") in 1648. Ortus medicinae was based on, but not restricted to, the material of Dageraad ofte Nieuwe Opkomst der Geneeskunst ("Daybreak, or the New Rise of Medicine"), which was published in 1644 in Van Helmont's native Dutch. His son Frans's writings, Cabbalah Denudata (1677) and Opuscula philosophica (1690) are a mixture of theosophy, mysticism and alchemy. Over and above the archeus, he believed that there is the