What is Van Helmont's experiment?

Measurement and rationale for the growth of a willow tree is Van Helmont's experiment.

Specifically, Jan Baptist van Helmont (1580-1644) was a Brussels-born scientist. He was an astute observer of objects and processes. One of his famous experiments was centered on the five-year study of the growth of a willow tree (Salixspp). His conclusions of a 164-pound weight gain due to the intake of water through the roots and soil were prefatory to understandings of the conservation of mass even though he ignored the role of atmospheric carbon dioxide.