Comment on Douglas Allchin’s “How Not to Teach History in Science”, David Hershey

David R. Hershey, Ph.D.

Hyattsville, MD


Allchin talked about the importance of proper historical context, and made various criticisms of my article on Helmont’s willow tree experiment (1991). My comments are:

1) Helmont’s conception of distilled water.

Helmont said he used distilled water in his willow experiment (Krikorian and Steward 1968), and distillation as a purification method was well known in Helmont’s era (Multhauf 1956). Helmont was aware that when he distilled most sources of water, he was left with solids in his still. Therefore, he knew that he needed to use distilled water or rain water otherwise his soil would have gained weight from the dissolved solids in the irrigation water. Determining whether the dry weight of the soil changed after a willow grew in it for five years was the crux of his experiment. Helmont also went to the trouble of covering his pot with a metal lid to prevent dust from adding to the weight of his soil.  Allchin also misattributed the statement “willows do not live by water alone” to me.

2) Boyle’s criticism of Helmont’s experiment.

Allchin’s assertion that “Helmont’s experiment was well designed and interpreted appropriately in the context of its own time” does not take into account early criticisms of the experiment. In 1666, Robert Boyle rejected Helmont’s conclusion that plant matter was formed from water only because it contradicted agricultural practices known since ancient times. Boyle wrote “And indeed experience shews us, that several plants, that thrive not well without rain water, are not yet nourish’d by it alone, since when corn in the field, and fruit-trees in orchards have consum’d the saline and sulphureous juices of the earth, they will not prosper there, how much rain soever falls upon the land, till the ground by dung or otherwise be supply’d again with such assimilable juices” (Hunter and Davis, 1999).

Allchin noted that “Helmont was also probably well aware that plants do not grow outside soil.” However, even in Helmont’s era floating aquatic plants, such as duckweed, were described in herbals (Gerard 1633) and grew in Europe. Helmont was a physician who studied herbals and the medicinal uses of plants. It is hard to believe Helmont was not aware of aquatic plants. Helmont also failed to acknowledge that Francis Bacon (1627) grew terrestrial plants in water.

3) “Meaning variance” of Carbon Dioxide.

Allchin wrote: “Carbon dioxide [was] a substance wholly outside his conception.” However, Helmont coined the term “gas”, discovered carbon dioxide and was the “real founder of pneumatic chemistry” (Leicester and Klickstein 1963). He also described several sources of carbon dioxide including belches, fermenting wine and burning of oak charcoal (Pagel 1972).

4) Hero and Fool

Allchin stated that it was “historically outlandish” to portray Helmont as both “hero and fool.” However, in his era, Helmont was regarded exactly that way because his “combination of mysticism, magic, alchemy, and new science irritated even his contemporaries” (Heinecke 1995). Even Robert Boyle had that hero-fool view because Boyle thought a mysticism-heavy treatise written by Helmont was misattributed to Helmont by his detractors (Heinecke 1995). Boyle couldn’t comprehend how Helmont, who made many important scientific discoveries, could also produce such unscientific nonsense. Helmont might also be considered a “fool” because he determined experimentally that oak charcoal consisted mainly of carbon dioxide gas but failed to use his own data to even consider that plants gained weight from air rather than water.

5) Teaching Purpose.

I chose to deal with Helmont’s very well known and straightforward willow experiment and objectively scrutinized it from the standpoint of experimental design and analysis. Focusing on his largely unscientific ideas on gases would have interfered with the focus on experimental design. My purpose was not to teach biography or Helmont’s mystical beliefs, but to use a familiar historical experiment as a basis for a discussion of modern experimental design. Helmont’s experiment is not just part of the history of science, but it is part of the scientific literature. Therefore, it can be used in a modern consideration of experimental design and analysis. My audience was teachers of high school or college introductory biology courses, not teachers of the history of science courses like Allchin.



Allchin, D. 2002. How Not to Teach History in Science. The Pantaneto Forum. 7: July 2002

Bacon, F. 1627. Sylva Sylvarum. London: J. Haviland.

Gerard, J. 1633. The Herbal or General History of Plants. New York: Dover.

Heinecke, B. 1995. The mysticism and science of Johann Baptista van Helmont (1579-1644). Ambix 42(2):65-78.

Hershey, D.R. 1991. Digging deeper into Helmont’s famous willow tree experiment. American Biology Teacher 53:458-460.

Hunter, M. and Davis, E.B. 1999. The Works of Robert Boyle. London: Pickering and Chatto.

Krikorian, A.D. and Steward, F.C. 1968. Water and solutes in plant nutrition: With special reference to van Helmont and Nicolaus of Cusa. BioScience 18:286-292.

Leicester, H.M. and Klickstein, H.S. 1963. A Sourcebook in Chemistry 1400-1900. Cambridge, MA: Harvard University Press.

Multhauf, R. 1956. The significance of distillation in Renaissance medical chemistry. Bulletin of the History of Medicine 30:329-346.

Pagel, W. 1972. Helmont, Johannes (Joan) Baptista Van Helmont. pp. 253-259, vol. 6. IN: C.C. Gillespie (ed.) Dictionary of Scientific Biography. New York: Scribner.

A version of this comment appeared in the Journal of College Science Teaching 31 (May, 2002) 6-7, a journal of the National Science Teachers Association.