Johann Baptista van Helmont (1577-1644) was a Belgian chemist and physician. He was born in Brussels, obtained his medical degree from the University of Louvain, and returned to Brussels to practice medicine. He moved later to Vilvorde and stayed there until his death. His work on chemical research made him infamous to the Inquisition and he was imprisoned for two years (Rook 1964). He started the inquiry which ultimately leads to the abandonment of the Humus Theory and to the understanding of the physiological process now called photosynthesis.
He applied exact quantitative methods in physiological research and may be considered as a pioneer in physiological chemistry. He introduced the word gas although he used it differently from the modern concept of gas, that is, a completely elastic fluid not liquid or solid at ordinary temperatures. He observed “fumes” but only described them as a stage in the conversion of water into substance and again to water (Rook 1964).
Before the advent of the term ‘carbon dioxide’, he called it ‘gas sylvestre’ and observed that it extinguishes the flame of a candle. For his report on gas sylvestre, Redgrove and Redgrove (1922) insisted that he should be credited for discovering carbon dioxide which was then also called carbonic anhydride and carbonic acid gas, but which credit went to Joseph Black.
His most important work, Ortus medicinae (or The Dawn of Medicine, published 1648 in Amsterdam; Donaldson 2009)), was published by his son after his death. The experiments were conducted at a time when the existing theory was that all things are composed of four “elements”: fire, air, water, and earth which originated from the early Greeks who first produced organized literature of science in Europe (Rook 1964). According to Windelspecht (2002), however, van Helmont’s experiment with the willow tree replicated that of Nicolas of Custa (1401-1464).
Van Helmont first prepared a clay pot with 200 pounds (lbs) or about 90.72 kg of soil that had been thoroughly dried by heat in a furnace. He moistened the soil with rainwater and inserted a stem of a willow tree that weighed 5 lbs (≈2.27 kg). Soil moisture was maintained by regular watering using rainwater or distilled water. To prevent the addition of dust to the soil, the mouth or open top of the pot was covered with an iron plate with perforations that allow the passage of water. After 5 years, he found that the tree (excluding the leaves that fell) weighed about 169 lbs and 3 ounces (≈169.19 lb or 76.74 kg) or an increment of 164.19 lbs (≈74.39 kg) while the soil, again dried, decreased in weight by a mere two ounces (≈56.7 grams or .0567 kg). He concluded that wood, bark, and roots arose out of the water (Rook 1964).
The result of this experiment convinced Van Helmont that the increase in the weight of the willow tree must be due to the added weight of water and, therefore, it is water that is the prime component of the plant body. His conclusions are found in the following lines in John Chandler’s 1662 translation of Ortus medicinae (Rook 1964):
“Therefore one hundred and sixty-four pounds of Wood, Barks, and Roots arose out of water onely. . . . Fire indeed destroyeth simple but it generates nothing. . . . Therefore Wood, since it is wholly of water, the ashes. . .shall be of water.”
Later works by various researchers would prove him wrong; that it was carbon dioxide fixed in the process of photosynthesis that was primarily responsible for the increase in size and weight of the tree.
John Woodward (1665-1728). An English scientist, he published in 1699 his hydroponics (soilless or water culture) experiments with mint grown in the water of different purity such as rainwater, river water, drainage water, and others. He found that most of the water that the plant absorbed was expelled through the pores and released into the atmosphere. He concluded that more than water is needed by plants and that soil is responsible for the increase in the weight of plants (Devlin 1975). While this seems to favor further the humus theory, on one hand, it eliminated the conclusion made by Van Helmont that it was water that contributed to the growth increment in plants.
Note: The lists of contributors and Literature Cited are on the History of Photosynthesis main page.