Jan Ingenhousz

Jan Ingenhousz (1730-1799) was a Dutch physician, chemist and engineer. Ingenhousz is most notable for his work on plants; he authored one of the earliest and most comprehensive demonstrations of the process of photosynthesis. Ingenhousz also discovered that plants, like animals, have cellular respiration. He also served as personal physician to Empress Maria Theresa of Austria, whom he successfully inoculated against smallpox.

Early Life and Work

Jan Ingenhousz was born on December 8, 1730, in Breda, Netherlands. He studied medicine at the University of Louvain in Belgium (also spelled Leuven) and graduated in 1752. After spending some years in several European capitals in the typical 18th-century tradition, he settled in London in 1779 and worked with the celebrated naturalist John Hunter.

In that year Ingenhousz published his important book, Experiments upon Vegetables - Discovering Their Great Power of Purifying the Common Air in the Sunshine and of Injuring It in the Shade and at Night. In this work, he anticipated by two years Joseph Priestley's discovery of the principles of what is now called photosynthesis, the process by which plants exude oxygen and absorb carbon dioxide, thus purifying the air for animals to breathe.


Achievements in Natural Science

Unlike Joseph Priestley and other chemists who were working on the characteristics of oxygen from a chemistry perspective, Ingenhousz addressed the question of fundamental balance in the animal and plant kingdoms. This led him to investigate the mutual interdependence of plants and animals. He introduced the concept that the leaves of plants function in part to cleanse and purify the air. He noted that oxygen emission is a daylight process performed by the underside of leaves, whereas in darkness plants emit small quantities of carbon dioxide, rather than absorbing it as they do during the day.

In his reflections at the end of the book, Ingenhousz said:

"If these conjectures were well grounded, it would throw a great deal of new light upon the arrangement of the different parts of the globe and the harmony between all its parts would become more conspicuous."

The book was soon translated into many languages and became the foundation for that kind of research which, in modern times, has led to a more basic understanding of the process of photosynthesis; however, his search for the concept of economy or balance in nature was not well understood by his contemporaries. As to the nature and origin of the oxygen which the plant emits, a controversy developed in the 1780s between Ingenhousz and Priestley. Ingenhousz thought that water which plants absorb changes into vegetation and that part of this water is then released as oxygen.


Other Work

Ingenhousz built electrical machines and invented the plate electric machine. He also wrote a two-volume treatise dealing with problems in medicine. Ingenhousz also opposed the theory of subtle electrical fluids and repeated some of the experiments on plant electricity to disprove the accepted view that positive electricity was good for the growth of plants and that negative electricity slowed it.

All of Ingenhousz's scientific work was motivated by a deeply religious attitude and the belief that balance in nature is the best expression of the harmony created by God. Ingenhousz died in Wiltshire, England, on September 7, 1799.

Further Reading on Jan Ingenhousz

Jan Ingenhousz was a pivotal figure in the history of science, particularly biology and botany. Many books on those subjects feature analysis of his work, including the following:
  • Howard S. Reed, Jan Ingenhousz: Plant Physiologist, with a History of the Discovery of Photosynthesis (1949).
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