The Dutch physician, chemist, and engineer Jan Ingenhousz (1730-1799) is noted for his demonstration of the process of photosynthesis in plants.

Jan Ingenhousz was born on Dec. 8, 1730, in Breda. He studied medicine at the University of Louvain 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 2 years Joseph Priestley's discovery of the principles of what is now called photosynthesis, that is, the process by which plants exude oxygen and absorb carbon dioxide, thus purifying the air for animals and man.

Unlike Priestley and other chemists who were working on the characteristics of oxygen from the point of view of chemical philosophy, Ingenhousz was preoccupied by the problem of the fundamental balance in the animal and vegetable kingdoms, and this led him to investigate the mutual interdependence of plants and animals. He introduced the concept that the leaves of plants are great laboratories for cleansing and purifying the air. He also noted that oxygen is emitted by the underside of the leaves and that this is a daylight process, whereas in darkness even plants emit small quantities of carbon dioxide instead of absorbing it. 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 of that kind of research which in modern times 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.

Ingenhousz built electrical machines and invented the plate electric machine. He also wrote a two-volume treatise dealing with problems in medicine which are relevant to the physicist and the medical man, and in a sense it could be said that his basic interest was in what is now called biophysics. 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 retarded 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 its Author. Ingenhousz died in Wiltshire, England, on Sept. 7, 1799.

Further Reading

• Howard S. Reed, Jan Ingenhousz: Plant Physiologist, with a History of the Discovery of Photosynthesis (1949), contains a study of Ingenhousz as well as the text of his famous book.