It was he who first used the word "gas"-- a word coined by him, along with many others that soon fell into disuse.
The principles of the Iatrochemical school were the use of chemical medicines, and a theory of pathology different from the prevailing "humoral" pathology.

The founder of this school was Sylvius (Franz de le Boe, 1614-1672), professor of medicine at Leyden.

He attempted to establish a permanent system of medicine based on the newly discovered theory of the circulation and the new chemistry, but his name is remembered by medical men because of the fissure in the brain (fissure of Sylvius) that bears it.

He laid great stress on the cause of fevers and other diseases as originating in the disturbances of the process of fermentation in the stomach.

The doctrines of Sylvius spread widely over the continent, but were not generally accepted in England until modified by Thomas Willis (1622-1675), whose name, like that of Sylvius, is perpetuated by a structure in the brain named after him, the circle of Willis.