Sylvius was a supporter of the Iatrochemical School. At Leiden he established the first university chemical laboratory in Europe. His extensive treatise on the diseases of children was first published as volume 4 of this set as De Morbis infantum et aliis quibusdam memoratu dignis affectibus. Editus cura Justi Schraderi (1674). In that work Sylvius expressed his ideas about gastro-intestinal acidity as the cause of infantile disease.