At Leiden, Utrecht, Groningen, Franeker, Breda, Nimeguen, Harderwyk, Duisburg and Herborn, and at the Catholic university of Louvain, Cartesianism was warmly expounded and defended in seats of learning, of which many are now left desolate, and by adherents whose writings have for the most part long lost interest for any but the antiquary.
Christoph Wittich (1625-1687), professor at Duisburg and Leiden, is a representative of the moderate followers who professed to reconcile the doctrines of their school with the faith of Christendom and to refute the theology of Spinoza.
He became (1649) professor of philosophy and theology at Herborn, but subsequently (1651), in consequence of the jealousy of his colleagues, accepted an invitation to a similar post at Duisburg,, where he died on the 31st of January 1665.
The largest iron and steel works are at Essen, Oberhausen, Duisburg, Dusseldorf and Cologne, while cutlery and other small metallic wares are extensively made at Solingen, Remscheid and Aix-la-Chapelle.
In the immediate vicinity are also extensive beds of iron ore, and this combination of mineral wealth has enabled the town to become a competitor with Essen, Oberhausen, Duisburg and Hagen in the products of the iron industry.