This fourth volume specifies more in particular the french papers which came to constitute the core of the monograph L?Histoire du concept de ł molču-le ? (2001). There are also more philosophical texts on the consequence of molecularism for the classical debate on determinism and free will, on the one hand, and on the essential role of Europe in its development, on the other. Great teachers (Ren ̌Taton and Reijer Hooykaas) are honored, as is the souvenir of a colleague (Hľn̈e Metzger). The pictural arts play an interesting role in the scientific contacts between France and the Netherlands, both in history (Escher; Rembrandt and Caravaggio) and in real time (Helmantel). This volume also celebrates, at the invitation of the Int. Union of Crystallography, the centenary of the discovery of the diffraction of Rn̲tgen rays by crystals, in 1912 by Laue et al. Microscopy has a place of its own; physics and the other sciences are omnipresent. In the ?Preface? the Brexit-problems are discussed in a broad context, while some recent bizarre developments at Groningen University Library are highlighted. The Reader will also find a sketch of Molecular Ecology as one of the trend-setting branches of contemporary science, where, indeed, molecularism calls the tune.