Rohault was not the only Cartesian natural philosopher interested into the mysteries of glass drops. Others have started to examine the object and to produce their own explanations. In this post, I shall refer to the cases of Henricus Regius and Nicolas Poisson.
Regius (1598-1679) was a former friend of Descartes, who eventually felt into disgrace in the 1640s as a result of the disputes stirred at the University of Utrecht (see Verbeek, Theo. 1988. La Querelle d’Utrecht, Paris: Les impressions nouvelles). Descartes’s accusations from the preface letter to the French edition of the Principia philosophiae (1647) are directed to Regius’s new treatise Fundamenta Physices (1646). The accusations and the dispute between the two are beyond the scope of this post. However, it is important to notice the context in which Regius has developed his natural philosophy. In subsequent editions of the Fundamenta, he explored a larger area of the natural realm, introducing a discussion of glass drops in the final part of the Philosophia naturalis (1661, the third edition of the Fundamenta, but not in the second edition from 1654). This part is an eclectic collection of phenomena, curiosities, and experiments, which is in many ways reminiscent of Francis Bacon’s Sylva sylvarum. And, quite surprisingly, Regius adds his own explanation regarding this new object.
It is evident from the very beginning that he is interested in uncovering the cause and it is not very clear whether he performed any experiments with glass drops. Unlike other explanations, Regius is not very concerned with the structure of the drop in its larger part (the area marked with 1-2 in his illustration), but rather with the structure of the tail. He refers to the structure of the object, which he seeks to explain in terms of glass particles and pores. Even if Regius’s explanation is not very elaborate, it still gives the general outlines found in other popular descriptions of the phenomenon.
Almost a decade later, Nicolas-Joseph Poisson (1637-17010) has published his Commentaire ou remarques sur la méthode de René Descartes (1670). Poisson attempts to defend Descartes’s views on method and he does this by taking each of the four rules expressed in the Discours de la méthode, which are correlated with new experiments and discoveries. In his discussion of the second rule – “La seconde Regle de la Logique de M. Desc. est de diviser ou faire une espece d’anatomie de la difficulté qu’il se propose d’examiner il la regarde d’abord en general, puis distinguant chaque partie, il les demesle les unes d’avec les autres pour les contempler chacune en particulier, & en connoistre la nature & les proprietez.” (see the 1987 reprint of Poisson’s Commentaire, p. 54) – Poisson introduces the explanation of glass drops. Relying mainly on other testimonies, he is concerned with the explanation of this phenomenon, which he eventually achieves. However, what is peculiar to his explanation is that he begins with a list of what is important in his search. Thus, he enumerates: (1) the material condition (only air and glass are contained in the object); (2) the position of glass; (3) cause of the structure (action of fire); (4) how glass interact; (5) the relation between hot and cold with respect to glass; (6) the existence of a so-called ‘motion of liberty’ of the air that is trapped inside the drops (Poisson refers here to the mechanical tradition); (7) the “elastic” virtue of the air (with reference to Boyle); (8) the heavy force of the trapped air, which he claims will burst with great noise, as soon as it is freed. This list is quite strange, because his explanation can easily be reduced to all these eight points. But this can be due to the interplay between a resolutive and constructive procedure, which is traditionally ascribed to Cartesian method.
These two cases of Cartesian philosophers dealing with a new object puzzling early modern philosophers reveals another layer of the relation between theory and experimentation. Both Regius and Poisson seems to rely on observations and reports produces by others. Experiment is not neglected, yet it is not central, because what both of them seek is to produce an explanation, which ultimately rests on the mechanical structure of invisible parts of matter. In the next post, we shall see how a similar explanation emerges in the case of an experimental philosopher, Robert Hooke.