A Cartesian challenge to the early modern philosophy of experiment

Much has been written about seventeenth-century experiments and experimental philosophy. My paper for the CELFIS seminar of October 8 aimed at engaging with that tradition. In particular, I was concerned with the recent discussion by Peter Anstey of the so called BBH model of the experimental philosophy (BBH stands for the name of Bacon, Boyle, and Hooke). As a reaction to Thomas Kuhn and Peter Dear, Peter Anstey’s article provides a very nice introduction into the Baconian experimentation and its main developments in the second half of the seventeenth century. Both Boyle and Hooke engage with a form of experimentation that is labelled here “Baconian.” It is not, however, the purpose of this small blog post to engage with the details of Anstey’s article, but rather to try to complement his analysis with a new example of experimentalism that can be found in a completely different source. This is the case of the experimental work of the Cartesian natural philosopher, Jacques Rohault.

In my lecture, I’ve referred to two experiments that were performed by Rohault: with pneumatic devices, on the one hand, and with glass drops, on the other hand. It is well known that Boyle was the main contributor to the pneumatic or baroscopic experiments of the 1660s. Hooke was among the first to examine glass drops and to provide an explanation for both the production of the small glass objects and for the curious phenomena produced by those. Interestingly, Rohault deals with both of these issues in experimental terms.

Now, one might very well wonder why is important that a Cartesian philosopher was providing an explanation for some intriguing experiments; after all, he is a Cartesian, therefore a speculative philosopher (see the Otago blog here and here), and he would explain all phenomena according to the principles of Cartesian physics. Yet, this classification of seventeenth-century philosophers into “experimental” and “speculative” should not be an impediment in searching for explanations in one’s writings. But there is more than that and I argued in my paper that it is precisely Rohault’s experimental approach to the study of the two phenomena that would make difficult to draw a clear boundary between his work and the works of the most representative experimenters of the BBH model.

I have argued elsewhere that Rohault treats the study of the properties of the air in experimental terms. He does not simply jump from the conclusions derived in the general part of Cartesian physics (which is most often claimed that he does), but actively engage in experiments and observations.

With respect to the study of glass drops, Rohault is also concerned to perform all the needed observations before providing an explanation. This is also what Hooke did in his Micrographia.

As a tentative conclusion for this very sketchy blog-post, I claim that based on these two experiments, Rohault should be placed in the same context with Boyle and Hooke, so as a representative of the BBH model. If, on the contrary, one would like to point to his “Cartesianism,” then, one simply overlooks his experiments and this would raise new worries for the use of historical categories: if one dismisses some experimental practices only on the basis of placing the practitioner to one or another camp, then, the problem is not any more with the use of experiment in natural philosophy, but with the way various natural philosophies of the period were classified in our histories.

IV. Seventeenth-Century Experiments with Glass Drops: Robert Hooke on glass drops

In the previous posts, I have introduced the problem of glass drops in the seventeenth-century natural philosophy, which I have further discussed in the case of Jacques Rohault’s Cartesian experimentalism and other Cartesian explanations provided by Henricus Regius and Nicolas-Joseph Poisson.

In the final post on this topic, I would like to refer to the explanation provided by an experimental philosopher par excellence, Robert Hooke (1635-1703).

Some of the earliest investigations of the drops took place in the Royal Society (see Brodsley, Laurel, Charles Frank, and John Steeds. 1986. “Prince Rupert’s Drops.” Notes and Records of the Royal Society of London 41 (1) (October): 1–26). Hooke was one of its most distinguished members and witnessed these inquiries from the very beginning (see Birch, Thomas. 1756. History of the Royal Society. Vol. I. London: A. Millar). After an initial report signed by Robert Moray in 1661, Hooke published his own observations in the celebrated Micrographia of 1665. He applies jointly direct observation, hypothetical thinking, and experiment in order to formulate an explanation “Of some Phenomena of Glass drops,” as it is said in the title of his seventh observation. As expected, glass drops and fragments of drops are examined through the microscope. Yet, Hooke notes the difficulties of his inquiry: “I could not find, either with my naked Eye, or a Microscope, that any of the broken pieces were of a regular figure, nor any one like another, but for the most part those that flaw’d off in large pieces were prettily branched” (p. 33). Trying to find the imperfection of glass, sets Hooke on a quest of successive trials, such as grinding the object in different parts and observe if there are new effects or immersing glass drops in various substances (e.g., Icthyocolla). Like in the prior cases of Rohault and Regius, Hooke complements his text with an illustration (see Fig. X at the bottom of this image):

Robert Hooke, Micrographia (1665).

The object is not only investigated empirically with various instruments, but the ‘conjectures’ formed in this process are further examined through analogy and transduction, which ports theoretical elements into Hooke’s analysis. Moreover, descending into the invisible structures of matter, forces him to leave the eye and the microscope and frame his explanation on top of other prior considerations about heat, fluidity, and matter in general. He concludes by ascribing the cause to the arrangement of particles of glass, which are gathered into a springy tension inside the drop.

It is not the place in this blog-post to go into the details of Hooke’s explanation of the phenomena produced by glass drops, but it is worth noticing that his experimental methodology overlaps with what I have presented in the post dedicated to Rohault. Theory and experimentation work together in finding the explanation, which, ultimately reduces to a mechanical model of the accepted theory of matter. These four cases of seventeenth-century natural philosophers interested in the study of glass drops make wonderful examples of how “new scientific objects” are discussed and explored with new methodological tools. I shall investigate this problem into a forthcoming article.

III. Seventeenth-Century Experiments with Glass Drops: Henricus Regius and Nicolas Poisson on glass drops

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.

Henricus Regius, Philosophia naturalis (1661), p. 516.

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.


II. Seventeenth-Century Experiments with Glass Drops: Jacques Rohault and his Cartesian experimentalism

In the previous post, I’ve only introduced glass drops as objects of philosophical study in the early modern Europe. I argued that they have become popular in the late 1650s and early 1660s, at the same time with the spread of a more extensive experimental attitude in the investigation of nature. In this post, I shall give a brief account of Jacques Rohault’s empirical investigation of these objects.

Rohault (1618-1672) was one of the leading Cartesian philosophers of his time. He hosted a famous salon in Paris, where he jointly discussed problems of natural philosophy in connection with experiments. Moreover, his book on physics – the Traité de physique (1671) – was quickly adopted as a textbook on natural philosophy in various universities. As I argue in a forthcoming article, much of Rohault’s experimental work has been done in the late 1650s and early 1660s (see Dobre, Mihnea. “Rohault’s Cartesian Physics.” In Cartesian Empiricisms, eds. Dobre, Mihnea and Nyden, Tammy. Springer Studies in History and Philosophy of Science. Springer). Among his experiments, Rohault performed some with “larmes de verre” (see Christiaan Huygens’s report from March 17, 1660: “Rohaut [sic], qui fit l’experience d’une larme de verre” in Huygens, Oeuvres XXII, p. 562).


Jacques Rohault. Traité de physique (1671), p. 173.

For Rohault, glass drops (“larmes de verre”) are wonders of nature, which means that they challenge natural philosophy. Stating the extraordinary character of the object from the very beginning allows him to test the explanatory power of his own natural philosophy. This comes with both a theoretical and experimental dimension. First, Rohault draws the boundaries of his explanation in terms of what is theoretically acceptable; in other words, a general description of how motion works. Then, Rohault performs several observations, showing: (1) the quick cooling of glass; (2) that glass-surface becomes cooler, which he already explains by the fact that it closes its pores; (3) matter inside is still agitated producing both glass powder and larger pores; and finally, (4) the question: why is not breaking?

All sorts of experimental trials accompany the first three steps of this investigation. Rohault applies various operations to the larme de verre (e.g., attempts to break the drop in other points than D or to immerse the object in different substances and observe changes in the properties of the glass), which can be considered variations in the experimental procedure. He concludes his empirical investigations by answering the fourth point from above. Glass drops are difficult to break on point D, because of the arrangement of matter: the inner part of the drop is formed by a mixture of air and glass – with some sort of arch-structures – while the external part is very dense. This is easily explained by the process of fabrication: when melted glass is left to plunge into a flask of cold water, it quickly cools, such that glass particles that are in contact with water will solidify immediately, while the inner parts will continue their motion.

Using jointly the principles of Cartesian natural philosophy and an experimental method, Rohault is forced at this point to delve into the hidden structure of bodies. He appeals to Cartesian theory of matter (body is identical with matter and space; void space is not possible) and mechanical modeling of interacting bodies. Yet, this is done in a similar manner with his contemporaries, the so-called experimental philosophers.

In the next post, I shall discuss the cases of Henricus Regius and Nicolas Poisson. That will conclude my overview of Cartesian natural philosophers dealing with the problem of glass drops.

I. Seventeenth-Century Experiments with Glass Drops: an introduction

One of the new ‘scientific’ objects that captured the attention of the seventeenth-century French and British experimentalists is what they call a ‘glass drop’: the result of dropping a bit of incandescent glass into a bucket of cold water. Peculiar to this object was its solidity, which was contrasted with the very minute fragmentation of glass that was produced when the drop was finally destroyed.

A representation of the glass drop can be observed in the following illustration from Jacques Rohault’s Traité de physique (1671).

Jacques Rohault. Traité de physique (1671), p. 173.

This is the first post of a series discussing some seventeenth-century attempts to explain the properties of glass drops. Considered as one of the most intriguing objects, glass drop became an interesting item of study for many natural philosophers, including Robert Boyle, Jacques Rohault, Henricus Regius or Robert Hooke. As its name suggests, the object was made of glass and its peculiar shape intrigued as much as its apparently contradictory properties. On the one hand, the glass drop was found very difficult to break when it was pressed on the large size of the drop. On the other hand, its tail was very easy to fracture, which produced the blow of the drop with a loud noise.

A modern replica of the phenomena – starting from the production process of the drop and continuing with an exemplification of the two properties described above – is possible to watch online on the website of the Corning Museum of Glass (see http://www.cmog.org/video/prince-ruperts-drop): http://youtu.be/6V2eCFsDkK0.

The glass drop became philosophically interesting only in the seventeenth century. References to its structure and properties appeared almost at the same time in several places of Europe. The drop had also different names: larme de verre, lacryma Batavica, chymical glass, Prince Rupert’s drop, vitrae lacrymae, globuli vitrei, Batavian tears, etc. Its origins are still controversial, but it is commonly held that it came from either German lands or the Dutch provinces. I shall not try in these blog-posts to discuss the origins of this object or to attempt to correct some misconceptions about who has the priority in the philosophical investigation of the drop (an overview of these problems is in Brodsley, Laurel, Charles Frank, and John Steeds. 1986. “Prince Rupert’s Drops.” Notes and Records of the Royal Society of London 41 (1) (October): 1–26.). Rather, by looking at some of the early discussions and explanations of the phenomena produced by glass drops, I would like to suggest that early modern authors were forced to tackle the problem in a manner that included both theoretical and experimental examinations. This makes the glass drop case important for any discussion about the sources of knowledge in early modern philosophy, complicating the traditional divide between Rationalists and Empiricists and shading a new light on other more recent attempts to use actor-category terms, such as experimental and speculative (see the research project on Early Modern Experimental Philosophy at the University of Otago: https://blogs.otago.ac.nz/emxphi/the-project/).

In the next post, I shall discuss Jacques Rohault’s explanation of the phenomena.