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.

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.