The Miraculous Meniscus of Mercury

| By on Reading the Book of Nature


The Miraculous Meniscus of Mercury


Diagram of Boyle’s barometer, with the mercury standing at 30 inches, from New Experiments and Observations touching Cold(London, 1665). It was in this book that Boyle coined the word “barometer,” in order “to avoid Circumlocutions,” when referring to “the whole Instrument wherein a Mercurial Cylinder of 29. or 30. Inches is kept suspended after the manner of the Torricellian Experiment” (The Works of Robert Boyle, vol. 4, p. 237). I described the experiment of Evangelista Torricelli, which gave rise to the barometer, in an earlier column.

Does God act in nature? If so, should scientific explanations occasionally include divine action as part of the causal nexus accessible to scientific inquiry? Surprisingly, Boyle’s experiments with air raised just such questions. Although Boyle himself didn’t initiate those conversations, his answers are important—and highly relevant to contemporary discussions of “naturalism” in science.

Boyle’s Barometer and God’s Absolute Power

Robert Boyle didn’t invent the barometer, but he was the first person to call it that. He also experimented extensively with it, even building a portable version of the instrument that worked tolerably well and—Boyle suggested hopefully—might perhaps be modified to be “serviceable at Sea,” though he “had no opportunity to try” one under those conditiions (A Continuation of New Experiments Physico-Mechanical, in Works of Robert Boyle, vol. 6, p. 93).

As I pointed out in an earlier column, some of Boyle’s experiments led to fascinating debates about the space above the meniscus in a mercury barometer: is it really completely empty of matter? It wasn’t a purely physical question; a metaphysical dimension was also prominently present, as Steven Shapin and Simon Schaffer have shown. Even today, metaphysical issues are not entirely separate from science (despite the reluctance of some scientists to acknowledge it), but during the Scientific Revolution they were almost ubiquitous.

One of Boyle’s most vociferous opponents on this occasion was Francis Line, an English philosopher who spent much of his life in exile on the Continent, partly as professor of mathematics and Hebrew at the College of the English Jesuits at Liège. Line was born into a Catholic family, and England had strict laws against propagating Catholicism, dating back to before his birth. Thus, as his biographer says, “no doubt, Francis was baptized in secret” (Conor Reilly, Francis Line, p. 5). He went on to become a Jesuit priest, making him persona non grata throughout the English realm and placing him effectively under a death sentence, if he were discovered to be pursuing his calling in his homeland. Nevertheless, Line quietly returned to England at some point in the 1650s, not long before Boyle started exploring the properties of air.


Book cover: “New Experiments Physico-Mechanicall, Touching the Spring of the Air”

New Experiments Physico-Mechanicall, Touching The Spring of the Air (1660), was the first of Boyle’s scientific books to be published—although a devotional book about the love of God was printed the previous year, and he had already written portions of other scientific books that came out later. When air is compressed and the applied force is removed, it will “spring” back to its original volume. This phenomenon, widely used in athletic shoes today, is related to many of the experiments described in this book. The “new pneumatical engine” mentioned in the title was the air pump, built by his extraordinary assistant Robert Hooke, an Oxford student who went on to become one of the greatest scientists of his generation.

When Boyle’s famous book about the air pump came out in 1660, Line read it right away. He had done some pneumatical experiments of his own at Liège, but he did not believe in the possibility of creating a vacuum so he interpreted them differently. The following year, he challenged Boyle’s views in a Latin treatise, Tractatus de corporum inseparabilitate (1661), or “A Treatise on the Inseparable Nature of Bodies,” arguing that a genuine vacuum is impossible. The apparently empty space above the meniscus in the barometer was the subject, and the need for nature to avoid a vacuum was the problem. What was the solution? Line proposed that something he called a “funiculus,” formed from the “rarefied and extended upper surface of the mercury,” functions like a string adhering to the top of the glass tube, upholding the column of mercury in the tube and preventing a vacuum from forming (translation from Reilly, Francis Line, p. 66). In this way, Line believed he could preserve the Aristotelian principle that “Nature abhors a vacuum.”

How could this be possible? How could the mercury itself expand almost magically in this way, as if on demand in order to prevent a vacuum? According to Line, such a thing was at least possible “divinitus”—in other words, by God’s absolute power—and therefore it had to be consistent with the nature of matter. The sound of Boyle taking a deep breath is audible even now. “None is more willing to acknowledge and venerate Divine Omnipotence” than me, he replied, entirely without exaggeration. “I say, that our Controversie is not what God can do, but about what can be done by Natural Agents, not elevated above the sphere of Nature. For though God can both create and annihilate, yet Nature can do neither: and in the judgment of true Philosophers I suppose our Hypothesis would need no other advantage to make it be preferred before our Adversaries, then that in ours things are explicated by the ordinary course of Nature, whereas in the other recourse must be had to miracles” (Defence Against Linus, in The Works of Robert Boyle, vol. 3, p. 48).

“Our Hypothesis would need no other advantage …” Those are very significant words, barring the door to the direct invocation of God’s absolute power as an explanation in natural philosophy. Boyle was fully persuaded that God had worked numerous miracles, and that even now God could set aside “his ordinary and general Concourse” (as Boyle liked to put it) for some special purpose. But natural philosophy could not be built on rare acts of God that we cannot control or replicate. Invoking God in that way was nothing more than a shell game, hiding our inability to know the real cause of a phenomenon.

For example, consider the cause of the dark skin of Africans. Boyle was not impressed by those who “would have the Blackness of Negroes an effect of Noah’s Curse ratify’d by God’s, upon Cham; But though I think that even a Naturalist may without disparagement believe all the Miracles attested by the Holy Scriptures, yet in this case to flye to a Supernatural Cause, will, I fear, look like Shifting off the Difficulty, instead of Resolving it; for we enquire not the First and Universal, but the Proper, Immediate, and Physical Cause” (Experiments and Considerations Touching Colours, in The Works of Robert Boyle, vol. 4, p. 88).

Boyle objected to traditional Aristotelian explanations for similar reasons: they were a cloak for ignorance. Aristotelian forms and qualities were said to explain how the physical properties of bodies were produced. Why does snow dazzle the eyes more than grass? Here is Boyle’s opinion of the traditional Aristotelian answer:

“to say, that these and the like Effects are perform’d by the substantial Forms of the respective Bodies, is at best but to tell me, what is the Agent, not how the Effect is wrought; and seems to be but such a kind of general way of answering, as leaves the curious Enquirer as much to seek for the causes and manner of particular Things, as Men commonly are for the particular causes of the several strang Things perform’d by Witchcraft, though they be told, that tis some Divel that does them all” (Origin of Forms and Qualities, in The Works of Robert Boyle, vol. 5, p. 352).

The devil, it seems, was not to be found in the details.

Methodological Naturalism

What Boyle was driving at is one of the most contentious ideas in the contemporary controversy over origins:methodological naturalism, the idea that scientific explanations ought to refer only to “natural” causes, such as “natural” laws, mechanisms, or forces. Basically, Boyle endorsed methodological naturalism. At the same time, he completely rejected ontological naturalism, the belief that nature is all there is, denying the existence of God and any other “supernatural” entities. All Christian thinkers reject ontological naturalism—at least they ought to, if they want to be taken seriously both as Christians and as thinkers—but many Christian scientists accept methodological naturalism. Proponents of Intelligent Design typically do not accept it. In their view, it collapses all too easily into ontological naturalism. I discussed this important issue more fully in a separate column that some may want to read in conjunction with this one.

Questions about the nature of scientific knowledge, including the limits of naturalism and the implications of God’s absolute power for our ability to understand the “natural” world, are not capable of easy answers that will be obvious to all. Science itself doesn’t answer normative questions like these. They go beyond science into metaphysics—a way of thinking that contemporary scientists tend to disparage and ignore, such that they sometimes find themselves in the unenviable state of being ignorant of their own ignorance. That is why conversations like this are still relevant.

I’ll close by stepping back into history from philosophy. Historians don’t actually know very much about the history of naturalism and the (philosophical and theological) debates surrounding it. Last winter I read a short paper at an academic conference about that very topic that might result in a book about it, but I don’t expect it to be the last word. Suffice it to say that scientists have talked about these things for thousands of years, and I doubt the conversation will end any time soon.

Looking Ahead

My next column, coming in early January, explores more fully a theme I’ve already touched on in this series: Boyle’s heartfelt commitment to the design argument. After that, we’ll see how Boyle did invoke God’s absolute power in support of experimental science—not as a component of scientific explanations themselves, but to justify doing experiments in the first place. If that intrigues you, come back to see the details. In the meantime, MERRY CHRISTMAS to all—celebrate the mystery of the Incarnation, the most shocking of all miracles.

References and Suggestions for Further Reading

This material is adapted from Edward B. Davis, “Robert Boyle’s Religious Life, Attitudes, and Vocation,”Science & Christian Belief 19 (2007): 117-38. Additional information is from Conor Reilly, Francis Line S.J.: An Exiled English Scientist 1595-1675 (Institutum Historicum Societatis Jesu, 1969); Steven Shapin and Simon Schaffer, Leviathan and the Air-Pump: Hobbes, Boyle, and the Experimental Life (Princeton University Press, 1985); and Edward B. Davis and Robin Collins, “Scientific Naturalism,” in Science and Religion: A Historical Introduction, ed. Gary B. Ferngren (Johns Hopkins University Press, 2002), pp. 322-34. Several quotations are from The Works of Robert Boyle (Pickering & Chatto, 1999-2000), 14 vols., ed. Michael Hunter and Edward B. Davis.




Davis, Ted. "The Miraculous Meniscus of Mercury" N.p., 19 Dec. 2013. Web. 17 January 2019.


Davis, T. (2013, December 19). The Miraculous Meniscus of Mercury
Retrieved January 17, 2019, from /blogs/ted-davis-reading-the-book-of-nature/the-miraculous-meniscus-of-mercury

About the Author

Ted Davis

Ted Davis is Professor of the History of Science at Messiah College. A former high school science teacher, Ted studied history and philosophy of science at Indiana University, where his mentor was the late Richard S. Westfall, author of the definitive biography of Isaac Newton. With the English historian Michael Hunter, Ted edited The Works of Robert Boyle, 14 vols. (London: Pickering & Chatto, 1999-2000), but his interests include the whole 2000-year interaction of Christianity and science. Author of dozens of scholarly articles and essays, Ted is one of few historians who have written extensively about both the Scientific Revolution and modern America. He and his wife Kathy enjoy theater, music, and traveling to new places.

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