§44. Boyle’s Law
in 1643, Evangelista Torricelli reported his experiment with a column of mercury, in effect, the first barometer. He proposed the new idea that we live in a “sea of air” surrounding the earth.
As James Bryant Conant observed, “What most of us today regard as a fact, namely, that the earth is surrounded by a sea of air that exerts pressure, was in the 1640s a new conceptual scheme that still had to weather a series of experimental tests before it would be generally adopted.” Five years later, Blaise Pascal had experiments carried out to compare barometric pressure at the foot and peak of Puy-du-Dome. At the same time as these new experiments, long-lost works of Alexandrian mechanics came to light. A Latin translation of Heron appeared in 1545, describing the action of a siphon, as well as Strato's experiments with vacua. These writings stimulated experiments with pumps to produce a vacuum, rediscovering what Alexandrians called the power of the void (§21).173Concepts of “pressure” (force per area unit of surface) and equilibrium were already familiar from Archimedes's writings on hydrostatics. Having studied the work of Pascal, Otto von Guericke, and Florentine experiments on the Torricelli vacuum, Boyle stepped into this European conversation with a working hypothesis that we did indeed live in a sea of air, an elastic fluid with a “spring,” exerting pressure and propagating sound. The endeavor to establish this hypothesis experimentally was the occasion for Boyle constructing his air pump, a transparent glass chamber from which air could be extracted, leaving a novel experimental space into which various materials (canary, burning candle, ringing bell) could be introduced, manipulated, and observed.
Boyle's pumps were as unruly as Galileo’s telescopes—expensive, difficult to operate, never well-enough sealed. However, Boyle did not use the pump in the experiments that led to “Boyle’s law.” He and probably Hooke were accumulating data from experiments with a mercury-filled J-tube.
Pouring in mercury from the top of the J produced a bubble of space in the last bit of the tube’s sealed end. Pouring in more mercury never abolished it. Was it a vacuum? Boyle began these experiments in an effort to refute an Aristotelian critic named Linus. But when he noticed the regularity of changes in the weight of the mercury column and the (empty?) volume in the closed tip, he followed the new lead, unsuccessfully seeking a formula for what seemed like a correlation, but publishing the data anyway. At least three others perusing the published data independently discovered the formula, which we now know as Boyle’s law. In modern terms, the pressure exerted by a given mass of a gas in a closed system is inversely proportional to the volume it occupies if temperature is constant. The “law” is only approximately true for gasses at atmospheric pressure; near the point of condensation, pressure and volume deviate widely.174Boyle would have been reluctant to claim to have discovered a “law of nature.” He was content to have found a contingent general truth. The usual view in his day was that moral law and physical law were two species of one genus of natural law. Boyle demurred, arguing that beings with no understanding cannot “act according to laws of nature.” His Christian Virtuoso proclaims, “Inanimate bodies are utterly incapable of understanding what a law is, or what it enjoins, or when they act conformably or unconform- ably to it; and therefore the actions of inanimate bodies, which cannot incite or moderate their own actions, are produced by real power, not by laws.” Those last words are the point. To say that laws are ineffective without understanding is to say that they are not powers (except for an intelligence), and therefore explain nothing about nature. Laws of nature cannot make things happen, cannot even dictate what should happen. Aristotle was still right: to understand, we have to have the cause, and no “law” can be a cause of mindless, mechanical action.175
Boyle defines a “law of nature” as “the course of nature,” whereby things are “brought to pass by their proper and immediate causes, according to the wonted manner and series or order of their acting.” He distinguishes universal laws, holding absolutely for all bodies, such as Galileo’s law of acceleration or Newton’s (later) law of gravitation, and less general “customs of nature,” which belong not to all bodies but “to this or that particular sorts of bodies,” of which the gas law would be an example.
Like the nominalist he unknowingly is, he denies that laws are immanent in nature or in matter. Indeed no causation exists in nature, causation being something our concepts impose on observed regularities. A law of nature is a conceptualized similarity in observed phenomena, an old nominalism newly made normative for modern empiricism.176Boyle is a mechanist, but despite his admiration for the mechanical clock at Strasburg, he did not imagine nature was one great machine. It is not automated, and the laws are not inherent in matter, but rather “depend upon the will of the divine author of things” For Newton, matter has powers, and the laws of nature are their manifestation. For Boyle, natural law does not lie in matter or its qualities; it comes from a will, as all law does, and its function in nature is to maintain the created order. “The nature of everything” is “only the law that it receives from the creator, and according to which it acts on all occasions.” If God were to cease imposing his will on the motion of corpuscles, corpuscles would exist and collide but nature would not exist, no beautiful cosmos expressive of intelligent purpose.177