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X-Ray Fluorescence

Casey Mallinckrodt at the Penn Museum, working with the XRF Analyzer– possibly the same device that will be pointed at an instance of the Motley Emblem.

The curators at the University of Pennsylvania are close to agreeing to spray a single copy of the motley emblem with some high-energy x-rays.  We’re hoping the page will give up a few secrets about the chemical composition of its pigments.  Just knowing the chemical composition won’t be enough to determine which pigments were used; two of the emblem’s colors are almost certainly botanicals, and organic compounds are difficult to tell apart just by knowing their atomic composition.  But getting this work done will go a long way towards eliminating many possibilities, and give some clues to isolate the ones that remain.

As I’ve been embarking on this part of this project, I have wondered what a contemporary chemist would have made of all this.  Laurence Sterne’s marbled page began as an extended exercise in practical chemistry, as British bookbinders tried to get up to speed with Continental skill, and doing the work of recreating it has meant relearning that chemistry.  But what we’re proposing to do, by putting one under an x-ray emitter: that’s a chemistry of a different sort, what Thomas Kuhn would call “incommensurable.”

William Morgan, by Thomas Lawrence (1818), Staples Hall, Institute of Actuaries. It is tempting to think that Morgan’s ashen skin—especially of his hands—is due to overexposure to x-ray radiation, but is more likely the result of a different chemical effect: the fading of the translucent lake pigments Lawrence was known to employ for flesh-tone effects.

When William Morgan, a Welsh physician who would later emerge as a pioneer in the actuarial sciences, reported in 1785 on the glow emitted when an electrical current was made to pass through a rarefied gas, he saw a sign of what he called “the conducting power of air.”  And when the glow ceased, when he had perfectly evacuated the tube, he concluded that the electrical fluid simply ceased to flow.  Had someone from our moment been there, armed with the knowledge available even in a high-school physics textbook, they might have seen it differently.  They probably would have seen signs that there was still an electrical current.  And they might even have been open to the idea that some sort of invisible radiation was being emitted.  One such modern witness was Sir Richard Gregory, writing 150 years after Morgan’s experiment.  Morgan “did not know it,” writes Gregory, but Morgan was filling the room with x-rays, “and his simple apparatus represented the first X-ray tube.”  It is just that Morgan’s metaphysical models didn’t admit the explanation of an absence as a different sort of energetic presence.

There can be a danger of being like that woman at a performance of Othello, who (Jonathan Lamb points out) cries out from the audience that Othello is a fool for not believing Desdemona innocent.  Someone like Gregory can wonder how Morgan repeatedly doesn’t see x-rays for x-rays– a site of non-seeing repeated (he notes) when Humphry Davies and then Michael Faraday differently confirmed his experiments.  But chemistry in England, in the moment of the motley emblem, was engaged in a different set of tasks.  It was mostly of the pragmatic sort.  England’s best chemists were manipulating materials that can be seen and felt, in order to produce other effects, also felt and seen.  The talented experimentalist William Lewis, for instance, confined his metaphysical mood to a couple of pages in the preface to his Commercium Philosophico-Technicum (London: 1763).  Though he firmly believed that chemical action was owing to “bodies considered as… insensible and dissimilar parts,” those interactions receive little discussion in his treatise.  His goal was to develop a practical course of chemistry of use in British manufactories, and while, from a theoretical perspective, it helps to commit to a plausible metaphysical theory of the stuff you’re working with, from a practical one, it was more important to develop a repertoire of procedures that might prompt chemical effects in certain situations—processes like heating or dissolving in solution.  Chemical reactions are, he concluded, “not investigable from any principles,” at least, any principles he knew.  They can be “discoverable by observation only,” and so an active program of practical research seemed preferable to theoretical or metaphysical explorations.  What was important, he might have said, was intervening; even if he had had the technical expertise and equipment to check for x-rays, representing by models would have been less of a concern.

The primary photons from the X-ray tube have high enough energy that they can knock electrons out of the innermost orbitals, creating a vacancy (1). An electron from an outer orbital will move into each newly vacant space at the inner orbital to regain stability within the atom (2).

Chemistry would have to wait for roughly a century for the metaphysical model involved with x-ray fluorescence.  But it’s worth saying that that working model, the Rutherford or planetary model of atomic physics, is also a metaphysical assumption not strictly favored by current theory, I mean as an accurate picture of things as they are.  In virtually every handbook or instruction manual on x-ray fluorescence, you will see an image of x-rays irradiating a solar-system-like atom.  Working the device means buying into the assumption that every atom has a dense atomic core orbited at some distance by negatively charged electrons—much as planets orbit the Sun.  The device registers the change when an atom’s electrons are punched out of low-level orbits by high-energy x-rays, and each atom promotes a replacement from a more concentrically distant position.  As the electron falls, it sheds a discrete amount of energy as it goes, emitting a compensatory x-ray in a process called “secondary fluorescence.”  That is what the XRF device reads; because each element, by this model, has its own arrangement of electrons, the precise amount of energy released in the reshuffling is a signal as unique as a monogram or heraldric crest.  It doesn’t matter that we should really be thinking about an atom as a set of probabilistic distributions, or an electron as a smear of electrical charge.  The Rutherford metaphysics of atomic structure gives rise to the technical apparatus of the XRF gun, and the interpretive programme that makes sense of its results.  We need that model to put xrays to work, and interpret what they do after their done, even if the model isn’t strictly accurate, from a metaphysical point of view.

But all this is by way of reflecting on the difficulty of knowing what Robert Dossie or William Lewis would have made of our efforts to put the device to work.  When we put the motley emblem under an x-ray fluorescence device, two radically different chemical paradigms will be made to meet.  On the one hand, there is the practical chemistry of Dossie and Lewis, not to mention the technical expertise of the apothecaries and colourmen who mixed the pigments of the page, which produced the page in the first place.  That expertise is what I am attempting to gather in my (comparatively modest) alchemical laboratory.  But, on the other, there are the different, incommensurable, and also essentially outdated theories of Ernest Rutherford and Wilhelm Röntgen, that will burst into brightness when we fire up the XRF apparatus.  We are not attempting to arrive at the Enlightenment from a position of greater enlightenment.  Rather, we are trying to arrive at their moment, with the assumptions that made their accomplishments possible, with the tools we have available, some of which imply metaphysics that would have seemed as unlikely to them as the woman yelling to Othello from the crowd.