Fugacity (foo-gah’- ci -tee)
That’s not really a word, you’re probably thinking. Fugacity would be the name of the hooker in a science fiction story set on Planet Xenophon Centauri. She would have five very acrobatic appendages and curves bursting out of her lamé space bikini. Fugacity would be the name of the bar where philosophical detectives and rebels hang out in the 23rd century. It sounds like somewhere you might meet a hashish-addled Samuel Taylor Coleridge in a poem.
I was told by a scientist, by email, that fugacity was the proper measure of carbon dioxide. Not the ppm we have all come to know; the number that keeps crossing each threshold we set for the end of the world as we know it.
“We’re all part of a huge experiment. We’ve never had a species on this planet before that took all the carbon in the earth and put it in the atmosphere” I heard a Scientist reply to the Earnest Young Graduate Student who asked about the “so what” of rising CO2 levels.
The concentration of carbon dioxide, it turns out, is not all that easy to measure at any place or time, particularly at the surface of the ocean where it is being exchanged between the air and the water. Absorbed into the water, it reacts chemically to form the free and wild hydrogen ions that send us into screaming Ocean Acidification fits about the death of everything shelled or coralline.
Scientists have wavered, I learned, using mole fractions (not the animal) and partial pressures. Consistency is needed and fugacity is best, say Benjamin Pfeil and Are Olsen at the Bjerknes Centre for Climate Research in Bergen, Norway.
“Over the last few decades several million measurements of the surface ocean CO2 concentration have been made, in particular following the advent of infrared based systems which determines the CO2 concentration in an air headspace in equilibrium with a continuous stream of sea water,” the plea for the consistent use of fugacity begins. “The concentration can be expressed as the fugacity of CO2 (fCO2) in the headspace, which takes into account the non-ideal behavior of CO2 gas.”
Precision, it seems, can still emerge from the failure of the most ethereal of matter to behave well. No substance, it seems, is truly ideal. Not gold, not diamonds. Gases tend to escape, but in measurable ways.
The meaning of fugacity is elusive. It’s derived from the Latin fugere, to flee, and often interpreted by the physicists as “the tendency to flee or escape.” Fugere is also the root of “fugitive,” which, in our modern world, means flight from custody of some legal authority, or as a noun, a long-running television show about someone who didn’t do it. The word has earlier roots in that which is difficult to grasp or retain, likely to evaporate, deteriorate, change, fade, or disappear. Botanists use “fugacious” to describe plant parts that wither or drop off early, like the petals of an orchid. “Fugacity” also means “fleeting.” It seems there are many words like “fleeting,” synonyms that all mean something of short duration – evanescent, transient, transitory, ephemeral, momentary. But the Merriam Webster dictionary informs me there are subtle distinctions. “Evanescent” suggests a quick vanishing and an airy or fragile quality. “Transient” applies to what is actually short in its duration or stay, like a night in a hotel, while “transitory” applies to what is by its nature or essence bound to change, pass, or come to an end, like fame. “Ephemeral” implies a striking brevity of life or duration, like the adult phase of the mayfly, but “momentary” suggests coming and going quickly and therefore being merely a brief interruption of a more enduring state, like a pang of guilt in the midst of joy. But both “fugitive” and “fleeting,” derived from fugere, imply passing so quickly as to make apprehending difficult. Ralph Waldo Emerson used “fugacity” in a literary sentence: “He (the poet) perceives the independence of the thought on the symbol, the stability of the thought, the accidency and fugacity of the symbol.” (The fugacious relationship between creative nonfiction and truth was before his time.)
What we actually feel from a gas is its fugacity, not the pressure we would feel if it was an ideal gas. So when we are mindful of our breath, we must really be experiencing its fugacity, like an image we are watching on the walls of Plato’s cave, like the weight of our sins on our soul, like our karma, like love, actually. Fugacity is the tendency to stay in the state you’re in, which for a gas is expansive. It’s the return of your attention to the hardness of your meditation cushion, the number of breaths you fail to mind, the sneak peek at the person next to you. Physicists are not idealists. There’s no evidence that they yearn for the ideal gas which they seem awfully certain can never exist. They would say the same about Truth, Kindness, and Beauty. Fugacity is really the measure of the change in pressure it would take to make a gas behave ideally – but where would that pressure come from?
Human consciousness and souls have a tendency to escape, we are told, because there has been a fall from somewhere, sometime in the distant past, into a less-than-ideal form. Spirit is imprisoned in matter, striving to escape. Zoroaster preached that the world was a Lie, that Truth and Light were imprisoned in the darkness of the matter that is makes up our body. The Gnostics called it Sophia, the goddess of wisdom, a divine emanation of the Supreme God who upset the order of Darkness and Light by her desire to know what lies beyond the limits of what can be known. The world was created with the Goodness of Light mixed into the Evil of Darkness and matter. The Sophia part of us is the longing of a divine spark of our soul to escape matter, the part beloved by philosophers. When all of us return to the perfect, divine realm, the world will end. Fugacity is the measure of how far we have to go to our lost home and merge into the Light.
Fugacity arises because substances prefer the phase of matter of greatest disorder. Substance tends toward entropy, the escape of order to greater disorder. Scientists call this tendency to escape the chemical potential of the substance, a potential which is not mathematically well behaved. It’s a measure of the deviation that is reality. It’s the dimensionless coefficient that would close the gap.
Fugacity. Just when you think you apprehend it, it escapes you, like the world, like your self.