A Garden of Marvels Page 15
With these questions in mind and using laboratory equipment invented fifty years earlier by Stephen Hales, he continued to experiment with fixed air, enclosing insects, frogs, and mice in upside-down jars of common and fixed air until they lost consciousness. (A softhearted man, he often removed his subjects and was able to revive them before they died.) Then one day in the summer of 1771, he idly put a small mint plant into an inverted jar of common air to see what would happen to it. No one had tried the experiment, but then, why would they? If plants breathed, certainly they did so like other living creatures, by exhaling poisonous fixed air. Priestley assumed that his mint plant would die, just as mice did.
Not only did the mint survive in the jar; it continued to grow for months. Even more surprising was his discovery that, after all that time, “the air [inside] would neither extinguish a candle, nor was it at all inconvenient to a mouse, which I put into it.” What was most astonishing is that if he put a plant in a jar in which a candle had guttered out or a mouse had suffocated, some days later a candle would burn and mice could breathe again.
Priestley repeated his experiments throughout the summer of 1771, to make sure he had made no mistake. He hadn’t. Somehow, mint cured bad air. Thinking that perhaps mint was the only life-giving plant, he tried balm, foul-smelling groundsel (in case only sweet-smelling plants performed this restorative function), and spinach. They all worked. God had arranged it, he concluded, so that plants “reverse the effects of breathing.” Priestley had the first glimmer that the living organisms and the nonliving environment are inextricably and reciprocally related—one of the essential principles of modern ecology. With remarkable intuition, he realized that the Earth’s environment is a closed system. The paper he presented to the Royal Society on his findings on plant-generated, breathable air was a sensation, and he won the Society’s Copley Medal in 1773. As Sir John Pringle concluded when presenting Priestley with the medal, “from these discoveries we are assured, that no vegetable grows in vain [and] every individual plant is serviceable to mankind; if not always distinguished by some private virtue, yet making a part of the whole which cleanses and purifies our atmosphere.”
How plants accomplished their miracle of purification was uncertain. Did they take something noxious from the air or add something beneficial to it? Priestley had also discovered that plants placed in air “strongly tainted with putrefaction” often grew particularly vigorously. With this in mind, he picked the first option: The plant took something out of the air. What he thought it took out was “phlogiston,” a weightless and invisible “principle” that was generally believed to depart a material when it burned or animals breathed it in. A mint plant, Priestley concluded, took phlogiston out of the air and left the air “dephlogisticated.” What we know as oxygen was to Priestley “dephlogisticated air” or “good air,” that is, air that had been purified of the phlogiston added by animals, decay, or fermentation.
From 1773 to 1777, Priestley focused on other scientific matters, as well as theological ones, but in early 1778, he again took up his research on plants and air. Reports had reached him that other scientists had been unable to reproduce his results, so he began repeating his earlier experiments. To his distress, this time his results were inconsistent. Sometimes his plants in jars produced good air and sometimes fixed air. Sometimes they barely altered the air at all. To clarify his muddled findings, he submerged little plants in vials of water, corked the vials, and analyzed the quality of the air that collected inside. He seemed at first to be making progress: The plants produced a particularly pure dephlogisticated air. But after he removed some plants from their vials, he noticed a “green matter” on the inside of the glass. To his astonishment, the green matter (which he assumed was neither animal nor vegetable) was releasing bubbles of good air.
He continued to experiment with vials of this interesting green matter, putting them near a stove and on sunny windowsills, and wrapping some in brown paper and afterward measuring the dephlogisticated air they produced. In the process, he arrived at his second great discovery: “As extraordinary as it will seem,” he wrote, light is essential to producing good air. Unfortunately, this insight led him to wonder if “plants had not, as I imagined, contributed anything to the production of this pure air.” Instead, he hypothesized, “light disposes water . . . to make a deposit of a greenish or brownish matter, and then to yield dephlogisticated air.” It was all very confusing, and by 1779 he could write only that “upon the whole, I still think it probable [his emphasis] that the vegetation of healthy plants, growing in situations natural to them, has a salutary effect on the air in which they grow.” Perplexed, he urged others to consider his results and investigate further.
sixteen
Leaves Eat Air
One of those who did was a Dutch physician, Jan Ingen-Housz. Ingen-Housz was born in 1730 in Breda, in the southern part of the Netherlands. His father was an educated man and the leading apothecary in town. Like Priestley, Jan had a gift for languages, and attended universities in Paris, the Netherlands, and Edinburgh. After completing his study of medicine, chemistry, and physics, he settled down to a career as a physician in his hometown.
His heart, however, was in experimental science. As a Catholic, he had no hope of a career at one of the Dutch universities, which were as off-limits to him as the Anglican universities were to Priestley. To his good fortune, an old connection opened a door for him. In the mid-1740s, the English army had helped the Dutch defeat a French invasion of Flanders, and the army had camped for some time outside Breda. The chief medical officer of the English army had been the young Dr. John Pringle. Pringle was fluent in Dutch, and became a frequent visitor to the Ingen-Housz residence, where he met the apothecary’s precocious teenage son. In 1764, Pringle—by then knighted, a physician to King George III, and head of the Royal Society—invited the thirty-four-year-old Jan to London and introduced him to London’s scientific and medical community.
Ingen-Housz found work at the London Foundling Hospital, where, among other duties, he inoculated the resident children against smallpox, a terrifying disease that infected more than a million Europeans annually, killing a third of its victims. Many of those who survived were horribly scarred; some were also blinded. Although by this time many physicians in England and the Netherlands recommended inoculation to their patients, there were few takers in those countries and almost none in other European states. The practice was banned in Paris. It is not hard to understand people’s reluctance, given the procedure, which involved a physician cutting a small vein and dropping in fluid or ground-up scabs taken from a victim of a mild case of the disease. Besides, while the procedure generally induced only a slight infection, it was not without risk: About 2 percent of those inoculated contracted full-blown disease.
Wealth and status provided little protection against smallpox; the virus is highly infectious and can remain viable for months in the environment. By 1767, Maria Theresa, empress of the Holy Roman Empire, and sovereign of a host of other central European states, as well as parts of modern Netherlands, France, and Italy, had lost two of her sixteen children and several other family members to the disease. That year, at the age of fifty, she contracted the disease herself, and although she survived, she was deeply scarred. Her daughter-in-law, Empress Maria Josepha of Bavaria, died of the illness that same spring. Maria Theresa took one of her daughters to pray with her at the dead empress’s unsealed tomb. When, days later, her daughter came down with smallpox and died, the empress blamed herself, although given the weeklong incubation period, she was certainly not responsible. The imperial court physician opposed inoculation (instead favoring bleeding and the ancient Japanese treatment of dressing the afflicted in red), but after two more imperial daughters were disfigured that fall, the empress overruled her doctor. She wrote to King George III to ask that a physician be sent to Vienna to inoculate her remaining family. It fell to Dr. Pringle to select the man.
The man who subjected the imp
erial family to a potentially lethal procedure would be taking a gamble. Success might make his career, but a failure could end it. Pringle thought of Ingen-Housz, both for his competence in the procedure and out of political considerations. If any of the empress’s family were to fall ill, Catholic royalty would not have died at the hands of an English Protestant. Pringle approached Ingen-Housz, who accepted the job.
On his arrival in Vienna, the empress first had him inoculate twenty-nine commoners’ children in her presence. When these human guinea pigs survived, he was allowed to treat the immediate imperial family, including the young Marie Antoinette. Again, everyone survived, and Ingen-Housz was honored, awarded a heap of gold ducats, and appointed court physician with a lifetime pension. His chief responsibility over the coming years would be to inoculate dozens of royal wives, husbands, cousins, nieces, nephews, and grandchildren at their villas and palaces across Europe—a great improvement in working conditions over those at the foundling hospital.
Ingen-Housz now had the opportunity to spread the word about comprehensive smallpox inoculation, a cause he fervently believed in. But welcome, too, was the time, status, and wherewithal he now had to pursue his scientific interests. In May 1771, he joined Ben Franklin and two American businessmen on a kind of applied-science tour of the English Midlands, visiting marble-milling, silver-plating, iron-smelting, and other manufacturing operations. He and his companions also stopped in on Priestley, who demonstrated some of his electrical experiments, which so deeply intrigued the Dutch physician he began his own electrical investigations. When passing through Livorno, Italy, on the Mediterranean coast, he hired a fishing boat and a crew of eighteen, who captured five electric rays for him. In a makeshift lab on board, he attempted to correlate their size to the intensity of the shocks they delivered, and to capture their electrical charge in a Leyden jar. Then he dissected his subjects, with the hope (disappointed) of discovering where and how they stored their energy. Naturally, he reported his findings to the Royal Society.
Ingen-Housz stayed current with Priestley’s research on air and plants, and in the summer of 1779 he took leave from his post in Vienna, rented a villa near London, and set himself a discrete task. He would attempt to clarify the muddled relationship among plants, sunlight, and air. Many of his experiments were similar to Priestley’s, but his approach was fundamentally different. Priestley tested whole plants, but the Dutch physician used only leaves. Priestley had let his experiments run for days or weeks; Ingen-Housz completed each test in a matter of days.
After completing 546 experiments in ninety days, Ingen-Housz had answers. Priestley had been right the first time. Leaves and other green parts of a plant turn fixed air into good, dephlogisticated air. Plants require sunlight to carry out the transformation. Moreover, Ingen-Housz proved that the brighter the light is, the more dephlogisticated air is produced. The good air emerges primarily from the stomata on the undersides of leaves. That “green matter” in the vials, he demonstrated, was vegetal in nature and also produced good air. Roots, flowers, and ripe fruit always produce only fixed air. Most remarkably, in the dark all parts of a plant, including leaves, produce fixed air. Although the amount emitted in darkness, he calculated, is only a hundredth of the amount of good air emitted in two hours of sunlight, he wondered if plants should be removed from a sickroom at night.
Ingen-Housz was able to explain Priestley’s inconsistent findings. The English minister hadn’t understood to what degree variations in the amount of sunshine and the changing daylight length affected the output of good air. Nor did he realize that the gradual senescence of leaves would reduce the output of good air. In addition, he hadn’t taken into account how root systems altered his experiments: the greater the mass of roots, the greater the production of fixed air. To put it in modern chemical terms, the amounts of carbon dioxide and oxygen in Priestley’s closed containers had fluctuated, depending on light intensity and duration in his lab, the time of day that he tested the air, the ratio of roots to leaves of a given plant, and the ever-changing size and health of his experimental subjects.
Ingen-Housz also had a far greater appreciation of painstaking lab procedures. Experiments, he realized, had to be repeated in precisely the same fashion and under exactly the same conditions before any valid conclusions could be drawn. Sloppy lab procedures would have measurably influenced Priestley’s results and those of others trying to reproduce his experiments.
Ingen-Housz discovered which parts of a plant produced good air and fixed air, but it was Jean Senebier, a Swiss pastor, librarian, and amateur botanist, who proved that the two gases were related. In 1781, he repeated his predecessors’ experiments, ran his own, and wrote up his results. The pastor’s prose was in want of a hedge trimmer (he wrote 2,100 pages on his plant experiments), but he made a major advance: in order for leaves to produce good air, they must have a supply of fixed air. To account for the difference between the two airs, he posited that phlogiston was liberated from air and incorporated in plants’ new growth.
Even while Senebier was imagining a new role for phlogiston, the concept was under attack by Antoine Lavoisier. Lavoisier was a brilliant French chemist and the youngest person ever elected to the French Academy of Sciences. Burning, according to Lavoisier, has nothing to do with a supposed phlogiston, but with an element—a term he redefined as a chemical substance that could not be further broken down into other chemical substances—that he named oxygène. Oxygen readily combined with many other elements; this process of combination, or oxidation, could occur rapidly with a sudden release of heat and light, as when wood is heated, or slowly, as when iron rusts.* Proof of his theory lay in the fact that while rusted iron appeared to involve a loss of a substance, it actually weighed more than untarnished iron. In fact, the mass of rusted iron was exactly equal to the iron plus the mass of incorporated oxygen. Modern chemists had to be bookkeepers, he wrote, measuring precise volumes and weights of liquids, solids, and gases, keeping track of inputs and outputs that always balanced.
Lavoisier presented his revolutionary chemistry to the Academy in 1778 and published his masterwork, Elements of Chemistry, in 1789. Priestley never accepted Lavoisier’s “new chemistry,” and Ingen-Housz accepted only a part. But by 1796, Senebier was a believer, adopting the new concepts and terminology. Leaves, he now asserted, absorb that small portion of air that is carbon dioxide and, in sunlight, decompose it into carbon and oxygen. The carbon from carbon dioxide becomes the organic matter of plants, and oxygen ends up in the atmosphere. He didn’t have the full story yet, but he was the first to put photosynthesis in modern chemical terms.
Nicolas-Théodore de Saussure, born in 1767 in Switzerland, was a young man when Lavoisier’s book appeared. His father, Horace-Benedict de Saussure, was a professor of natural sciences at the Academy of Geneva, the man who coined the New Latin word geologia, discovered fifteen minerals, and became a famously intrepid mountaineer. He tutored his eldest son at home and brought the boy along on his scientific treks through the Alps. These adventures were so rugged and at such heights—the purpose was to study the relationship of altitude to the density of air, among other puzzles—that one time their porters threw away the expedition’s provisions in a desperate effort to convince the pair to descend. The unusual education suited Nicolas-Théodore, who found his calling in the new chemistry, although it perhaps had a less beneficial effect on his social development. He grew up to be such a painfully shy, self-effacing man that while his work won him an early professorship at the Academy of Geneva, he was never able to stand before students and lecture. He was a masterful experimenter, however; a double-entry chemical bookkeeper par excellence. Thanks to the better instruments of his day and his extraordinary skill in using them, he was able to measure the volume and weight of gases exchanged by plants to the hundredths of an ounce.
Saussure’s passion was the chemistry of plant physiology, and he put together the modern description of the basic process of photosynthesis. He demon
strated that, in sunlight, fixed carbon appears in leaves simultaneously with the disappearance of carbon dioxide and its replacement by oxygen in the surrounding air. Moreover, the green parts of plants actively transform the carbon into organic matter. In other words, leaves eat air. When plants grow larger, he discovered, atmospheric carbon dioxide is by far the largest source of that increase. Minerals from the soil—which were still assumed to provide the bulk of new plant material—actually comprise well less than 5 percent of a plant’s mass. It was Saussure who explained why in darkness all parts of a plant release carbon dioxide. Plants—like animals—respire, meaning they use oxygen to burn carbon-based sugars to fuel growth, concoct scents, manufacture insect-killing resins, and carry out other essential activities. In essence, roots and seeds “breathe” as we do. In fact, even leaves respire a little bit during the day. It’s just that they take up so much more carbon dioxide in sunlight than they release through respiration that their respiration is easy to overlook.
The Swiss chemical bean counter was able to make such minute and precise measurements that he discovered an error in the chemical books. The mass of the carbon taken in by leaves and the mass of minerals taken up by roots did not equal a plant’s total increase in mass. A plant weighs more than the sum of the carbon and minerals. The extra weight, he realized, comes from water.
Van Helmont had been right, or at least a little bit right, after all: Plants are made of water. Although 99 percent of the water rising from a plant’s roots exits its leaves as water vapor, about 1 percent is incorporated into the substance of a plant. It is a minuscule but crucial amount. H2O, split apart by the energy of sunlight, contributes the hydrogen for those molecules of C6H12O6 (the simple sugar, glucose) that plants make and use for energy. And although Saussure didn’t know it, the oxygen produced by plants, the oxygen that all multicellular life on Earth requires, comes from the oxygen in water, not, as everyone assumed, the oxygen in carbon dioxide. Remarkably, this last fundamental fact of photosynthesis was not uncovered until the 1930s by Stanford professor C. B. van Niel.