by Sarah Dry
Tyndall was dissatisfied with the stalemate the glacier debate had reached, but he was a restless thinker and, above all, a doer. He soon found a new project into which he could sink his prodigious energies and which would again unite his passion for the grandeur of nature and the precision of laboratory experimentation. He considered the new project, quite naturally, to be continuous with the old one. He was still fundamentally interested in the role played by heat in matters both of basic physics and in the complicated, messy reality of the earth itself. His trips to the Alps made to study the glaciers and how they moved started him thinking about gases, heat, and the sun’s rays. It was impossible for him to be there, in the mountains, without thinking about the endless transfer of energy from one substance to another. In his own words, his work on glaciers had “directed my attention in a special manner to the transmission of solar and terrestrial heat through the atmosphere.”39 His new project would be an experimental investigation of how heat affects not solid objects, such as ice, but gases, including those found in the earth’s atmosphere. It is this work for which he has now regained a measure of remembrance, as an early discoverer, along with Joseph Fourier and Svante Arrhenius, of what we today call the greenhouse effect.
He spent time in early 1859 in the basement of the Royal Institution with a specific question in mind: How much heat could different gases absorb? To answer this question, Tyndall created a complete and controlled artificial environment, part electrical apparatus, part cloud chamber. It never got a name of its own. It was a complicated thing, comprising a sealed glass tube within which he could release gases of different types, a steady source of artificial heat (provided by a gas flame and a cube of boiling water), and a precise method for measuring the absorption of that heat by the gas, via an instrument (only recently invented) called a galvanometer, which would measure the difference between the current transmitted through the tube with or without the gas inside it.40
In concept, simple enough (perhaps). In fact, the device spun off problems like a Catherine wheel. Even when no charge was present, the galvanometer’s needle turned of its own accord, as much as thirty degrees from neutral. After much tinkering, Tyndall finally figured out that the copper used to make the wire of the coil was tainted by magnetic metals. A purer, less magnetic copper reduced the deviation from thirty degrees to three. But it was still not good enough. The absorptive properties Tyndall hoped to measure might be very small indeed. Three degrees of error in the instrument could obscure any effect he might be trying to measure. He finally thought to unwrap the green silk that covered the copper wire. Some compound containing iron had been used to dye the silk green. The bare wire, wrapped in white silk with clean hands, gave no deflection of the needle.
At first, despite the improvement in the apparatus, he saw nothing in the gases that he studied. Contriving a constant source of heat was a major challenge. He spent weeks in the spring of 1859 trying to get a result, during which time he often despaired. “The course of the inquiry during this whole period was an incessant struggle with experimental difficulties.” It was as different as could be from the moments of instant epiphany he experienced in the mountains, where all was revealed to him as abruptly as the shifting of a cloud from the face of the sun.41 And then, on May 18, 1859, after nearly two months of constant work with the device, he had a breakthrough: “Experimented all day; the subject is completely in my hands!” The next day he continued, “Experimenting, chiefly with vapours, coal gas wonderful—ether vapour still more so.”42
FIG. 2.8. The apparatus John Tyndall used to measure the absorption of heat by different gases, including water vapor, which could explain “all the mutations of climate which the researches of geologists reveal.” John Tyndall, “The Bakerian Lecture: On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connexion of Radiation, Absorption and Conduction,” Philosophical Transactions of the Royal Society 151 (1861): 36.
But then, seemingly unaccountably, Tyndall broke off his experimental labors, returning to the Alps to make more glacier studies. Though he was not employed at an academic institution, he kept academic timetables—lecturing and experimenting in the fall and spring, with summers off. The break he took in June 1859 was a recurrent and natural one for Tyndall. Summers were devoted to the Alps, and to the Alps he went in the summer of 1859. It was only in September 1860 that he would return to the apparatus, spending seven weeks fine-tuning it, trying and rejecting new sources of heat. He experimented up to ten hours a day. Over the course of the next seven weeks, he worked nonstop, experimenting from eight to ten hours a day. He studied sulfuric ether, ozone, olefiant gas, carbon bisulfide, ethyl iodide, methyl iodide. The list went on, stretching to dozens of substances. By late October, he had almost run his way through the long list of elements he’d set as his primary course of work. Slowly, he’d learned how to still the motley assortment of molecules in the room sufficiently so that the miniscule effects he sought were sensible to the instrument. For it had been a matter of increasing frustration that the simple substances he investigated were almost to a fault extremely poor absorbers of the heat that radiated from the boiling cube. They varied, it was true, and Tyndall worked hard to capture the silent music in these jittery numbers.43 It was still not good enough. This work, too, was ultimately unsuccessful, and he rejected all of his findings. It was a trying time: “a continued struggle against the difficulties of the subject and the defects of the locality in which the inquiry was conducted.”44
One problem was getting a steady heat source. In November 1860 he had better luck. The air of the laboratory, freed from its moisture and carbonic acid, produced a deflection of about one degree. Oxygen obtained from chlorate of potash and peroxide of manganese produced the same deflection, as did nitrogen, hydrogen produced from zinc and sulfuric acid, and hydrogen obtained from the electrolysis of water. He worked especially hard to obtain a pure specimen of oxygen, first obtaining a sample from electrolysis, and then sending it through a series of eight bulbs containing a strong solution of iodide of potassium, depriving the oxygen of its ozone. This too produced a deflection of just one degree. Then he tried the oxygen that had not been passed through the successive baths of iodide of potassium and still contained its complement of ozone. The needle jumped to four degrees. What this meant was that ozone was three times more potent an absorber of radiant heat than oxygen alone.
On November 20, something even more surprising happened. He first measured the absorption of heat produced by air that had been rid of its moisture and its carbonic acid. This was a negligible amount. That was unsurprising, given the readings he’d been gathering for other elements. But then came the unexpected result. Air that had been taken direct from the laboratory deflected the needle an incredible fifteen times more. Tyndall subtracted the effects of the carbonic acid from this and was still left with an amazing result. The invisible moisture carried by undried air was responsible for blocking thirteen times more heat than oxygen alone.
After a total of fourteen weeks of experimentation, Tyndall was able to report results in his Bakerian lecture of 1861. He saved his biggest discovery for the tail end of the paper. After describing the small deflections produced by substances such as chloroform and alcohol, he came to a point of “considerable interest” having to do with the relationship between the atmosphere and what he called solar and terrestrial (i.e., earthly) heat. The curious result was this: Air that had been rid of all moisture and other constituents absorbed very little heat, while air that had been taken directly from the laboratory produced an absorption up to fifteen times greater.
Even very small alterations in the amount of the key gases—water vapor, carbon dioxide, and hydrocarbon vapors—could change the amount of heat trapped by the atmosphere dramatically, thereby warming the planet. This was a mechanism that could, potentially, explain both the ice ages and the warmer periods whose existence was suggested by the fossil record. It explai
ned why mountaintops were so cold, even though they were closer to the sun, and why the sun was hotter at midday than evening. The key to it all was the split in the nature of aqueous vapor. Though water vapor exercised a “destructive action,” to use Tyndall’s phrase, on the rays of radiant heat emitted by a cooling earth, it was completely transparent to light rays. This made all the difference. The light that reached Earth from the sun passed through the aqueous vapor easily and was absorbed by the earth, which then radiated heat back outward, as any rock warmed by the sun will do. That heat was then trapped by the aqueous vapor, which acted like a great blanket, swaddling the earth in heat which would otherwise be lost to space. Variations in the amount of aqueous vapor, Tyndall surmised, could account for many, if not all, of the changes in climate that fossil records and geological strata revealed. No longer would it be necessary to theorize changes in the density or height of the atmosphere or in the elevation of entire continents to explain the different degrees of heat reaching the earth. Instead, “a slight change” in the amount of water vapor in the atmosphere was sufficient to produce “all the mutations of climate which the researches of geologists reveal.”45 He needed to repeat the experiment in other locations, with other samples of atmospheric air, to eliminate any possible interference caused by dust or other particles in the air. But the implication was startling. “It is exceedingly probable,” wrote Tyndall, “that the absorption of the solar rays by the atmosphere . . . is mainly due to the watery vapour contained in the air.”46
Tyndall’s patient work in a quiet basement in London had produced results that could explain how changes in the earth happened at the largest scales imaginable—in terms of both time and space. The absorption of heat by water vapor in the atmosphere clearly affected the entire climate of the globe. He was not shy about saying so in both the lecture he gave and the paper he published summarizing his results. His paper was read aloud to the fellows of the Royal Society and was chosen to be the Bakerian lecture for the year—a special honor.
He didn’t have much time to enjoy his success before an unwelcome letter came from a German physicist named Heinrich Gustav Magnus, asserting priority. Tyndall was ready for it. He’d placed a preliminary notice before the Royal Society back in May 1859 for just such an eventuality. The notice didn’t contain all of his subsequent results, but enough to stake a claim. But there was more serious trouble. Magnus had come up with results that were different from Tyndall’s—diametrically so—on the matter of water vapor. Magnus had found that dried air absorbed more (only slightly, but still more) heat than moist air.47
Tyndall’s response was hard work. “Self-chastening,” he went so far as to call it, deliberately calling up the religious comparison. The research had already been an “immense and arduous toil,” but that was as nothing to the next period of work, spurred by the presence of an unwelcome competitor.
He spent every weekday for the next four months proving Magnus wrong. With every improvement in his technique, Tyndall could see the effects of aqueous vapor more clearly. For moist air, he obtained deflections of the galvanometer’s needle of forty-eight or even fifty degrees, while the dried air shifted the needle by only one degree.
To his relief, the more he became a master of his equipment, the greater the difference between the absorptions. He developed new and elaborate methods for drying the air. A massive block of glass was ground to dust in a mortar, boiled in nitric acid, washed with distilled water, and finally dried before being moistened by pure sulfuric acid. These fragments were then introduced into a U-shaped tube to prevent any contact between the sulfuric acid and the cork which stoppered the tube and which could otherwise undo the good effects of the drying process previously achieved. Pure white marble was ground for use with the caustic potash. Tyndall prepared new drying tubes daily to ensure they were equally effective. With these tubes, Tyndall had invented a way to rid the air of carbonic acid and moisture independently. And with his galvanometer and tube, he could measure the different absorptive capacities of various molecules.
Magnus remained unconvinced, questioning whether the findings would hold for air from locations other than Tyndall’s laboratory. Tyndall took up the challenge. He had plenty of friends who were only too happy to help. Very quickly, he managed to obtain samples of air from places that were much clearer in atmosphere than the very center of London, if not as pellucid as the Swiss skies. His friends sent him air from Hyde Park, Primrose Hill, Hampstead Heath, and the Epson racecourse. He also received samples from two locations on the Isle of Wight, one a beach near Blackgang Chine. Consistently, Tyndall found that these samples all absorbed sixty to seventy times more heat with their water vapor than without it. Once again, Tyndall had managed to show that findings determined in one location—be it the Alps, his Albemarle laboratory, or Hampstead Health—could be duplicated elsewhere.
Having taken on the physicists, Tyndall was bold with the meteorologists. He was unafraid to direct them to his results, which offered what he termed “absolute certainty.” Tyndall stated categorically that “the withdrawal of the sun from any region over which the atmosphere is dry, must be followed by quick refrigeration.” This was, Tyndall freely admitted (even bragged), “simply an a priori conclusion.” Such were the fruits of laboratory experimentation. He was confident enough to state that be believed no meteorological experience would contradict it. Once gained in the laboratory, truths could not be lost. He boldly asserted that ten percent of the “entire terrestrial radiation is absorbed by the aqueous vapour which exists within ten feet of the earth’s surface.”
His conclusions about aqueous vapor accounted for all sorts of climatological findings. It explained that the great twist of cloud that commonly mimics the course of the great rivers, the Nile and the Ganges, owes its existence to the “chilling of the saturated air above the river by radiation from its vapour.” It accounted for the huge differences in temperature at high altitudes between the air, which often remained very cool, and the ground, which could warm up readily in the sunshine, which his friend Hooker had noticed in the Himalayas.48 The effect held in Europe. A descent from Mont Blanc, hip-deep in snow, had been suffered in blazing sunlight and almost unbearable heat, despite the snow all around. It also explained how, in the arid regions of central Australia, temperature fluctuations of forty degrees were quite common, more than double the range found in damp London. Correspondingly, it explained the intense daytime heat of the Sahara and its nighttime cold.
As impressive as it was to be able to provide what Tyndall considered conclusive proof about the causes of the earth’s climate, this was not, and never had been, Tyndall’s primary goal. As with his studies of the glacier, Tyndall was really interested in molecular physics, with how heat affected molecules in their most basic form. When he described the alteration of the climate, he said that it revealed the “effect of our atmosphere on solar and terrestrial heat.” Heat was always his primary focus, and the atmosphere was of interest primarily in its role as an interruption in the journey of heat. The work he did on gases was important to Tyndall because it revealed a “purer case of molecular action” than had ever been studied before, a way of understanding not why the history of the earth had unfolded the way it had, but why “a ray of heat is stopped by one molecule and unimpeded by another.”
Tyndall did not go as far as Croll in trying to connect the absorptive capacities of water vapor in the atmosphere. He did not suggest the mechanisms by which the amount of water vapor in the earth’s atmosphere might fall or rise. What he had done, instead, was expand the toolkit to which Croll had already pointed. There were more ways than one to cool or warm the earth that did not rely on the Lyellian emergence or subsidence of continents. Though neither Tyndall nor Croll said as much, it was clear from their work that not only did paying attention to the action of water in the earth’s climate provide sufficient means for explaining past climates, it could do so at potentially much shorter timescal
es than geologists such as Lyell always required. Tyndall had shown that water vapor could effect dramatic changes in local weather extremely quickly—from day to day if not hour to hour.
Tyndall repeatedly wrote about the changeability of the skies. This description of his attempt to scale the Galenstock mountain, chosen almost at random from his book recounting his mountain adventures, Hours of Exercise in the Alps, demonstrates his almost symphonic sensitivity to the pervasive presence of water in the environment. “The sky was clear and the air pleasant as we ascended; but in the earth’s atmosphere the sun works his swiftest necromancy, the lightness of air rendering it in a peculiar degree capable of change. Clouds suddenly generated came drifting up the valley of the Rhone, covering the glacier and swathing the mountain-tops, but leaving clear for a time the upper névé of the Rhone. Grandeur is conceded while beauty is sometimes denied to the Alps. But the higher snow-fields of the great glaciers are altogether beautiful—not throned in repellent grandeur, but endowed with a grace so tender as to suggest the loveliness of woman.” Tyndall here demonstrates how closely his appreciation of atmospheric and glacial phenomena was linked to the possibility of change. It was the sense of a landscape undergoing constant transformation, an erotic veiling and unveiling, that kept Tyndall endlessly attentive. The phenomena themselves—the clouds, the wind, the snow and ice—are given full meaning and force by the sense of connections between them, by their potential to be transformed into each other according to laws of nature which, though regular and fundamental, gave rise to phenomena that were infinite and sublime.
