by Will Durant
Our current explanation of combustion was prepared by the work of Hales, Black, and Scheele on the chemistry of the air. Stephen Hales paved the way by devising a “pneumatic trough,” or air receptacle, into which gases could be collected in a closed vessel over water. He pointed out that gases (which he called “airs”) were contained in many solids, and he described air as “a fine elastic fluid, with particles of a very different nature floating in it.”25 The decomposition of air and water into diverse substances put an end to the agelong conception of air, water, fire, and earth as the four fundamental elements. In the next generation the experiments of Joseph Black (1756) proved that one constituent of air was what, after Hales, he called “fixed air”—i.e., air contained in, and removable from, solid or liquid substances; we now term it carbon dioxide, or “carbonic-acid gas.” Black further cleared the way to the discovery of oxygen by showing experimentally that this gas is contained in human exhalations. But he still believed in phlogiston, and oxygen, hydrogen, and nitrogen were still mysteries.
Sweden contributed lavishly to eighteenth-century chemistry. Torbern Olof Bergman, whom we shall meet again as a pioneer in physical geography, was primarily a chemist, famous and loved as a professor of that science in the University of Uppsala. He was the first to obtain nickel in a pure state and the first to show the importance of carbon in determining the physical properties of carbon-iron compounds. In his relatively short life of forty-nine years he studied, with over thirty thousand experiments, the chemical affinities of fifty-nine substances, and reported his findings in Elective Attractions (1775). He died before completing this task, but meanwhile he had passed on to Scheele his devotion to chemical research.
English historians of science now gallantly concede that a Swedish chemist, Karl Wilhelm Scheele, anticipated (1772) Priestley’s discovery (1774) of what Lavoisier (1779) was the first to call oxygen. Scheele lived most of his forty-three years in poverty. Beginning as an apprentice to an apothecary in Göteborg, he rose no higher than to be a pharmacist in the modest town of Köping. His teacher, Torbern Bergman, obtained a small pension for him from the Stockholm Academy of Science; Scheele spent eighty per cent of it on chemical experiments. He performed most of these at night after the day’s work, and with the simplest laboratory equipment; hence his early death. Yet he covered nearly the whole field of the new science, and defined it with his usual simplicity: “The object and chief business of chemistry is skillfully to separate substances into their constituents, to discover their properties, and to compound them in different ways.”26
In 1775 he sent to the printer a manuscript entitled Chemical Treatise on Air and Fire; its publication was delayed till 1777, but nearly all the experiments it described had been carried out before 1773. Scheele, while holding till his death the belief in phlogiston, laid down the basic proposition that unpolluted atmosphere is composed of two gases: one of these he named “fire-air” (our oxygen), as the main support of fire; the other he called “vitiated air” (our nitrogen), as air that has lost “fire-air.” He prepared oxygen in several ways. In one method he mixed concentrated sulfuric acid with finely ground manganese, heated the mixture in a retort, and collected the resultant gas in a bladder that had been pressed nearly free of air. He found that when the gas so produced was played over a lighted candle, this “began to burn with a larger flame, and emitted such a bright light that it dazed the eyes.”27 He concluded that “fire-air” was the gas that supported fire. “There is little doubt but that he obtained the gas two years before Priestley.”28
This was but a fraction of Scheele’s achievement. His record as a discoverer of new substances is probably unequaled.29 He was the first to isolate chlorine, barium, manganese, and such new compounds as ammonia, glycerine, and hydrofluoric, tannic, benzoic, oxalic, malic, and tartaric acids. His discovery that chlorine would bleach cloth, vegetables, and flowers was put to commercial uses by Berthollet in France and James Watt in England. In further researches Scheele discovered uric acid by analyzing stone in the bladder (1776). In 1777 he prepared sulfuretted hydrogen, and in 1778 molybdic acid; in 1780 he proved that the acidity of sour milk is due to lactic acid; in 1781 he obtained tungstic acid from calcium tungstate (now known as scheelite); in 1783 he discovered prussic (hydrocyanic) acid, without realizing its poisonous character. He produced also arsine gas (a deadly compound of arsenic), and the arsenic pigment now known as Scheele’s green.30 He helped to make photography possible by showing that sunlight reduces chloride of silver to silver, and that the diverse rays that compose white light have different effects upon silver salts. The incredibly fruitful labor of this brief life proved of endless importance in the industrial developments of the nineteenth century.
2. Priestley
Joseph Priestley, rather than Scheele, for a long time received the credit for the discovery of oxygen because he discovered it independently of Scheele, and announced his discovery in 1775, two years before Scheele’s retarded publication. We honor him nevertheless because his researches enabled Lavoisier to give chemistry its modern form; because he was among the pioneers in the scientific study of electricity; and because he contributed so boldly to British thought on religion and government that a fanatical mob burned down his house in Birmingham, and induced him to seek refuge in America. He touched the history of civilization at many points, and is one of its most inspiring characters.
He was born in Yorkshire in 1733, son of a Dissenter cloth-dresser. He studied voraciously in science, philosophy, theology, and languages; he learned Latin, Greek, French, German, Italian, Arabic, even some Syriac and Chaldee. He set up as a Dissenting preacher in Suffolk, but an impediment in his speech lessened the appeal of his eloquence. At twenty-five he organized a private school, whose curriculum he enlivened with experiments in physics and chemistry. At twenty-eight he became tutor in a Dissenting academy at Warrington; there he taught five languages and yet found time for researches that won him a fellowship in the Royal Society (1766). In that year he met Franklin in London, and was encouraged by him to write The History and Present State of Electricity (1767), an admirable survey of the whole subject up to his own time. In 1767 he was appointed pastor of Mill Hill Chapel at Leeds. He recalled later that “it was in consequence of living for some time in the neighborhood of a public brewery that I was induced to make experiments in fixed air”31—the brewery mash emitted carbonic-acid gas. He dissolved this in water, and liked its bubbling tang; this was the first “soda water.”
In 1772 he was relieved of economic worry by appointment to the post of librarian to Lord Shelburne. In the house provided for him at Colne he performed the experiments that won him international renown. He improved upon Hales’s pneumatic trough by collecting over mercury, instead of over water, the gases generated by diverse mixtures. So in 1772 he isolated nitric oxide, nitrous oxide (“laughing gas”), and hydrogen chloride; in 1773 ammonia (independently of Scheele); in 1774 sulfur dioxide; in 1776 nitrogen peroxide. On March 15, 1775, he communicated to the Royal Society a letter announcing his discovery of oxygen. In Volume II of his Experiments and Observations on Different Kinds of Air (1775) he described his method. Using a strong burning lens, he said,
I proceeded … to examine, by the help of it, what kind of air a great variety of substances would yield [when so heated], putting them into … vessels … filled with quicksilver and kept inverted in a basin of the same. With this apparatus, … on the first of August, 1774, I endeavored to extract air from mercurius calcinatus per se [mercuric oxide]; and I presently found that, by means of this lens, air was expelled from it very readily.… What surprised me, more than I can well express, was that a candle burned in this air with a remarkably vigorous flame.32
Noting, like Scheele, that a mouse lived much longer in this “dephlogisticated air” (as he called oxygen) than in the ordinary atmosphere, he thought he might safely sample the new air himself.
My reader will not wonder that, after having ascertained the superio
r goodness of dephlogisticated air by mice living in it, and the other tests above mentioned, I should have the curiosity to taste it by myself. I have gratified that curiosity by breathing it, drawing it through a glass siphon; and by this means I reduced a large jar full of it to the standard of common air. The feeling of it to my lungs was not sensibly different from common air, but I fancied that my breast felt peculiarly light for some time afterward. Who can tell but that, in time, this pure air may become a fashionable article of luxury? Hitherto only two mice and I have had the privilege of breathing it.33
He predicted some forms of this future luxury:
From the greater strength and vivacity of the flame of a candle in this pure air, it may be conjectured that it might be peculiarly salutary to the lungs in certain morbid cases, when the common air would not be sufficient to carry off the phlogistic putrid effluvium [carbon dioxide] fast enough. But perhaps we may also infer from these experiments that though pure dephlogisticated air [oxygen] might be very useful as a medicine, it might not be so proper for us in the usual healthy state of the body; for as a candle burns out faster in dephlogisticated air than in common air, so we might, as may be said, live out too fast, and the animal power be too soon exhausted, in this pure kind of air.34
Priestley’s experimental work was brilliant with fruitful hypotheses and alert perceptions, but his theoretical interpretations were mostly traditional. Like Stahl and Scheele, he supposed that in combustion a substance, phlogiston, was emitted by the burning material; this substance, in his view, united with one constituent of the atmosphere to form “vitiated air,” or “phlogisticated air” (our nitrogen); the other constituent was in his nomenclature “dephlogisticated air,” which Lavoisier was to name oxygen. While Lavoisier argued that a material in process of combustion absorbed oxygen from the air instead of expelling phlogiston into it, Priestley to the end of his life retained the old conception.
In 1774 he traveled with Lord Shelburne on the Continent, and told him of the oxygen experiments. In 1780 Shelburne retired him with an annuity of £ 150. Priestley settled in Birmingham as junior minister of a large Dissenting congregation known as the New Meeting Society. He joined James Watt, Josiah Wedgwood, Erasmus Darwin, Matthew Boulton, and others in a “Lunar Society” that discussed the latest ideas in science, technology, and philosophy. He was popular with nearly all classes, admired for his cheerful spirit, his modesty and generosity, and “the unspotted purity of his life.”35 But some of his neighbors questioned his Christianity. In Disquisitions relating to Matter and Spirit (1777) he reduced everything, even the soul, to matter. This, he insisted, was perfectly orthodox,
it being well known to the learned … that what the ancients meant by an immaterial being was only a finer kind of what we should now call matter; something like air or breath, which first supplied a name for the soul.. . . Consequently the ancients did not exclude from mind the property of extension and local pressure. It had, in their idea, some common properties with matter, was capable of being united with it, of acting and being acted upon by it.… It was therefore seen that … the power of sensation or thought … might be imparted to the very grossest matter, … and that the soul and body, being in reality the same kind of substance, must die together.36
In a further publication of the same year, The Doctrine of Philosophical Necessity Illustrated, Priestley, following Hartley and Hume, enthusiastically denied the freedom of the will. And in a History of the Corruptions of Christianity (1782), he rejected miracles, the Fall, the Atonement, and the Trinity; all these doctrines he considered to be “corruptions” developed in the evolution of Christianity; they were not to be found in the teachings of Christ or the twelve Apostles. All that was left of Christianity in Priestley was the belief in God, based on the evidences of divine design. Not quite reconciled to mortality, he suggested that at the Last Day God would re-create all the dead. His real hope, however, was not in a heaven above but in a utopia that would be built on this earth by the victory of science over superstition and ignorance. Seldom has the eighteenth-century religion of progress been more fervently expressed:
All knowledge will be subdivided and extended; and knowledge, as Lord Bacon observes, being power, the human powers will in fact be increased; nature, including both its materials and its laws, will be more at our command; men will make their situation in this world abundantly more easy and comfortable; they will probably prolong their existence on it, and will daily grow more happy, each in himself, and more able (and, I believe, more disposed) to communicate happiness to others. Thus, whatever was the beginning of this world, the end will be glorious and paradlsaical beyond what our imaginations can now conceive.37 … Happy are they who contribute to diffuse the pure light of this everlasting gospel.38
Part of this glorious progress, in Priestley’s vision, was to be political, and would be based upon a simple humanitarian principle: “The good and happiness of the … majority of the members of any state is the great standard by which everything relating to that state must finally be determined”;39 here Bentham, according to Bentham, found one source of his utilitarian philosophy. The only just government, said Priestley, is one that aims at the happiness of its citizens, and it is quite consistent with Christianity that an obviously unjust government should be overthrown by the people. To St. Paul’s caution that “the powers that be are ordained of God” Priestley replied that “for the same reason the powers which will be will be ordained of God also.”40
It was natural that such a rebel should sympathize with the colonies in their protest against taxation without representation. Still more warmly did he acclaim the French Revolution. When Burke denounced it Priestley defended it; Burke, in Parliament, branded him as a heretic. Some of Priestley’s friends shared his radical views. On July 14, 1791, the “Constitutional Society of Birmingham” met in the Royal Hotel to celebrate the anniversary of the fall of the Bastille. Priestley did not attend. A crowd gathered before the hotel, listened to its leaders’ attacks upon heretics and traitors, and stoned the hotel windows; the banqueters fled. The crowd moved on to Priestley’s house, and joyously burned it down, including his laboratory and instruments, his library and manuscripts. Then for three days it ranged through Birmingham, swearing to kill all “philosophers”; terrified citizens scrawled on their windowpanes, “No philosophers here.” Priestley fled to Dudley, then to London. Thence on July 19 he addressed a letter to the people of Birmingham:
MY LATE TOWNSMEN AND NEIGHBORS,
After living with you eleven years, in which you had uniform experience of my peaceful behavior in my attention to the quiet duties of my profession, and those of philosophy, I was far from expecting the injuries which I and my friends have lately received from you.… Happily the minds of Englishmen have a horror of murder and therefore you did not, I hope, think of that.… But what is the value of life when everything is done to make it wretched? …
You have destroyed the most truly valuable and useful apparatus of philosophical instruments.… You have destroyed a library … which no money can repurchase except in a long course of time. But what I feel far more, you have destroyed manuscripts which have been the result of the laborious study of many years, and which I shall never be able to recompose; and this has been done to one who never did, or imagined, you any harm.
You are mistaken if you imagine that this conduct of yours has any tendency to serve your cause, or to prejudice ours.… Should you destroy myself as well as my house, library, and apparatus, ten more persons, of equal or superior spirit and ability, would instantly spring up. If those ten were destroyed, an hundred would appear. …
In this business we are the sheep and you the wolves. We will persevere in our character, and hope you will change yours. At all events, we return you blessings for curses, and pray that you may soon return to that industry, and those sober manners, for which the inhabitants of Birmingham were formerly distinguished.
I am, your sincere well-wisher,
/> J. PRIESTLEY.41
Nevertheless he sued the city for damages, estimating his loss at £4,500; Charles James Fox helped his suit; Birmingham awarded him £ 2,502. He tried to establish a new domicile in England, but churchmen, royalists, and his fellows in the Royal Society shunned him.42 The French Académie des Sciences, through its secretary, Condorcet, sent him an offer of a home and laboratory in France. On April 8, 1794, aged sixty-one, he emigrated to America. He made his new home in the town of Northumberland, in Franklin’s Pennsylvania, on the banks of that lovely Susquehanna River about which Coleridge and Southey were soon to dream. He resumed his experiments, and discovered the composition of carbon monoxide. He was welcomed by learned societies, and was offered the chair in chemistry at the University of Pennsylvania. In 1796 he delivered before the Universalists of Philadelphia a series of discourses on “The Evidences of Christianity”; his audience included Vice-President John Adams and many members of Congress. From those meetings a Unitarian Society took form. Two years later Timothy Pickering, Secretary of State under President Adams, proposed to deport Priestley as an undesirable alien. The election of Jefferson (1800) ended Priestley’s insecurity, and he was allowed four years of peace. In 1803 he wrote his last scientific paper, still defending phlogiston. He died at Northumberland on February 6, 1804. In 1943 the Pennsylvania legislature designated his home as a national memorial.
While Thomas Paine took up Priestley’s campaign as a rebel Christian, Henry Cavendish pursued the chemistry of gases. Son of a lord, nephew of a duke, Cavendish at forty inherited one of the greatest fortunes in England. Timid, hesitant in speech, careless of dress, he lived as a recluse in his laboratory at Clapham Common, London, and made no overtures to fame. His research was distinguished by meticulous measuring and weighing of all materials before and after an experiment; these measurements enabled Lavoisier to formulate the principle that in chemical changes the amount of matter remains constant.