* The hold of alchemy on chemical thought was gradually loosened during the 17th and 18th centuries. What can now be recognized as modern chemistry emerged late in the 18th century, through the work of a French genius named Antoine Lavoisier.
* Despite the introduction of new ideas, many of the old ideas of alchemy lingered on. Late in the 17th century, a German researcher named Johann Joachim Becher (1635:1682) constructed a refinement of the "sulfur principle", noting that burning materials seemed to give off some sort of vaporous substance, which he called "terra pinguis (fatty earth)". His student Georg Ernst Stahl (1660:1734) took up the idea, suggesting there was a substance in materials that escaped into air when the materials were burned or rusted -- incidentally, rust was called "calx" in those days. Stahl called this material "phlogiston". Stahl suggested that phlogiston was absorbed by plants from the air and released when the plants were burned.
Phlogiston is regarded in hindsight as one of the worst ideas in the history of science, but Stahl had taken a major step forward by articulating the idea that burning and rusting were different aspects of the same process; he realized correctly that rusting was just a "slow burn". Phlogiston seemed to make sense of other chemical processes as well, but in hindsight it had an obvious flaw: materials that rusted gained weight, which would be strange if they were releasing phlogiston in the process. At the time, however, due to various confounding effects in experiments, chemists weren't entirely sure if rusting metals gained or lost weight.
* Although chemical theory still had a long way to go at the beginning of the 18th century, chemical practice was making clear progress. Improved techniques helped isolate a series of new metals. In the 1730s, a Swedish researcher named George Brandt (1694:1768) investigated a bluish mineral that miners who had found it believed to be copper ore, until it proved indifferent to attempts to smelt the copper out of it. They thought the ore had been cursed by "kobolds", or Nordic earth spirits, but Brandt managed to isolate the material into a distinct metal, which he named "cobalt".
Sweden seemed to be a haven for metallurgists at the time, with Swedish researchers discovering a set of other "new" metals. Axel Fredric Cronstedt (1722:1765) isolated "nickel" in 1751; Johann Gottlieb Gahn (1745:1818) isolated "manganese" in 1774; and Peter Jacob Hjelm (1746:1813) isolated "molybdenum" in 1782. Cronstedt also invented the "blowpipe", a glass pipette used to blow on and focus a flame, which remains a standard item of kit for high school chemistry classes. In addition, he attempted to come up with a more modern and rigorous scheme for classifying minerals.
Cronstedt's work was followed by that of a Swedish mineralogist named Torbern Olof Bergman (1735:1784), who was puzzled by the ways in which some substances would react with each other, while others wouldn't. Bergman created a set of tables listing the "affinities" of substances for each other. He had no clue as to the reason for these affinities, but his tables provided a good deal of predictive power. His tables would go into widespread use, and be extended by others.
* The most important chemical tinkering of the time was in the isolation of gases. In 1727, a British physiologist named Stephen Hales (1677:1761) developed a means of collecting gases -- or "airs" as they were known at the time -- by bubbling them through water to be collected in an inverted flask.
In the 1750s, a Scots researcher named Joseph Black (1728:1799) used these techniques to isolate carbon dioxide (CO2), or what he called "fixed air", by heating limestone (what we now call "calcium carbonate" or CaCO3) and trapping the gas, leaving lime ("calcium hydroxide" or CaOH) behind. He could then recombine the fixed air with the lime to obtain limestone again. Lime itself would be restored to limestone at a much lower rate if it were left in the open air, which demonstrated that fixed air was a minor component of ordinary air. The traditional wisdom believed that gases were simply locked into a material and released by heating, but Black had demonstrated they were capable of entering into chemical reactions. Black also performed pioneering studies into the nature of heat, for example showing that different materials had different "specific heats": a given amount of one material would demonstrate a lower change in temperature for a given amount of heating than another.
* Black had noticed in the course of his experiments that if he placed a candle in a sealed vessel, it would quickly go out, even though there still seemed to be air in the chamber. Black didn't follow up the matter himself, passing it on to his student, Daniel Rutherford (1749:1819), for further investigation. In 1772, Rutherford announced the results of his experiments along this line. He showed that once a candle in a sealed vessel burned out, any lit candle placed in the vessel would immediately go out as well; mice placed in such air would quickly die as well. Rutherford, believing the phlogiston theory, assumed that once the level of phlogiston in the air inside the vessel reached a certain level, the air was then saturated; no more phlogiston could be released, and so combustion was stymied. Rutherford called the saturated gas "phlogistonated air".
In 1766, a wealthy and reclusive English researcher named Henry Cavendish (1731:1810) discovered a gas given off by the action of sulfuric acid on iron. Others had found it before him, but none had taken the time to investigate it in detail. The gas was very light and highly explosive; he called it "flammable air" and wondered if he had actually discovered phlogiston. Cavendish's discovery meant that three distinct gases were now known: air itself, fixed air (carbon dioxide), and flammable air (hydrogen).
It was yet another Briton, a Unitarian minister named Joseph Priestley (1733:1804), who pushed the isolation of gases into high gear. Priestley came up with the idea of collecting water-soluble gases by bubbling them through mercury, allowing researchers to broaden their range of investigation, and during the 1770s he used mercury collection to identify almost a dozen more gases. Priestley had found what we now know as carbon monoxide (CO), ammonia (NH3), nitrous oxide (N2O), sulfur dioxide (SO2), and most significantly oxygen.
Priestley discovered oxygen in experiments conducted in 1774, in which he heated mercury inside a vessel using a large glass lens and found a reddish powder, what we would now call mercuric oxide, forming on the surface of the mercury. Priestley skimmed off the powder and then heated it to a higher temperature in another vessel, causing the powder to gradually disappear. It was clearly being broken down into other substances -- one of them certainly mercury since mercury droplets formed on the walls of the vessel. Assuming that the breakdown of the powder had also released a gas, he performed follow-up experiments to investigate, for example shoving a smoldering splinter of wood into the vessel. It promptly burst into flame. Candles burned brightly in the gas, and mice seemed to become very energetic. Priestley inhaled the gas to find it made him feel pleasantly light-headed.
That is exactly what we would now expect for oxygen. Priestley had isolated the gas and characterized it very well, but he was still under the spell of phlogiston and so he called his discovery "dephlogistonated air". He believed that this "air" had been depleted of phlogiston, and so combustion processes proceeded very rapidly in it. Priestley also found that placing a plant in a vessel full of phlogistonated air -- which, as Rutherford had found, would suffocate mice -- dephlogistonated it again. In 1779, a Dutch researcher named Jan Ingenhousz (1730:1799) found the plant would only perform this trick if it were exposed to sunlight. Priestley and Ingenhousz were on the trail of the fundamental plant process of "photosynthesis", but understanding photosynthesis would take generations.
BACK_TO_TOP* Now the French scientist Antoine Laurent Lavoisier (1743:1794), one of the towering minds of chemistry, entered the picture. Lavoisier believed as Boyle did, that chemistry needed to become a quantitative science. He was from a prosperous upper-class family, which opened doors for him into realms where his brilliance was quickly appreciated. In 1764, he made a scientific name for himself by an analysis of the mineral gypsum, carefully heating it and collecting the water driven off for precise measurement. He acquired membership in the prestigious French Academy of Sciences in his early twenties, where he assisted in researching reports on a wide range of practical subjects, including the water supply of Paris, manufacture of black powder explosive, mesmerism, street lighting, and hydrogen balloons.
These studies were useful, but they established Lavoisier as no more than a clever chemical handyman. He then began to consider more fundamental issues, leveraging off his inclination to question the conventional wisdom in the sciences and back up his challenges with meticulous experiments. Alchemists had long claimed that the residue left behind by boiling water amounted to water converted into "earth". In 1770, Lavoisier boiled water in a closed experimental system for 101 days, capturing the water vapor and returning it to the boil. At the end of the test, he turned off the heat, let the water cool off, and found a mass of sediment, just as had the alchemists. Lavoisier had measured the weights of the water and the experimental apparatus beforehand, however, and he found that he had exactly the same weight of water as existed at the end of the test. The weight of the experimental apparatus had been reduced slightly -- but the deficit matched the weight of the residue. The residue was simply a portion of the experimental apparatus, dissolved by the water.
Lavoisier's next series of experiments were on combustion, setting him on a collision course with the phlogiston theory. In 1772, he and a group of colleagues bought a small diamond, which he heated to high temperature in a closed vessel using sunlight focused through a large lens. Somewhat surprisingly, the diamond generated "fixed air" -- carbon dioxide. Diamonds seemed to be made of carbon, making them not too different in principle from a lump of coal.
Lavoisier then started heating metals in a closed vessel. For example, he placed a piece of tin in a vessel, then sealed and weighed the vessel. He heated the vessel, observing that the tin formed a white calx or rust. Chemists had realized by this time that rusting caused metals to gain weight, which caused problems for the phlogiston theory. One chemist suggested that phlogiston had negative weight or "levity" -- an only too appropriate term, since nobody took the idea seriously. Lavoisier weighed the vessel containing the rusty tin again, finding that its weight was the same as before. What if, so he thought, the rust had been formed from the air inside the sealed vessel? If that was so, then the loss of some of the air would lower the pressure inside the vessel. He opened the vessel, and sure enough air rushed in with a pop.
Lavoisier understood that the rusting amounted to a combination of a metal with some part of the air, but he didn't know which part. In 1774, Priestley paid Lavoisier a visit and told him of his discovery of dephlogistonated air. Lavoisier quickly grasped that it was what was forming the rust. The phlogiston theory had it backwards -- burning and rusting were not due to a loss of a material, but to the gain of one. Wood lost weight in combustion because it gave off gases. He published his ideas in 1775.
Lavoisier conducted further experiments, finally determining that the dephlogistonated air amounted to about a fifth of ambient air. If that fifth were used up by combustion or rusting in a closed vessel, candles would not burn inside the vessel, and mice would die there. Lavoisier concluded that air consisted of two gases in proportions of 1:4. He called the minority gas "oxygen", meaning "acid former" -- he was under the mistaken perception that oxygen was an inherent component of acids -- and the majority gas "azote", though it would later be called "nitrogen".
* Many other chemists were skeptical of Lavoisier's proposals: he was maintaining that phlogiston was a literally backwards idea, but some of his colleagues didn't see that it as any improvement just to turn things around. To be sure, the concept of oxidation was consistent with observed measurements while phlogiston theory was not, but Lavoisier tried to measure the results of human "metabolism" -- the chemical process by which a organism keeps itself alive -- in terms of his oxygen theory, and couldn't make the inputs and outputs balance.
Matters then shifted back to England. When Priestley discovered dephlogistonated air, Cavendish got to wondering what would happen if he combined his flammable air with dephlogistonated air. If the flammable air was really phlogiston, he should get ordinary air in the end. In 1781, he created flammable air, collected it, drove it as a jet of gas through a tube into a vessel of dephlogistonated air, and lit the jet with a spark. The end result was very surprising: droplets of water formed on the wall of the vessel.
Cavendish didn't quite understand the implications of this experiment, but Lavoisier saw it immediately, realizing that water, long thought to be an "elemental", was really what would now be called an "oxide" -- a combination of oxygen gas with the flammable air, which Lavoisier named "hydrogen", meaning (correctly) "water former". Of course, the fact that the end result of the experiment was water and clearly not phlogistonated air was a major blow to the phlogiston theory. The formation of water also helped Lavoisier figure out where the discrepancies in his observations of human metabolism were coming from.
* Support for the phlogiston theory faded out over the following decades. Ironically, while Lavoisier destroyed one bogus theory, he created another elsewhere by supposing that heat itself was composed of a similarly mysterious substance, an invisible, weightless fluid that he called "caloric", which to a modern point of view followed a logic along the lines of the phlogiston theory. The reign of caloric was short-lived. The investigations of Benjamin Thompson (1753:1814) or Count Rumford -- an American physicist and inventor who backed the British Crown in the American Revolution, and had to flee to Europe when his side lost -- struck it a fatal blow in a paper published in 1798.
Thompson was working in an armory in Munich and was boring out cannon, to notice that more heat was produced by grinding out cannon with a dull tool than a sharp one. Thompson realized that he could get more and more heat out of the process by continuing to make the tool duller, until in the end the cannon seemed to be providing an indefinite supply of caloric. Thompson realized instead that heat was a product of the work performed boring the cannon: he could obtain heat as a direct consequence of the work performed on the job. Heat was clearly a form of energy, which later physicists would show to be due to the motion of the particles making up a system.
* Incidentally, some years later, Henry Cavendish would follow up work in isolating oxygen from air by isolating nitrogen as well, using sparks to convert the nitrogen in the air of a sealed vessel to nitric acid (HNO3). After he could convert no more, he still found that there was some remainder left, about 1% of the original. He wasn't sure if it was just an error in his experiment or if it was some sort of unreactive or "inert" gas, but in any case he had no means to identify what it was. It would take about another century to figure it out.
As another footnote, in 1765 a Russian polymath named Mikhail Vasilevich Lomonosov (1711:1765), regarded as the founder of Russian science, published a document that rejected phlogiston and suggested that combustion was caused by a combination of a substance with a portion of the air. Lomonosov also had advanced concepts on atoms and heat, but unfortunately he wrote in Russian, which few in the West could read, and his work was overlooked.
It is common in the history of science and technology that, once a certain basic enabling knowledge becomes widespread, the same discoveries or inventions are made in parallel in a number of different places. While Britons and Frenchmen were isolating gases, another Swede, Karl Wilhelm Scheele (1742:1786), had been coming up with much the same discoveries. Scheele was an apothecary's apprentice when, in modern terms, Torbern Bergman "discovered" him and sponsored his researches. Scheele discovered a wide range of acids; isolated a number of gases, including the toxic gases hydrogen fluoride (HF), hydrogen sulfide (H2S), and hydrogen cyanide (HCN); assisted his Swedish colleagues in the discovery of new metals; and most significantly, isolated nitrogen and oxygen in 1771 and 1772.
Unfortunately, his publisher was negligent and Scheele's work wasn't published until 1777, robbing him of priority and unfairly placing him historically in a lower rank than he deserved. That was not the worst of his misfortunes: his willingness to taste chemical samples helped put him into an early grave. He wasn't the only chemist of his era who cut his life short through such carelessness. Priestley, as noted, also used himself as a guinea pig, but managed to live to a ripe old age.
BACK_TO_TOP* Lavoisier remains one of the dominating figures in the history of chemistry, but his career was cut short, in an unfortunately literal fashion, by the French Revolution. Although he supported revolutionary reform, he was a member of the upper class, with his affluence supporting his experiments with the best gear he could buy. His wealth might have not been held too much against him, but he had also acquired an interest in and active participation in a business named the "Ferme General". The "fermiers (farmers)" were tax farmers, meaning they were private companies that collected taxes on behalf of the government. Such organizations were of course widely disliked, all the more so because the scheme was so prone to corruption.
The officials of the Ferme General were arrested by the French revolutionary government in late 1793. Although there was no evidence that Lavoisier had engaged in corrupt practices in his work with the Ferme General -- indeed, it seems he had worked to make sure the organization remained within the law -- Lavoisier's association with the despised fermiers, and the fact that he had extensive correspondences with scientific colleagues in countries then at war with France, was damning enough. He went under the guillotine on 8 May 1794. The mathematician Joseph Lagrange commented: "It took only a second to cut off his head, and likely a hundred years won't be enough to produce another one like it."
The impact of Lavoisier's work did not die out with him; he had killed off phlogiston and, in doing so, had helped chemistry shed the lingering dead weight of alchemy. He acquired enthusiastic advocates for his views, such as Torbern Bergman, though not everyone was excited about his ideas; Priestley and Cavendish maintained their faith in phlogiston to the end of their days. Although they had given Lavoisier much of the ammunition he needed to take down phlogiston, he had an arrogant streak and avoided crediting them in the slightest. Possibly the snub had something to do with their conservatism.
Priestley, incidentally, was an outspoken anti-establishment sort. He supported the American Revolution, the French Revolution, and radical reform in general. Such an attitude had its hazards, which were demonstrated when a mob burned down his house in 1791. He decided that life would be better in America, leaving England in 1794 to settle in Pennsylvania, where he died in 1804. Seventy years later, a hundred years after his discovery of oxygen, American chemists met at Priestley's house to form the American Chemical Society.
Lavoisier's critique of phlogiston was only one of the reasons he would later be generally honored as the founder of modern chemistry. His other accomplishments included:
Some scholars assert the significance of Lavoisier has been overstated. Chemistry had been advancing rapidly for a century before him, and if he was arguably the brightest star of his era, he wasn't the only star in the sky. Along with his great successes, he also made some great blunders -- caloric being the most famous of them, but he also believed that light itself was a fluidic substance, a notion that went nowhere.
Lavoisier's reputation as the founding father of modern chemistry was partly self-created. As noted, he was not always inclined to credit the work of others, implicitly overstating his own importance, and his introduction of modern chemical nomenclature ensured, by plan or not, that later generations of chemists would find the writings of those who came before him archaic and quaint. However, even without overstatement Lavoisier was still a giant, and if he stood on the shoulders of other giants, his work did represent something of a watershed. From the modern point of view, from his time on, chemists seemed scientists worthy of the name, the science having clearly outgrown its alchemical roots.
BACK_TO_TOP* The French revolutionary government formally established the metric system in 1799; it was another break with the past, and its (gradual) adoption by chemists would rationalize chemical calculations. Basic metric units were introduced earlier, along with fundamental concepts of physics; it is useful to complete the discussion of metric by introducing the concepts of "exponential notation" and "metric prefixes".
Science generally deals with quantities that may vary over a wide range, leading to very large or very small values, for example:
1,470,000,000,000 0.000000000000592
Using long strings of zeroes is clumsy and it is much more convenient to use exponential notation, with the strings of zeroes changed to a power of 10:
1,470,000,000,000 = 1.47 * 1,000,000,000,000
= 1.47 * 10^12
= 1.47E12
0.000000000000592 = 5.92 * 0.000000000001
= 5.92 * 10^-12
= 5.92E-12
It should be fairly obvious that "10^12" is 10 multiplied by itself 12 times, or 1 followed by 12 zeroes. The "E12" is a shorthand way of representing this. Similarly, "10^-12" is 1/10 multiplied by itself 12 times, and "E-12" is a shorthand format.
The "metric prefixes" are another shorthand scheme that give a specific multiplying factor for a particular unit of measurement. For example, a "millimeter" is a thousandth of a meter, a "centimeter" is a hundredth of a meter, and a "kilometer" is a thousand meters. The standard metric prefixes include:
exo (E) = 1E18 = 10^18 peta (P) = 1E15 = 10^15 tera (T) = 1E12 = 10^12 giga (G) = 1E9 = 10^9 mega (M) = 1E6 = 10^6 kilo (k) = 1E3 = 10^3 hecto (h) = 1E2 = 10^2 deka (d) = 1E1 = 10 centi (c) = 1E-2 = 10^-2 milli (m) = 1E-3 = 10^-3 micro (mu) = 1E-6 = 10^-6 nano (n) = 1E-9 = 10^-9 pico (p) = 1E-12 = 10^-12 femto (f) = 1E-15 = 10^-15 atto (a) = 1E-18 = 10^-18
Chemistry will often take a slightly different read on units than physics. For example, for student lab experiments, a kilogram is too big to be convenient, and so "grams", of course meaning a thousandth of a kilogram, are generally used instead. For industrial chemistry, a kilogram is too small, so the "tonne", metric ton or a thousand kilograms, is generally used instead. Calculations using different sets of units have to be rescaled accordingly, which can be a tricky and troublesome issue.
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