Carbon Dioxide and Nitrogen
The explanation of the puzzling changes in weight during combustion was to be found, of course, in the gases that appeared or disappeared while the products were forming. Despite the slowly growing knowledge of gases since the time of Van Helmont, a century earlier, there was still no attempt in Stahl's day to take them into account in any way except to note their existence. In thinking of weight changes in combustion, the investigators had eyes only for solids and liquids. Ash was lighter than wood, but what about the vapors given off by the burning wood? Not considered. Rust was heavier than metal, but had rust gained anything from the air? Not considered.
Before this deficiency could be corrected, chemists had to grow more familiar with gases. The terrors of a substance that seemed so hard to hold, confine, and study had to be overcome.
The English chemist Stephen Hales (1677-1761) made a step in the right direction, in the early eighteenth century, when he collected gases over water. The vapors formed as a result of a chemical reaction could be led, through a tube, into a jar of water that had been upended in a basin of water. The gas bubbled upward into the jar, displacing the water and forcing it out through the open bottom. In the end, Hales had obtained a jar of the particular gas or gases formed in the reaction.
He himself did not distinguish between the different gases he had prepared and trapped, or study their properties. The mere fact that he had devised a simple technique for trapping them, however, was of first-rate importance.
Another important step forward was taken by a Scottish chemist, Joseph Black (1728-1799). The thesis that earned him a medical degree in 1754 dealt with a chemical problem (this was the era when mineralogy and medicine were closely intertwined), and he published his findings in 1756. What Black did was to heat, strongly, the mineral limestone (calcium carbonate). This carbonate decomposed, giving off a gas and leaving behind lime (calcium oxide). The gas given off could be made to recombine with calcium oxide to form calcium carbonate again. The gas itself (carbon dioxide) was identical with Van Helmont's "gas sylvestre", but Black called it "fixed air" because it could be combined ("fixed") in such a way as to form part of a sold substance.
Black's findings were important for a number of reasons. First, he showed that carbon dioxide could be formed by the heating of a mineral, as well as by the burning of wood, so that an important connection was made between the animate and inanimate realm.
Again, he showed that gaseous substances were not merely given off by solids and liquids, but could actually combine with them to produce chemical changes. This discovery made gases that much less mysterious and presented them, rather, as a variety of matter possessing additional properties in common (chemically at least) with the more familiar solids and liquids.
Still further, Black showed that when calcium oxide was allowed to stand in air, it turned slowing to calcium carbonate. From this, he deduced (correctly) that there were small quantities of carbon dioxide in the atmosphere. Here was the first clear indication that air was not a simple substance and, therefore, despite Greek notions, that it was not an element by Boyle's definition. It consisted of a mixture of at least two distinct substances, ordinary air and carbon dioxide.
In studying the effects of heat on calcium carbonate, Black measured the loss of weight involved. He also measured the quantity of calcium carbonate that would neutralize a given quantity of acid. This was a giant step toward the application of quantitative measurement to chemical changes, a method of analysis that was soon to come to full maturity with Lavoisier.
In studying the properties of carbon dioxide, Black found that a candle would not burn in it. A candle burning in a closed container of ordinary air would go out eventually, and the air that was left would then no longer support a flame. This behavior certainly seemed reasonable, since the burning candle had formed carbon dioxide. But when the carbon dioxide in the trapped air was absorbed by chemicals, some air remained unabsorbed. This air that was left and that was not carbon dioxide would still not support a flame.
Black turned this problem over to one of his students, the Scottish chemist Daniel Rutherford (1749-1819). Rutherford kept a mouse in a confined quantity of air till it died. He then burned a candle in what was left until the candle went out. He then burned phosphorus in what was left after that, until the phosphorus would no longer burn. Next, the air was passed through a solution that had the ability to absorb carbon dioxide. The air remaining now would not support combustion; a mouse would not live in it and a candle would not burn.
Rutherford reported this experiment in 1772. Since Rutherford and Black were both convinced of the validity of the phlogiston theory, they tried to explain their results in terms of this theory. As mice breathed and as candles and phosphorus burned, phlogiston was given off and entered the air, along with the carbon dioxide that was formed. When the carbon dioxide was later absorbed, the air left behind still contained much phlogiston. In fact, it contained so much phlogiston as to be saturated with it; it would accept no more. That was why objects no longer burned in it.
On this reasoning Rutherford called the gas he had isolated "phlogisticated air". Nowadays, we call it nitrogen, and give Rutherford the credit for its discovery.
Hydrogen and Oxygen
Two other English chemists, both upholders of the phlogiston theory, further advanced the studies of gases at this time.
One of these was Henry Cavendish (1731-1810). He was a wealthy eccentric who did research in a number of fields, but kept to himself and did not always publish the results of his work. Fortunately, he did publish the results of his work with gases.
Cavendish was particularly interested in a gas that was formed when acids reacted with certain metals. This gas had been isolated before by Boyle and Hales and perhaps others, but Cavendish, in 1766, was the first to investigate its properties systematically. He usually gets the credit, therefore, for its discovery. Eventually the gas was named hydrogen.
Cavendish was the first to measure the weight of particular volumes of different gases so that he might determine the density of each gas. He found hydrogen to be unusually light, with only one-fourteenth the density of air. (It is still the least dense gas known.) It had a second unusual property for, unlike carbon dioxide and air itself, it was easily inflammable. Cavendish, considering its extreme lightness and inflammability, speculated on the possibility that he had actually isolated phlogiston itself.
The second chemist was Joseph Priestley (1733-1804). He was a Unitarian minister who was deeply concerned with chemistry as a hobby. In the late 1760's, he took over a pastorate in Leeds, England, next door to which, as it happened, was a brewery. Fermenting grain produces carbon dioxide, which Priestley thus could obtain in quantity for experiments.
In collecting carbon dioxide over water, he found that some of it dissolved and lent the water a pleasantly tart taste. This is what we call "seltzer" or "soda water" today. Since it requires only added flavoring and sugar to produce "soda pop", Priestley may be viewed as the father of the modern softdrink industry.
Priestley went on to study other gases in the early 1770's. At the time only three gases were known as distinct individuals; air itself, the carbon dioxide of Van Helmont and Black, and hydrogen of Cavendish. Rutherford was about to add nitrogen as a fourth. Priestley, however, proceeded to isolate and study a number of additional gases.
His experience with carbon dioxide having showed him that gases could be soluble in water, and so lost to his experiments, he tried collecting them instead over mercury. By this method he was able to collect and study such gases as nitrogen oxide, ammonia, hydrogen chloride, and sulfur dioxide (to give them their modern names), all of which are too soluble in water to survive passage through it.
In 1774, the use of mercury in his work with gases was the occasion of Priestley's most important discovery. Mercury, when heated in air, will form a brick-red "calx" (which we now call mercuric oxide). Priestley put some of this calx in a test tube and heated it with a lens that concentrated sunlight upon it. The calx broke down to mercury again, for the metal appeared as shining globules in the upper portion of the test tube. In addition, the decomposing calx gave off a gas with most unusual properties. Combustibles burned more brilliantly and rapidly in this gas then in air. A smoldering splint of wood thrust into a container of the gas burst into flame.
Priestley tried to explain this phenomenon in terms of the phlogiston theory. Since object burned so easily in this gas, they must be capable of giving off phlogiston with unusual ease. Why should this be so, unless the gas was a sample of air from which the usual content of phlogiston had been drained, so that it accepted a new supply with special eagerness? Priestley therefore called his new gas "dephlogisticated air". (A few years later, however, it was renamed oxygen, the name we use today.)
Priestley's "dephlogisticated air" did, indeed, seem to be the opposite of Rutherford's "phlogisticated air". Mice died in the latter, but were particularly active and frisky in the former. Priestley tried breathing some "dephlogisticated air" and found himself feeling "light and easy".
But both Rutherford and Priestley had been anticipated by a Swedish chemist, Karl Wilhelm Scheele (1742-1786), one of a group of chemists who brought Sweden to the forefront of science in the eighteenth century.
One of these Swedes, George Brandt (1694-1768), had studied, about 1730, a bluish mineral that resembled copper ore but which, to the exasperation of the miners, yielded no copper when put through the usual treatment. The miners thought it was ore that had been bewitched by the earth-spirits they called "kobolds". Brandt was able to show that the mineral contained no copper, but contained, rather, a new metal (resembling iron in its chemical properties) which he named cobalt after the earth-spirits.
In 1751, Axel Fredric Cronstedt (1722-1765) discovered the very similar metal, nickel; Johann Gottlieb Gahn (1745-1818) isolated manganese in 1774, and Peter Jacob Hjelm (1746-1813) isolated molybdenum in 1782.
The discovery of these new elements by Swedes demonstrated the strides mineralogy was making in that nation. Cronstedt, for instance, introduced the blowpipe into the study of minerals. This was a long, narrowing tube which, when blown into at the wide end, produced a concentrated jet of air at the narrow end. This jet, directed into a flame, increased its heat.
When the heated flame impinged on minerals, information concerning the nature and composition of the mineral could be gathered from the color of the flame, the nature of the vapors formed, the oxides or metallic substances left behind, and so on. The blowpipe remained a key tool in chemical analysis for a century.
Enough knowledge was gained about minerals through new techniques such as that of the blowpipe, the Cronstedt felt justified in suggesting that minerals be classified not only according to their appearance but also according to their chemical structure. A book detailing this new form of classification was published in 1758.
This work was carried further by another Swedish mineralogist, Torbern Olof Bergman (1735-1784). Bergman evolved a theory to explain why one substance reacted with a second but not with a third. He supposed the existence of "affinities" (that is, attractions) between substances in varying degree. He prepared elaborate tables listing various affinities, and these tables were very influential during his lifetime and for a few decades afterward.
Scheele, who began life as an apothecary's apprentice, attracted the attention of Bergman, who befriended and sponsored him. Scheele discovered a variety of acids, including tartaric acid, citric acid, benzoic acid, malic acid, oxalic acid, and gallic acid in the plant kingdom; lactic acid and uric acid in the animal; and molybdic acid and arsenious acid in the mineral.
He prepared and investigated three highly poisonous gases; hydrogen fluoride, hydrogen sulfide, and hydrogen cyanide. (His early death is supposed to have been the result of slow poisoning by the chemicals he works with, which he would routinely taste.)
Scheele was involved in the discovery of most of the elements for which credit is given to his Swedish friends. Most important of all, he prepared oxygen and nitrogen in 1771 and 1772. He prepared oxygen by heating a number of substances that held it loosely, including the mercuric oxide used by Priestley, a couple of years afterward.
Scheele described his experiments carefully but, through the negligence of his publisher, the descriptions did not appear in print until 1777. By that time the work of Rutherford and Priestley had appeared, and they gained the credit for the discoveries.
The Triumph of Measurement
As the eighteenth century drew toward its close, the numerous important discoveries made in connection with gases needed to be drawn together into some over-all theory. The man to do that was on the scene. He was the French chemist Antoine Laurent Lavoisier (1743-1794).
From the very beginning of his chemical researches, Lavoisier recognized the importance of accurate measurement. This, his first important work, in 1764, lay in an investigation of the composition of the mineral gypsum. This he heated to drive off the water content, and then measured the quantity of water so given off. He joined the company of those who, like Black and Cavendish, were applying measurement to chemical change. Lavoisier, however, went about it more systematically, and used it as a tool with which to break down the ancient theories which were no longer useful and which merely encumbered, if they did not stifle, chemical advance.
There were still some, for instance, who, even in 1770, clung to the old Greek notion of the elements and held that transmutation was possible because water could be turned to earth on long heating. This supposition seemed reasonable (even to Lavoisier at first) for water, heated for a period of many days in a glass container, did develop a solid sediment.
Lavoisier decided to test this alleged transmutation by more than eyesight. For 101 days he boiled water in a device that condensed the water vapor and returned it to the flask so that no substance was permanently lost in the course of the experiment. And, of course, he did not forget to measure. He weighed both water and vessel before and after the long period of boiling.
The sediment did appear, but the water did not change its weight during the boiling. Therefore, the sediment could not have been formed out of the water. However, the flask itself, once the sediment had been scraped away, proved to have lost weight, a loss just equal to the weight of the sediment. In other words, the sediment was not water turning to earth, it was material from the glass, slowly etched away by the hot water and precipitated in solid fragments. Here was a clear example where measurement could lead to a demonstration of reasonable fact, while the testimony of the eyes alone led to a false conclusion.
Lavoisier was interested in combustion, first, because it was the great chemical problem of the eighteenth century, and second, because one of his early triumphs had been an essay in the 1760's on improved methods for street-lighting. He began in 1772, when he clubbed together with other chemists to buy a diamond which he then heated in a closed vessel until it disappeared. Carbon dioxide was formed, the first clear demonstration that diamond was a form of carbon and therefore closely related to, of all things, coal.
He went on to heat metals such as tin and lead in closed containers, with a limited supply of air. Both metals formed a layer of "calx" on the surface up to a certain point and then rusted no further. The phlogistonists would say that the air had now absorbed all the phlogiston from the metal that it could hold. As was well known, however, the calx weighed more than the metal itself, and yet when Lavoisier weighed the entire vessel (metal, calx, air, and all) after the heating, it weighed precisely the same as it had before the heating.
It followed from this result that if the metal had gained weight on being partially turned to a calx, then something else in the vessel must have lost an equivalent amount of weight. That something else, it seemed, would have to be air. If that were so, then a partial vacuum must exist in the vessel. Sure enough, when Lavoisier opened the vessel, air rushed in. Once that had happened, the vessel and its contents proved to have gained in weight.
Now it was possible for him to advance a new explanation for the formation of metals from ores. Ores were a combination of metal and gas. When an ore was heated with charcoal, the charcoal took the gas from the metal, forming carbon dioxide and leaving the metal behind.
Thus, whereas Stahl said the process of smelting involved the passage of phlogiston from charcoal to ore, Lavoisier said it involved the passage of gas from ore to charcoal. But were not these two explanations the same thing, with one equal to the other backwards? Was there any reason to prefer Lavoisier's explanation to Stahl's? Yes, there was, for by Lavoisier's theory of gas-transfer, one could explain the weight changes that resulted in combustion.
A calx was heavier than the metal from which it formed, by the weight of the added portion of the air. Wood also burned through addition of air to its substance, but it did not appear to gain weight, because the new substance formed (carbon dioxide) was itself a gas and vanished into the atmosphere.
The ash left behind was lighter than the original wood. If wood were burned in a closed vessel, the gases formed in the process would remain in the system, and then it could be shown that the ash, plus the vapors formed, plus what was left of the air, would retain the original weight of wood plus air.
In fact, it seemed to Lavoisier in the course of his experiments that if all the substances taking part in a chemical reaction and all the products formed were taken into consideration, there would never be a change in weight (or, to use the more precise term of the physicists, a change in mass).
Lavoisier maintained, therefore, that mass was never created or destroyed, but was merely shifted from one substance to another. This concept is the law of conservation of mass, which served as the very cornerstone of nineteenth-century chemistry. With the opening of the twentieth century, this law was shown to be incomplete, but the correction made necessary by the increased sophistication of twentieth-century science is an extremely small one and can be neglected in the ordinary reactions occurring in the chemical laboratory.
Lavoisier's achievements through the use of measurement were so great, as you can see, that chemists accepted the principle of measurement wholeheartedly from his time forward.
Lavoisier was not yet entirely satisfied. Air combined with metal to form a calx and with wood to form gases, but not all the air combined in this fashion. Only about a fifth of it did. Why was this?
Priestley, discoverer of "dephlogisticated air", visited Paris in 1774 and described his discoveries to Lavoisier. Lavoisier saw the significance at once and in 1775 published his views.
Air is not a simple substance, he said, but is a mixture of two gases in a 1 to 4 proportion. One-fifth of the air was Priestley's "dephlogisticated air" (though Lavoisier unfortunately neglected to give Priestley due credit). It was this portion of the air, and this portion only, that combined with burning or rusting materials, that was transferred from ore to charcoal, and that was essential to life.
It was Lavoisier who gave this gas its name, oxygen. This was from Greek words meaning "acid producer", Lavoisier having the idea that oxygen was a necessary component of all acids. In this, as it turned out, he was mistaken.
The remaining four-fifths of the air, which could not support combustion or life (Rutherford's "phlogisticated air"), was a separate gas altogether. Lavoisier called it "azote" (from Greek words meaning "no life") but later the term nitrogen replaced it. This word means "forming niter", since niter, a common mineral, was found to contain nitrogen a part of its substance.
Lavoisier was convinced that life was supported by some process that was akin to combustion (In this, he proved to be right.), for we breathe in air rich in oxygen and low in carbon dioxide, but breathe out air that is lower in oxygen and considerably richer in carbon dioxide. He and a co-worker, Pierre Somon de Laplace (1749-1827) - who was later to become a famous astronomer - attempted to measure the oxygen taken in and the carbon dioxide given off by animals. The results were puzzling, for some of the oxygen that was inhaled did not appear in the carbon dioxide exhaled.
In 1783 Cavendish was still working with his inflammable gas. He burned some of it and studied the consequences. He found that the vapors produced by the burning condensed to form a liquid that, on investigation, proved to be nothing more nor less than water.
This was a crucially important experiment. In the first place, it was another hard blow at the Greek theory of the elements, for it showed that water was not a simple substance but was the sole product of the combustion of two gases.
Lavoisier, hearing of the experiment, named Cavendish's gas, hydrogen ("water-producer") and pointed out that hydrogen burned by combining with oxygen and that therefore water was a hydrogen-oxygen combination. It seemed to him that the substance of food and of living tissue contained both carbon and hydrogen in combination, so that when air was inhaled the oxygen was consumed, not only by forming carbon dioxide out of carbon, but also water out of hydrogen. This explanation disclosed the fate of that part of the oxygen he had not been able to account for in his early experiments on respiration.
(In his theories Lavoisier had been anticipated by a Russian chemist, Mikhail Vasilievich Lomonosov (1711-1765) who, in 1756, nearly twenty years before Lavoisier's work on combustion, had rejected the phlogiston theory and had suggested that objects combined with a portion of the air on burning. Unfortunately, he published in Russian, and the chemists of western Europe, including Lavoisier, were unaware of his work. Lomonosov also had startlingly modern views on atoms and on heat, which were fifty to a hundred years ahead of his time. He was a most remarkable man who suffered under the misfortune of having been born in eastern Europe at a time when scientific advance was concentrated in western Europe).
Lavoisier's new theories involved a complete rationalization of chemistry. All mysterious "principles" were done away with. Henceforward, only materials that could be weighed or otherwise measured were of interest to the chemist.
Having established this foundation, Lavoisier went on to raise the superstructure. During the 1780's, in collaboration with three other French chemists, Louis Bernard Guyton de Morveau (1737-1816), Claude Louis Berthollet (1748-1822), and Antoine Francois de Fourcroy (1755-1809), he worked out a logical system of chemical nomenclature. This was published in 1787.
No longer was chemistry to be a farrago of names as in alchemical days, each writer using his own system and puzzling everyone else. There was to be a recognized system that all were to use; a system based on logical principles, so that one could tell from the name of a compound the elements that made it up. For instance, calcium oxide was made up of calcium and oxygen; sodium chloride of sodium and chlorine; hydrogen sulfide of hydrogen and sulfur; and so on.
A careful system of prefixes and suffixes was set up so that one could tell something about the proportions in which the different elements were present. Thus, carbon dioxide contained more oxygen than did carbon monoxide. Again, potassium chlorate contained more oxygen than potassium chlorite, while potassium perchlorate contained still more oxygen, while potassium chloride contained no oxygen at all.
In 1789, Lavoisier publish a book (Elementary Treatise on Chemistry) which served to supply the world with a unified picture of chemical knowledge on the basis of his new theories and nomenclature. It was the first modern chemical textbook.
Among other things, the book included a list of all the elements known up to that time (or, rather, all the substances which Lavoisier judged to be elements on the basis of Boyle's criterion - that they could not be broken down to simpler substances). It is a credit to Lavoisier's judgement that in the thirty-three items he listed, only two were completely wrong. These were "light" and "caloric" (heat), which, as was to become plain in the decades after Lavoisier, were not material substances at all, but forms of energy.
|New Names||Old Names|
|Caloric||Heat - Principle or element of heat - Fire. Igneous fluid - Matter of fire and of heat|
|Oxygen||Dephlogisticated air - Empyreal air - Vital air, or base of vital air|
|Azote||Phlogisticated air or gas - Mephitis, or its base|
|Hydrogen||Inflammable air or gas, or the base of inflammable air|
|Phosphorus||The same names|
|Fluoric radical||Still unknown|
|New Names||Old Names|
|New Names||Old Names|
|Lime||Chalk, calcareous earth - Quicklime|
|Magnesia||Magnesia, base of Epsom salt - Calcined or caustic magnesia|
|Barytes||Barytes, or heavy earth|
|Argill||Clay, earth of alum|
|Silex||Siliceous or vitrifiable earth|
Of the remaining thirty-one items, some were indeed elements according to present views. These included the substances, such as gold and copper, that had been known to the ancients, as well as others, such as oxygen and molybdenum, that had been discovered only in the years just prior to the publication of Lavoisier's book. Eight of the substances listed (lime and magnesia, for example) are no longer accepted as elements because, since Lavoisier's time, they have been broken down into simpler substances. In every case, however, one of those simple substances proved to be a new element.
There was some opposition to the new views of Lavoisier (views that have been retained to the present time), notably from some diehard phlogistonists, Priestley among them. Others, however, accepted the new chemistry enthusiastically. Bergman, in Sweden, was one of these. In Germany, the chemist Martin Heinrich Klaproth (1743-1817) was an early convert. His acceptance of Lavoisier's view was important for, since Stahl had been a German, there was a tendency among Germans to cling to phlogiston as a patriotic gesture. (Klaproth made his name later as a discoverer of elements. He discovered uranium and zirconium, in 1789).
In the same year that Lavoisier's textbook was published, the French Revolution broke out, degenerating quickly into wild excesses of the Terror. Lavoisier, unfortunately, was connected with a tax-collecting organization that the revolutionists considered a vicious tool of the hated monarchy. They executed, by guillotine, all its officers whom they could seize. One of them was Lavoisier.
In 1794, then, this man, one of the greatest chemists who ever lived, was needlessly and uselessly killed in the prime of life. "It required only a moment to sever that head, and perhaps a century will not be sufficient to produce another like it," said Joseph Lagrange, the great mathematician. Lavoisier is universally remembered today as "the father of modern chemistry".