Despite its advance, chemical knowledge in certain respects lagged behind other branches of science.

In astronomy the importance of quantitative measurements and of the application of mathematical techniques had been understood since ancient times. One reason was that the astronomical problems tackled by the ancients were relatively simple, and certain of them could be handled reasonably well even with plane geometry.

The application of mathematics and of careful measurement to physics was dramatized by the Italian scientist Galileo Galilei (1564-1642), who, in the 1590's, studied the behavior of falling bodies. The results of his work led, nearly a century later, to the important conclusions of the English scientist Isaac Newton (1642-1727). In his book Principia Mathematica, publish in 1687, Newton introduced his three laws of motion, which served for over two centuries as the basis of the science of mechanics. In the same book Newton advanced his theory of gravitation, which also served for more than two centuries as an adequate explanation of the workings of the universe and holds true today within the limits of our personal observations and attainable velocities. In connection with this theory, he made use of calculus, a new and powerful branch of mathematics which he himself had worked out.

The Scientific Revolution reached its climax in Newton. There was no question thereafter of deferring to the Greeks or to any of the ancients. Western Europe had far surpassed them and there was to be no more looking back.

But an equivalent change from mere qualitative description to careful quantitative measurement did not take place in chemistry for a full century after Newton's climactic work. In fact, while Newton was building the modern structure of astronomy and physics with a beauty and solidity that amazed the scientific world, he remained immersed in alchemy. He sought ardently throughout Europe for recipes whereby he might make gold by transmutation.

This persistence in the wrong approach was not entirely the fault of chemists. If they were slow to adopt the quantitative mathematical techniques of Galileo and Newton, it was because the material they dealt with was more difficult to represent in a fashion simple enough to be amenable to mathematical treatment.

Nevertheless, chemists made progress, and faint signs of a forthcoming chemical revolution were not wanting, even in Galileo's time. Such signs were present in the work of a Flemish physician, Jan Baptista Van Helmont (1577-1644). He grew a tree in a measured quantity of soil, added water periodically, and carefully weighed the tree as it grew. Since he hoped to discover the source of the living tissue formed by the tree, he was applying measurement to a problem in chemistry, and in biology as well.

Until Van Helmont's time the only air-like substance known and studied was air itself, which seemed sufficiently distinctive and unlike other substances to serve as one of the elements of the Greeks. To be sure, alchemists had frequently obtained "airs" and "vapors" in their experiments, but these were elusive substances that were hard to to study and observe, and easy to ignore.

The mystery of these vapors was implicit in the very name given to liquids that vaporized easily. They were termed "spirits", a word originally meaning "breath" or "air", but carrying also an obvious sense of the mysterious and even of the supernatural. We still speak of "spirits of alcohol" and "spirits of turpentine". Alcohol is so much the oldest and best-known of the volatile liquids that "spirits" has come to refer to alcoholic liquors in particular.

Van Helmont was the first chemist to consider the vapors he produced and to study them. He found that they resembled air in physical appearance but not in all properties. In particular, he obtained vapors from burning wood that resembled air, but did not behave quite like air.

To Van Helmont these air-like substances, without fixed volume or fixed shape, were something akin to the Greek "chaos"; the original material, unshaped and unordered, out of which the Universe (according to Greek myth) was created. Van Helmont called the vapors by the name of "chaoes", but spelled the word in accordance with its phonetic sound in Flemish, which made it gas. This word is still used today for all air-like substances.

The particular gas which Van Helmont obtained from burning wood and which he studied with particular care, he called "gas sylvestre" ("gas from wood"). It is what we call today carbon dioxide.

It was the study of gases, the simplest form of matter, that first lent itself to the techniques of careful measurement; it served as a highway to the world of modern chemistry.

Boyle's Law

Toward the end of Van Helmont's life, gases - air, in particular, since it was the most common gas - were attaining a new and dramatic importance. The Italian physicist Evangelista Torricelli (1608-1647) was able to prove, in 1643, that air exerted pressure. He showed that air could support a column of mercury thirty inches high and, in so doing, he invented the barometer.

Gases at once became less mysterious. They were matter, possessing weight as did the more easily studied liquids and solids. They differed from liquids and solids chiefly in their much lower density.

The pressure exerted by the weight of the atmosphere was demonstrated in an astonishing manner by the German physicist Otto von Guericke (1602-1686). He invented an air pump with which he could pull the air out of containers, so that the air pressure on the outside was no longer equalized by air pressure on the inside.

In 1654 Guericke prepared two metal hemispheres that fit together along a greased flange. When the hemispheres were put together and the air within was removed by the air pump, air pressure from without held the hemispheres together. Teams of horses attached to each of the hemispheres and whipped into straining their utmost in opposite directions, could not pull the hemispheres apart. When air was allowed to re-enter the joined hemispheres, however, they fell apart of themselves.

Demonstrations such as this roused great interest in the properties of air. In particular, the curiosity of an Irish chemist, Robert Boyle (1627-1691), was roused. He devised an air pump of his own that was even better than Guericke's. Then having, so to speak, pulled air apart in sucking it out of the container, he went on to try the opposite procedure of compressing it - that is, of pushing it together.

In his experiments Boyle found that the volume of a sample of air varied with pressure according to a simple inverse relationship. He discovered this by dropping mercury into a very long, specially constructed tube and trapping a sample of air in the short, closed end which was fitted with a stopcock. By adding more mercury to the long open end he could increase the pressure on the trapped air. If he added enough mercury to place the trapped air under doubled pressure ( a doubled weight of mercury), the volume of the trapped air was halved. If pressure was tripled the volume was reduced to a third. On the other hand, if pressure was eased off the volume expanded. This relationship whereby volume decreased in proportion as pressure increased was first published in 1660 and is still referred to as Boyle's Law.

This was the first attempt to apply exact measurement to changes in a substance of particular interest to chemists. It must be pointed out, though, that the change studied by Boyle was not a chemical one. Air, however, it might be compressed or expanded, remains air. Such change in volume is a physical change. He was therefore involve in physical chemistry, the study of the physical changes of chemicals. This was not to come into its own for two centuries after the time of Boyle, but he laid the groundwork.

Boyle did not specify that temperature must be held constant if Boyle's law is to be valid. Probably he realized this and supposed it would be taken for granted. The French physicist Edme Mariotte (1630-1684), who discovered Boyle's Law independently, about 1680, did specify that temperature must be held constant. For this reason, Boyle's Law is often referred to as Mariott's law in continental Europe.

Boyle's experiments offered a focus for the gathering numbers of atomists. As stated earlier, Lucretius's poem, introduced in a printed edition, had brought Greek views on atomism to the attention of European scholars. A French philosopher, Pierre Gassendi (1592-1655), was a convinced atomist as a result, and his writings impressed Boyle, who thereupon also became an atomist.

As long as one concentrated on liquids and solids only, the evidence for atomism was no better in Boyle's time than in that of Democritus. Liquids and solids cannot be compressed by more than insignificant amounts. If they consist of atoms, those atoms must be in contact and cannot be pushed closer together. It is therefore hard to argue that liquids and solids must be made up of atoms, for if they were made up of continuous substances they would also be very difficult to compress. Why bother with atoms, then?

Air as had been observe even in ancient times, and as Boyle had now made dramatically clear, can easily be compressed. How could this be unless it consisted of tiny atoms separated by empty space? Compressing air simply would mean, from that point of view, the squeezing of empty space out of the volume, pushing the atoms closer together.

If this view of gases is accepted it becomes easier to believe that liquids and solids are composed of atoms, too. For instance, water evaporates. How can that be unless it disappears tiny bit by tiny bit, and what could be simpler, then, than to suppose that it turns into vapor atom

by atom? If water is heated it boils and vapor is visibly formed. The water vapor has the physical properties of an air-like substance and therefore, it is natural to suppose, is composed of atoms. But if water is composed of atoms in its gaseous form, why not in its liquid form as well, and in its solid form of ice? And if this is true of water, why not of all matter?

Arguments of this sort were impressive, and for the first time since atoms were first imagined two thousand years before, atomism began to win numerous converts. Newton, for instance, became an atomist.

Nevertheless, atoms remained a misty concept. Nothing could be said about them except that if they were assumed to exist, it was easier to explain the behavior of gases. Another century and a half had to pass before atomism came into sharp focus.

The New View of Elements

Boyle's career marks the passing of the terms "alchemy" and "alchemist". Boyle dropped the first syllable of the term in writing a book, The Sceptical Chymist, publish in 1661. From that time on, the science was chemistry and workers in the field were chemists.

Boyle was "sceptical" because he was no longer willing to accept, blindly, the ancient conclusions that had been deduced from first principles. In particular, Boyle was dissatisfied with ancient attempts to identify the elements of the universe by mere reasoning. Instead, he defined elements in a matter-of-fact, practical way. An element, it had been considered ever since Thales' time, was one of the primal simple substances out of which the universe was composed. Well, then, a suspected element must be tested in order to see if it were really simple. If a substance could be broken into simpler substances it was not an element, but the simpler substances might be - until such time as chemists learned to break them down to still simpler substances.

Furthermore, if two substances were each an element, they might be intimately combined to form a third substance called a compound. If so, then that compound should lend itself to breakdown into the two original elements.

The term "element" in this view, had only a practical meaning. A Substance such as quartz, for instance, could be considered an element until such time as experimental chemists discovered a way of converting it into two or more still simpler substances. In fact, no substance could ever be an element except in a provisional sense, according to this view, since one could never be certain when advancing knowledge might make it possible to devise a method for breaking down a supposed element into still simpler substances.

It was not until the coming of the twentieth century that the nature of elements could be defined in a non-provisional sense.

The mere fact that Boyle wanted an experimental approach in defining elements (an approach that was adopted eventually) does not mean that he knew what the different elements were. It might have turned out, after all, that the experimental approach would indeed have proved Greek elements of fire, air, water, and earth to be elements.

Boyle was convinced, for instance, of the validity of the alchemical viewpoint that the various metals were not elements and that one metal could be converted into another. In 1689, he urged the British government to repeal the law against the alchemical manufacture of gold (they, too, feared the upset to the economy) because he felt that by forming gold out of base metal, chemists could help to prove the atomic view of matter.

But Boyle was wrong there; the metals did prove to be elements. In fact, nine substances which we now recognize as elements had been known to the ancients; the seven metals (gold, silver, copper, iron, tin, lead, and mercury) and two non-metals (carbon and sulfur). In addition, there were four substances now recognized as elements that had become familiar to the medieval alchemists: arsenic, antimony, bismuth, and zinc.

Boyle, himself, came within a hair of being the discoverer of a new element. In 1680 he prepared phosphorus from urine. Some five to ten years before that, however, the feat had been accomplished by a German chemist, Hennig Brand (?-1692). Brand is sometimes called the "last of the alchemists", and, indeed, his discovery came while he was searching for the philosopher's stone which he thought he would find in (of all places) urine. Brand was the first man to discover an element that had not been known, in a least some form, before the development of modern science.


The seventeenth-century discoveries concerning air pressure and the unusual feat that one could perform by producing a vacuum and allowing air pressure to work, had important results. It occurred to several people that a vacuum might be formed without the use of an air pump.

Suppose you boiled water and filled a chamber with steam, then cooled the chamber with cold water on the outside. The steam within the chamber would condense into a few drops of water, and a vacuum would exist in its place. If one of the walls of the chamber were movable, air pressure on the other side would then drive that wall into the chamber.

The movable wall could be pushed outward again if more steam were formed and allowed to enter the chamber, and then be pushed inward again if the steam were once more condensed. If you imagine the movable wall to be part of a piston, you can see that the piston will move in and out and that this in-an-out motion could be used to run a pump, for instance.

By 1700, such a steam engine had actually been produced by an English engineer, Thomas Savery (c.1650-1715). It was a dangerous device because it used steam under high pressure at a time when high-pressure steam could not be safely controlled. However, another Englishman, Thomas Newcomen (1663-1729), working in partnership with Savery, devised a steam engine that would work on low-pressure steam. The device was improved and made really practical, toward the end of the eighteenth century, by the Scottish engineer James Watt (1736-1819).

The result of these labors was that, for the first time, mankind was no longer dependent upon its own muscles or upon the muscles of animals. Nor was man dependent upon the hit-or-miss force of the wind, or upon the spottily located energy of running water. Instead, he had a source of energy he could call upon at any time and in any place merely by boiling water over a wood or coal fire. This was the chief factor marking the start of the "Industrial Revolution".

The increasing interest from 1650 onward in the possibility of turning fire to new uses and, by way of the steam engine, making it do the heavy work of the world, brought to chemists a new awareness of fire. Why do some things burn and others not? What is the nature of combustion?

By old Greek notions something which could burn contained with itself the element of fire, and this something was released under the proper conditions. Alchemical notions were similar, except that a combustible was thought of as containing the principle of "sulfur" (though not necessarily actual sulfur).

In 1669, a German chemist, Johann Joachim Becher (1635-1682), tried to rationalize this notion further, by introducing a new name. He imagined solids to be composed of three kinds of "earth". One of these he called "terra pinguis" ("fatty earth"), and felt this to be the principle of inflammability.

A follower of Becher's rather vague doctrines was the German physician and chemist Georg Ernest Stahl (1660-1734). He advanced a newer name still for the principle of inflammability, calling it phlogiston, from a Greek word meaning "to set on fire". He went on to devise a scheme, involving phlogiston, that would explain combustion.

Combustible objects, Stahl held, were rich in phlogiston, and the process of burning involved the loss of phlogiston to the air. What was left behind after combustion was without phlogiston and therefore could no longer burn. Thus, wood possessed phlogiston, but ash did not.

Stahl maintained further that the rusting of metals was analogous to the burning of wood, and he considered a metal to possess phlogiston while its rust (or "calx") did not. This was an important insight, which made it possible to advance a reasonable explanation of the conversion of rocky ores into metals - the first great chemical discovery of civilized man. The explanation consisted of this: A rocky ore, poor in phlogiston, is heated with charcoal, which is very rich in phlogiston. Phlogiston passes from the charcoal into the ore, so that the phlogiston-rich charcoal is turned into phlogiston-poor ash, while the phlogiston-poor ore is turned into phlogiston-rich metal.

Air itself was considered by Stahl to be only indirectly useful to combustion, for it served only as a carrier, holding the phlogiston as it left the wood or metal and passing it on to something else (if something else were available).

Stahl's phlogiston theory met with opposition at first, notable from a Dutch physician, Hermann Boehaave (1668-1738), who argued that ordinary combustion and rusting could not be different versions of the same phenomenon.

To be sure, there is the presence of flame in one case and not in the other, but to Stahl the explanation was that in the combustion of substances, such as wood, phlogiston left so rapidly that its passage heated its surroundings and became visible as flame. In rusting the loss of phlogiston was slower and no flame appeared.

Despite Boerhaave's opposition, then, the phlogiston theory gained popularity throughout the eighteenth century. By 1780 it was almost universally accepted by chemists, since it seemed to explain so much so neatly.

Yet a difficulty remained that neither Stahl nor any of his followers could explain. Most combustible objects, such as wood, paper, and fat, seemed largely to disappear upon burning. The remaining soot or ash was much lighter than the original substance. This is to be expected, perhaps, since phlogiston had left that original substance.

However, when metals rusted, they also lost phlogiston, according to Stahl's theory, yet the rust was heavier than the original metal (a fact which had been noted by alchemists as early as 1490). Could phlogiston have negative weight, then, so that a substance that lost it was heavier than before, as some eighteenth-century chemists tried to maintain? If so, why did wood lose weight in burning? Were there two kinds of phlogiston, one with weight and one with negative weight?

This unanswered problem was not as serious in the eighteenth century as it seems to us today. We are used to measuring phenomena accurately, and an unexplained change in weight would disturb us. The eighteenth-century chemists, however, had not yet accepted the importance of accurate measurements, and they could shrug off the change in weight. As long as the phlogiston theory could explain changes in appearance and properties, changes in weight, they felt, could be ignored.