James Richard Fromm
Between 1500 and 1600, we enter perhaps the most interesting phase of the development of chemical concepts, a period of transition which represents the transition from earlier ideas, and from alchemy, to a chemistry whose ideas are essentially those of modern chemistry. This occurred by about 1800, so that this period begins with Johann Van Helmont and ends with Antoine L. Lavoisier (AD 1743 - 1794). Much of this study involved gases and the nature of combustion. The erroneous concept of phlogiston is perhaps the best-known concept of the period.
Johann Baptista Van Helmont (1579-1644) of Brussels studied several subjects before finally choosing to follow a career in medicine. He became a medical chemist as well as an ardent follower of Paracelsus. The chemical side of medicine finally took over from the medical side and he devoted much of his life to chemical experiments. He has been described as the last alchemist and the first chemist, because although he believed in alchemy, including the production of gold from lead which he claimed to have performed, his emphasis upon experiment rather than argument is a great advance.
Van Helmont is an important figure in the development of chemical concepts because it is impossible to separate an understanding of the nature of air from an understanding of the nature of combustion, or burning in air. Air had been considered by Aristotle and the Greek philosophers as one of the four elements, with real "airs" or what we call gases all being more or less contaminated ideal air. The concept of different gases was not clearly understood; all known gases or vapors were considered as different mixtures of air and earth or air and fire. This Greek understanding of the nature of air persisted through the Middle Ages and through the period of alchemy.
Van Helmont was probably the first to recognize, and was the first to state in print, that there existed or could be created several specific different kinds of gas each with different properties. Indeed, the word gas was first used by Van Helmont. Many of the 15 or so "different" gases mentioned by Van Helmont we now know to be mixtures of gases, or were carbon dioxide obtained in different ways, but the major advance of recognition of the existence, and some of the properties, of different gases we owe to him. We owe also other advances in quantitative measurements, including the use of the balance for precise weighing, to him, as well as some advances in medicine. (Van Helmont is also known for an experiment in which he planted a tree in a pot of earth and weighed both the earth and the tree after five years. Since the weight of the earth had decreased by at most a few ounces while the weight of the tree was about 170 pounds, Van Helmont concluded that the tree arose from water only, since he had added nothing else to the pot!) The period of Van Helmont's chemical contributions began in the seventeenth century, probably around 1609, but his work was not generally published until 1648.
Robert Boyle (1627 - 1691) is best known as the discoverer of Boyle's Law which relates the pressure upon a sample of gas to its volume, as we shall see in Chapter 6. Boyle was also responsible for the first clear statement of the modern definition of a chemical element: a chemical element is a pure substance which cannot be broken down into any simpler substance by chemical means.
We now know that the air we breathe is a mixture of gases, primarily oxygen and nitrogen. In the eighteenth century, however, the discovery that air was such a mixture, and the characterization of its components, was modern chemical research. Van Helmont's understanding that there were different gases with different chemical properties led to attempts to separate gases from air and react gases with air. These studies are the basis of our modern understanding of the nature of the atmosphere.
The erroneous doctrine of phlogiston, introduced by Georg Ernest Stahl, enabled chemists to explain metal reduction and oxidation by the same mechanism. It was held by most eighteenth-century chemists including Joseph Priestley (1733 - 1804) and the Swedish apothecary Carl Wilhelm Scheele (1742 - 1786). The doctrine of phlogiston did not fade until its replacement by the oxygen chemistry approach of Lavoisier and his colleagues around 1800.
Although Scheele and Priestley did not use a symbol for phlogiston, it is convenient for us to write chemical reactions using symbols. If phlogiston is symbolized by X, the oxidation or rusting of iron as understood by Priestley would be written Iron --> Calx of iron + X. The reaction of carbon is similar, Carbon --> Calx of carbon + X. The reduction of the calx of tin can be, then, Calx of tin + X --> Tin, and the description of the actual smelting process of tin or iron is:
Calx of tin + Carbon Tin + Calx of carbon
Calx of iron + Carbon Iron + Calx of carbon
Scheele interpreted his experiments in the light of this phlogiston theory. He thought that hydrogen might be phlogiston, or perhaps a compound of phlogiston with some unknown substance, for it was clear that many metals could be used in the reaction:
Calx of metal + Hydrogen Metal + Heat.
For example, when Scheele burned a hydrogen flame in a glass globe of air standing over water, he observed that the water rises. He reasoned, correctly, that this meant that combustion must use up the fire air (oxygen). His further interpretation of this experiment is an interesting piece of erroneous reasoning: since no product of this reaction was observed other than heat (water did not condense since hot water was used in his water bath), the reaction taking place would have to be fire air + X heat. It is therefore reasonable to attempt to reverse the reaction, to decompose heat and produce fire air. To do this, one must present to heat a substance having a greater attraction for phlogiston than does fire air. Such a substance is nitric acid, because it readily attacks metals taking out their phlogiston:
(Calx of M + X) + nitric acid Calx of M + (nitric acid + X)
where (Calx of M + X) is equivalent to the metal M itself and (nitric acid + X) is taken as the red fumes of nitrogen dioxide, NO2(g). Then, since nitric acid boils away on heating, one must fix it by combination with potash:
H+NO3- + KOH KNO3 + H2O
and then distill it with sulfuric acid ("oil of vitriol", H2SO4) in a retort:
4KNO3 + H2SO4 4NO2 + O2 + 2K2SO4 + H2O
If one then absorbs the nitrogen dioxide gas in limewater, while the K2SO4 remains behind in the flask, an empty bladder can be filled with the fire air gas. This erroneous interpretation of a correctly planned experiment is worthy of some thought on the part of the reader.
Other methods of preparing oxygen were also used by Scheele, who interpreted the results of heating the red oxide of mercury according to the phlogiston theory using (fire air + X) as equivalent to heat and (calx of mercury + X) as equivalent to mercury metal:
Calx of mercury + (X + fire air) fire air + (calx of mercury + X)
The work of chemists from Boyle onward has been based on the idea that a chemical element is a pure substance which can not be broken down into any simpler substances by chemical means. Although we now know that nuclear reactions can actually convert one element into another, these lie outside the normal province of chemistry. Most of the substances found on earth are mixtures containing several different substances. Mixtures can be separated by physical means into the different pure substances which are their components. The trinity of principles of the alchemists, mercury, sulfur, and salt, could all be prepared as pure substances, as could water and metals. Reactions of pure metals with air produced pure metal oxides, and reactions of these oxides with water produced other pure compounds. Through the seventeenth century, it became clear that most pure substances are not elements but compounds. Compounds could be made or synthesized from their pure component elements, and in many cases broken back down to elements again or analyzed. The number of pure compounds known grew quickly, and the number of elements known grew more slowly, through both the seventeenth and eighteenth centuries.
Pure substances, both elements and compounds, engage in well-defined chemical reactions with each other to produce new pure substances or, in many cases, mixtures of substances. Pure substances which engage in chemical reactions came to be known as reagents. Substances of the highest available purity, which are the most desirable for the quantitative study of chemical reactions, became known as reagent-grade materials. The specifications for modern reagent-grade chemicals are published in most countries by their national governmental standards office or by their national society of chemists because they are so important.