STORM TRACK: March 31, 1982 (Volume 5 Issue 3)

With the coming of instrumentation during the 15th through 17th centuries, science began to move forward as it hadn't since the time of Aristotle. And while meteorologists went about the business of defining the atmosphere, chemists were involved in their own projects, one of which was discovering the nature of air. Now a legitimate question might be 'what does the chemistry of air have to do with thunderstorm forecasting?' The answer is probably 'very little.' But the story is an interesting one, and those fascinated with forecasting might also have an interest in air. On this tenuous thread, the following is presented.

The process of unraveling the constituents of the atmosphere began, as do many other scientific investigations, with the ancient Greeks. Between the 6th and 4th centuries B.C., Greek philosophers were embroiled in a great controversy concerning the nature of the Universe. All seemed to agree that some basic substance existed, a primitive sort of material from which all matter was composed, but agreement could not be reached as to what the material was. Convincing arguments were presented for water, for air, and for fire. Ideally, in about the middle of the 4th century B.C., Empedocles suggested a compromise; namely, that some forms of 'water', 'air', and 'fire' as well as some solid, such as 'earth' (an addition of his own) might all logically be considered as the basic components of matter. This suggestion provided the basis for the doctrine of the 'four elements.' Various combinations of these four basic elements were thereafter utilized to describe all substances found in nature. The subsequent acceptance of this concept by Aristotle caused the elemental 'oneness' of air to become a casually accepted fact, by even the most careful scientists, for the next 2,000 years!

The modern reader is often tempted to entertain smug thoughts when reading early scientific notions, but cautious thought might be more appropriate. The theories presented were postulated by the most learned men of the time and were limited by the state of their art. Modern readers, though possessing more refined notions on the nature of the Universe, would be hard put to perform the demonstrations requisite to illustrating those principles we now accept as 'true'. Furthermore, some of these ancient ideas were not, as far wrong as at first glance they might seem. Suppose, for example, one considers the basic properties of each of the Greeks 'four elements'. With a little thought, it can be seen that these men actually chose a rather logical scheme for categorizing matter; namely: solids, liquids, gases, and energy. In lieu of later sophisticated data, could more be expected?

Eventually, however, repeated purposeful experimentation toppled erroneous theory. Alchemists had worked throughout the Dark Ages, attempting to create gold and had thereby performed uncountable chemical experiments on various solids ('earth'). Certain substances (e.g. copper, iron, etc.) had resisted centuries of attempts to break them into simpler substances. Others (e.g. bronze, etc.), however, could be resolved into more basic forms. As the Renaissance approached, an updating of chemical theory became inevitable. The first major break with Aristotelian dogma came when the chemist Robert Boyle (1627-91) published a new definition of the term 'element.' He decided to call a substance an element if, in actual experimentation, it could not be broken into two or more simpler substances. Thus, the term element went from an untested abstract to a more comfortable reality.

Various interesting aspects of 'air' were also being noted at about this time. The Flemish chemist van Helmont (1579-1644) noted that the 'air' given off by heated water must be different than ordinary 'air,' since it could be turned back into water. Thus, he gave the water byproduct a different name: vapor. This represented a major deviation from the Greek ideas and, for its time, was a courageous step. But the real breakthrough came via the substance called Phlogiston.

About 1700, a theory of combustion was published by the German chemist G.E. Stahl (1660-1734) which, though itself one of the more famous 'near-misses' of science, led eventually to the unraveling of the composition of air. His theory, briefly stated, suggested that combustibles were capable of burning because they contain a substance called 'Phlogiston' (Greek 'to set fire'). As a material burns, it gives off phlogiston as visible fire. Air became an important part of his process by acting as a Phlogiston receiver. Stahl also showed that the rusting process is merely a slow 'burning' of metal. In rusting, he felt, Phlogiston is passed to the air very slowly; and while normal metal can be combusted, rust cannot.

In attempting to isolate Phlogiston, Joseph Black (1728-99) heated limestone and found that an 'air' was given off which, when exposed to pure lime, recombined to form limestone again. Now 'ordinary' air did not have this property, so -behold- another new gas. Because this new gas could be 're-fixed' to lime, Black called it 'fixed air' as opposed to 'ordinary air.' It was further noted that fixed air would not accept phlogiston (i.e. would not support combustion). Black proposed that fixed air might actually be 'Phlogisticated air', or air filled to capacity with phlogiston. But the chemist Daniel Rutherford (1749-1819) was suspicious. He decided to make some 'phlogisticated air' by burning a candle in a closed container until the flame expired, then compare the result to Black's fixed air. The two gases were found to exhibit very different proper- ties. Furthermore, when he next removed fixed air from his phlogisticated air sample (by bubbling it through a lime solution), his theory was confirmed. Fixed air and phlogisticated air were two different gases. Black stumbled across the fact that when water condenses, it gives off a small amount of heat in the process.

In the early 1770s, an amateur scientist named Joseph Priestly (1733-1804) stumbled upon the fact that when mercury is heated in air it rusts, and that when this rust is heated in a test tube, it breaks down to mercury again, giving off a rather unusual new gas in the process. He found that this particular new gas supported combustion better than air and must, therefore, be a much more efficient phlogiston receiver. He suggested that ordinary air must contain a certain amount of phlogiston by nature, while the new gas must be completely 'dephlogisticated' air. Another new gas had been found.

However, the phlogiston theory was about to fall into disrepute. The French chemist Antoine Lavoisier (1734-94), emphasized careful and accurate observation in his work. Consequently when he began to study the rusting process, he soon learned that rusted metal GAINS WEIGHT. This was completely unexpected. It implied that if a substance (phlogiston) was given off during rusting, that it, must have negative weight. So Lavoisier tested for negative weight. He repeated the rusting experiment but utilized a sealed container. Again, he found that, the rusted metal weighed more, but he also found that the weight of the metal plus that of the air remained constant.' Two possibilities suggested themselves; (1) Negatively weighted phlogiston might be mixing with the air to reduce its weight by the amount gained by the metal; or (2) Some of the air (i.e. some component of the air) might be combining with the metal during the rusting process. Which was true? Well, when Lavoisier had completed the experiment and opened the sealed container, air had rushed IN. It was evident that the volume of gas in the container had, therefore, decreased and that some component of air had combined with the metal during combustion. The concept of phlogiston became unnecessary.

An extremely bothersome fact regarding combustion still troubled Lavoisier, however. It seemed that no matter what techniques he employed, only about one fifth part of a given volume of air could be made to combine in burning. It was not until 1774 that he finally realized the combination of gasses that made up ordinary air. He went on to show that air was composed of (by volume) about 78% Rutherford's 'phlogisticated' air (now called Nitrogen) and 21% Priestly's 'dephlogisticated' air (oxygen). It had already been shown by Black that air contained a small percentage of 'fixed air' (now called carbon dioxide) and, in subsequent years, trace quantities of other gasses were dis- covered to be part os this 'ordinary air.'