Читать книгу The Fontana History of Chemistry - William Brock J. - Страница 29

A rational reconstruction of what seem to have been the essential features of the ‘chemical revolution’ would draw attention to six necessary and sufficient conditions. First, it was necessary to accept that the element, air, did participate in chemical reactions. This was first firmly established by Hales in 1727 and accepted in France by Rouelle and Venel. Although Hales tried to explain the fixation of air by solids by appealing to the attractions and repulsions of Newtonian particle theory, there was no satisfactory explanation for its change of state. Secondly, it was necessary to abandon the belief that air was elementary. This was essentially the contribution of the British school of pneumatic chemists. Beginning in 1754 with Black, who showed that the ‘fixed air’ released from magnesia alba had different properties from ordinary air, and continuing through Rutherford, Cavendish and Priestley, it was found possible to prepare and study some twenty or more ‘factitious airs’ that were different from ordinary air in properties and density. Their preparation and study were made possible by the development of apparatus by Hales for washing air, the pneumatic trough, thus extending the traditional ‘alchemical’ apparatus of furnaces and still-heads that had hitherto largely sufficed in chemical investigations. Whether factitious airs were merely modifications of air depending upon the amounts of phlogiston they contained, or distinct chemical species in an aerial condition, or the expanded particles of solid and liquid substances, was decided by Lavoisier’s development of a model of the gaseous state.

The concept of a gas was a necessary third condition for the reconstruction of chemistry. By imaging the aerial state as due to the expansion of solids and liquids by heat, or caloric, Lavoisier brought chemistry closer to physics and made possible the later adoption of the kinetic theory of heat and the development of chemical thermodynamics. The balance pan had always been the principal tool of assayers and pharmacists, while the conservation of mass and matter had always been implicit in chemists’ rejection of alchemical transmutation and their commitment to chemistry as the art of analysis and synthesis. With the conceptualization of a whole new dimension of gaseous-state chemistry, however, it was necessary that chemical analysis and book-keeping should always account for the aerial state. Here was a fourth necessary condition that raised problems for phlogistonists when Guyton demonstrated conclusively in 1771 that metals increased in weight when they were calcined in air. Many historians, like Henry Guerlac, saw this as the ‘crucial’ condition for effecting a chemical revolution and the event that set Lavoisier on his path to glory.

Largely for pedagogic reasons, generations of historians, chemistry teachers and philosophers of science have interpreted the chemical revolution as hinging upon rival interpretations of combustion – phlogiston theory versus oxygen theory. More recently, those historians who have seen Lavoisier’s chemistry as literally an anti-phlogistic chemistry have had a wider agenda than combustion in mind. In particular, it now seems clear that the interpretations of acidity was a major issue for Lavoisier and the phlogistonists. Indeed, it could be argued that, once Lavoisier had the concept of a gas, it was the issue of acidity, not combustion, that led him to oxygen – as its very name implies. The transformation of ideas of acidity, therefore, formed a fifth factor in the production of a new chemistry.

Finally, and not least, the sixth necessary condition was a new theory of chemical composition and organization of matter in which acids and bases were composed from oxygen and elements operationally defined as the substances that chemists had not succeeded in analysing into simpler bodies. Oxygen formed the glue or bond of dualistic union between acid and base to form salts, which then compounded in unknown ways to form minerals. To make this more articulate and to avoid confusion with the unnecessary thought patterns of phlogiston chemistry, a new language was required – one that reflected composition and instantly told a reader what a substance was compounded from. After 1787 chemists, in effect, spoke French, and this underlined the new chemistry as a French achievement.

Although he pretended at the beginning of the Traité that it had been his intent to reform the language of chemistry that had forced the reform of chemistry itself, it was clearly because he had done the latter that a new language of composition was needed. As historians have stressed, the new nomenclature was Lavoisier’s theoretical system. He justified its adoption in terms of Condillac’s empirical philosophy that a well constructed language based upon precise observation and rationally constructed in the algebraic way of equal balances of known and unknown would serve as a tool of analysis and synthesis.

Observation itself involved chemical apparatus – not merely the balance, but an array of eudiometers, gasometers, combustion globes and ice calorimeters, which would enable precise quantitative data to be assembled. In this way chemical science would approach the model of the experimental physicists that Lavoisier clearly admired and with whose advocates he frequently collaborated.

This last point has led some historians to question whether Lavoisier was a chemist at all and whether the chemical revolution was instead the result of a brief and useful invasion of chemistry by French physicists. Others, while admitting the influence of experimental physics on Lavoisier’s approach, continue to stress Lavoisier’s participation in a long French tradition of investigative analysis of acids and salts to which he added a gaseous dimension. Even Lavoisier’s choice of apparatus, though imbued with a care and precision lacking in his predecessors’ work, was hallmarked by the investigative procedures of a long line of analytical and pharmaceutical chemistry. All historians agree, however, that until about 1772, when events triggered a definite programme of pneumatic and acid research in his mind, Lavoisier’s research was pretty random and dull, as if he were casting around for a subject (‘une belle carrière d’expériences à faire’) that would make him famous. Seizing the opportunity, the right moment, is often the mark of greatness in science. Priestley and Scheele believed that science progressed through the immediate communication of raw discoveries and ‘ingenious simplicity’. Lavoisier’s way, to Priestley’s annoyance, was to work within a system and to theorize in a new language that legislated phlogiston out of existence.

Like Darwin’s Origin of Species, Lavoisier’s Traité was a hastily written abstract or prolegomena to a much larger work he intended to write that would have included a discussion of affinity, and animal and vegetable chemistry. Like Darwin’s book, it was all the more readable and influential for being short and introductory. If more information was required, Fourcroy’s encyclopedic text and its many English and German imitations soon provided reference and instruction. But this was not the end of the chemical revolution. To complete it, Lavoisier’s elements had to be reunited with the older corpuscular traditions of Boyle and Newton. This was to be the contribution of John Dalton.