extremely probable we know nothing at all about them; however, if instead we apply the term elements or principles of bodies, to express our idea of the last point which analysis is capable of reaching, we must admit as elements, all the substances into which we are capable, by any means, to reduce bodies during decomposition. Not that we can be certain that these substances we consider as simple may not be compounded of two, or even a greater number of principles; but, since these principles cannot be separated, or rather since we have not hitherto discovered the means of separating them, they act with regard to us as simple substances, and we ought never to suppose them compounded until experiment and observation has proved them to be so.
For the same reason, although Dalton believed in physical atoms, most of his interpreters were content with a theory of chemical atoms – the ‘minima’ of the experimentally defined elements. Whether these chemical atoms were themselves composed from homogeneous or heterogeneous physical atoms was to go beyond the evidence of pure stoichiometry.
Stoichiometry was a subject invented by the German chemist Jeremias Richter (1762–1807), who had studied mathematics with the great philosopher, Immanuel Kant, at the University of Königsberg, and for whom he wrote a doctoral thesis on the use of mathematics in chemistry. This was, in practice, nothing grander than an account of the determination of specific gravities, from which Richter calculated the supposed weights of phlogiston in substances. Just as Kepler had searched for mathematical relations and harmony in astronomical data gathered by Tycho Brahe, so Richter spent his spare time as a chemical analyst in the Berlin porcelain works searching for arithmetical relations in chemistry. As Partington noted sardonically, Richter spent his entire life finding ‘regularities among the combining proportions where nature had not provided any’.
The exception was his discovery in 1792, while investigating double decompositions, that, because neutral products were formed, the reactants must ‘have amongst themselves a certain fixed ratio of mass’.
If, e.g., the components of two neutral compounds are A – a, a and B – b, b, then the mass ratios of the new neutral compounds produced by double decomposition are unchangeably A – a:b and B – b:a.
This law of neutrality was a special case of what came to be known as the law of reciprocal proportions. Richter referred to the study of these ratios as ‘stoichiometry’ and went on to examine how a fixed weight of an acid was neutralized by different weights of various bases. This investigation led him to claim, erroneously, that combining proportions formed arithmetical and geometrical series. It was Ernst Fischer, a Berlin physicist, who, when translating Berthollet’s Recherches sur la lois de l’affinité into German in 1802, pointed out that Richter’s results could be tabulated to show equivalent weights of a series of acids and bases. If 1000 parts of sulphuric acid was taken as a standard and the base equivalents needed for neutralization arranged in one column, and the amounts of other acids needed to neutralize these bases in another, then an analyst could gather at a glance how much of a particular base would neutralize a particular acid:
Thus, 672 equivalents of ammonia neutralized 427 of fluoric, 577 of carbonic and 712 of muriatic acids. Analysts now had a definite method of controlling the accuracy of their work and of calculating beforehand the composition of salts under investigation.
Dalton’s atomic theory was to provide a rational explanation for these regularities. There has been some debate as to whether Dalton was directly influenced by Richter. He certainly knew of Richter’s investigations, but probably not until after he had derived his own explanation from other sources.
What was ‘new’ in John Dalton’s A New System of Chemical Philosophy? The obvious reply seems to be the introduction of chemical atomism – the idea that each of Lavoisier’s undecompounded bodies was composed from a myriad of homogeneous atoms, each element’s atom differing slightly in mass. The surprising thing, however, is that only one chapter of barely five pages in the 916-page treatise was devoted to the epoch-making theme. These five pages, together with four explanatory plates, appeared at the end of the first part of the New System, which was published in Manchester in 1808 and dedicated to the professors and students of the Universities of Edinburgh and Glasgow, who had heard Dalton lecture on ‘Heat and the Chemical Elements’ in 1807, and to the members of Manchester’s Literary and Philosophical Society, who had ‘uniformly promoted’ Dalton’s researches. A second, continuously paginated, part of the New System, dedicated to Humphry Davy and William Henry, was published in 1810. Astonishingly, the third part, labelled as a second volume, did not appear until 1827. Even then the design was incomplete and a promised final part concerned with ‘complex compounds’ was never published.
Dalton’s apparent dilatoriness is easily explained by the fact that he earned his living as a private elementary teacher, which left him little time for the exacting experimental work and evidence upon which he based the New System. For it was a ‘new’ approach that he was taking, familiar though his scheme has become. Dalton recognized his innovation as being a ‘doctrine of heat and general principles of chemical synthesis’. A theory of mixed gases, which he developed in 1802, led him in 1803 to ‘new views’ on heat as a factor in the way elements (or, rather, atoms) combined together, a process he referred to as ‘chemical synthesis’. The fact that chemical compounds, or compound atoms (molecules), might be binary, ternary, quaternary, and so on up to a maximum of twelve atoms, gave Dalton a structure for his text: a detailed experimental examination of heat and the gaseous state, a theory of atomism and combination, which included the measure of atomic mass as a relative atomic weight, followed by a detailed account of the properties of the known elements, their binary combinations, ternary combinations and so on. Thus, although the exegesis of the atomic theory was limited to five pages, the whole of the New System was, in fact, imbued with a new stoichiometric approach to chemistry – that elements compounded together in fixed proportions by weight because of attractions and repulsions between the tiny particles of heat and elementary forms that made up laboratory chemicals. Inevitably, because Dalton was a slow worker and unable to spare time from teaching for research and writing, it was left largely to others, notably Thomas Thomson and Jacob Berzelius, to exploit the full consequences of Dalton’s insight.
John Dalton (1766–1844) was born at Eaglesfield in Cumbria, the son of a weaver, and, like most contemporary members of the Society of Friends, was a man of some learning. The highly efficient Quaker network of schooling and informal education ensured that Dalton received a good schooling; he himself began to teach village schoolchildren when he reached the age of twelve. In his teens he mastered sufficient geometry to be able to study Newton’s Principia. At the age of fifteen, Dalton and his brother moved to Kendall, in the English Lake District, where they acquired their own school, which offered Greek, Latin, French and mathematics. At Kendall, Dalton was befriended by the blind Quaker scholar, John Gough, who further encouraged Dalton’s mathematical abilities and knowledge of Newtonian natural philosophy, including the work of Boyle and Boerhaave. The constant stimulation of rapidly changing weather conditions among the mountains and lakes of Westmorland and Cumberland (present-day Cumbria) interested him in meteorology. The records he kept over a five-year period were published in Meteorological Essays in 1793. In the same year, on Gough’s recommendation, Dalton moved to Manchester as tutor in mathematics and natural philosophy at New College, a Dissenting academy that had begun its distinguished life elsewhere as the Warrington Academy. Here Priestley had taught between 1761 and 1767.
Although Manchester New
