by a small circle, with some distinctive mark; and the combinations consist in the juxta-position of two or more of these.
The synthesis of water and ammonia were represented as:
Such symbols referred to the atom and were therefore conceptually very different from alchemical symbols or those of Hassenfratz and Adet, which only had a hazy or qualitative meaning. Earlier symbols had been a shorthand; Dalton’s circles conveyed a theoretical meaning as well as being a convenient abbreviation.
Dalton was never to become reconciled to the symbols introduced by Berzelius, even though he himself used alphabetical abbreviations within circles for elements such as iron, sulphur, copper and lead. In 1837, soon after the British Association for the Advancement of Science had persuaded British chemists to adopt Berzelius’ symbols, Dalton wrote a testimonial for Thomas Graham’s application for the Chair of Chemistry at University College London.
Berzelius’s symbols are horrifying: a young student in chemistry might as soon learn Hebrew as make himself acquainted with them. They appear like a chaos of atoms. Why not put them together in some sort of order?…[They] equally perplex the adepts of science, discourage the learner, as well as to cloud the beauty and simplicity of the Atomic Theory.
Clearly Dalton felt strongly about his innovation and was prepared to criticize a professorial candidate with one hand while supporting him with another. Indeed, Dalton suffered the first of his two strokes in April 1837 after angrily discussing symbols with a visitor.
Dalton’s symbols did not survive, mainly one suspects because they were an additional printing expense, but both they, as well as Berzelius’ simplification, encouraged people to acquire a faith in the reality of chemical atoms and enabled chemists to visualize relatively complex chemical reactions. As in mathematics, chemistry could advance only to a certain degree without an adequate symbolism for its deeper study. Between them, Lavoisier and Dalton completed a revolution in the language of chemistry.
Dalton’s hieroglyphs also reveal that he had a three-dimensional geometrical model of combination in mind. When three or more particles combined, he conceived that like particles stationed themselves as far apart as possible. This conception offers not only an important clue concerning the origins of Dalton’s atomic theory, but an explanation of his opposition to the notion derived from the volumetric combination of gases that equal volumes contained equal numbers of particles.
THE ORIGINS OF DALTON’S THEORY
How did Dalton come to think of weighing atoms? There have been many different attempts by chemists and historians to explain this. Dalton supplied three, mutually inconsistent, accounts of his voyage of discovery. Reconstruction has been made difficult by the fact that most of Dalton’s surviving papers were destroyed during the Second World War, and, but for the fact that Henry Roscoe and Arthur Harden quoted from them in a historical study published in 1896, historians would have been hard-pressed for evidence. Although the debate over influences remains unresolved, all historians agree that Dalton must have come to his ‘new views’ through the study of the physical properties of gases, which in turn depended upon his youthful interest in meteorology. For, once air had been shown to be heterogeneous, and not a homogeneous element, the question arose whether oxygen, nitrogen, carbon dioxide and water vapour were chemically combined in air (perhaps a compound actually dissolved in the water vapour?) or merely mixed together. The fact that atmospheric air appeared to be homogeneous and that its gaseous components were not stratified according to their specific gravities (itself an indication that chemists like Priestley were prepared to think in terms of the specific weights of gas particles long before Dalton) made most late-eighteenth-century chemists believe that atmospheric gases were chemically combined.
Dalton thought differently. His long study of Newton’s Principia had made him familiar with Newton’s demonstration that Boyle’s law relating pressure and volume could be derived from a model in which homogeneous air particles were self-repulsive with a power inversely proportional to the distance. As a result of his meteorological studies, Dalton had become convinced by 1793 that water vapour could not possibly be chemically combined in air; instead, it was diffused among the other aerial particles and so freely available for precipitation or condensation as rain or dew. But if water was not chemically combined, why should the other constituents of air be?
If Newton’s model of the self-repulsion of air particles was translated into a model of self-repulsive constituents of air, what would be the consequences? Provided particles of one kind did not repel particles of a different gas, as Dalton showed, each gas or vapour would behave as if in a vacuum. The net effect would be a homogeneous mixture as the different gas particles repelled their own kinds. As to the cause of their self-repulsion, Lavoisier’s model of a gas supplied a satisfactory candidate: caloric. By imagining that the particles of oxygen, nitrogen, carbon dioxide and water vapour were surrounded by atmospheres of heat, Dalton arrived at a theory of mixed gases and, incidentally, a law of partial pressures that proved essential in quantitative work in gas analysis and barometry.
Dalton’s ‘New theory of the constitution of mixed aeriform fluids and particularly of the atmosphere’ was published in Nicholson’s Journal in 1801. It proved controversial, but this was no bad thing for Dalton’s British and European reputation. Most chemists who believed in the chemical theory of air wondered how it was that caloric atmospheres in different particles did not repel one another. Why suppose that there were ‘as many distinct kinds of repulsive powers, as of gases … and that heat was not the repulsive power in any one case’?
Dalton’s ingenious reply to this difficulty was published in full in the second part of the New System and was premised on differences of size of gaseous particles, the size being a function of both the atom’s volume and the radius of its atmosphere of heat. Using diagrams that look like the later magnetic force diagrams popularized by Faraday, Dalton showed visually that ‘no equilibrium can be established by particles of different sizes pressing against each other’. It followed that different particles would ‘ignore’ one another even when surrounded by the repulsive imponderable of heat. Such a static model remained the only satisfactory explanation of gaseous diffusion, partial pressures and atmospheric homogeneity until it was replaced in the 1850s by the kinetic theory of gases.
As historians of chemistry have shown, this second model of mixed gases, which was dependent on the sizes of atoms, was first developed by Dalton in September 1804, a full year after he had developed the first list of particle weights. The question of size offers a clue to his thinking during the previous year.
One of Dalton’s few supporters for the first theory of mixed gases was his Mancunian friend, William Henry (1774–1836), the owner of a chemical works for the manufacture of the pharmaceutical, milk of magnesia, used in the treatment of digestive complaints. Henry had at first opposed Dalton, only to be converted when he found that ‘water takes up the same volume of condensed gas as of a gas under ordinary pressure’. Henry’s law that the solubility of a gas at a given temperature depended upon pressure, which he discovered in 1803, was powerful evidence that solution was a purely mechanical effect. If chemical affinity was not involved, it seemed equally unlikely to be involved in the atmosphere. Moreover, as Henry found, a mixture of gases dissolved in water was ‘retained in its place by an atmosphere of no other gas but its own kind’.
Henry’s experiments were intriguing. Why, Dalton wondered, did different gases have different solubilities in water? Why were light and elementary gases such as hydrogen and oxygen least soluble, whereas heavier compound gases such as carbon dioxide were very soluble? If his first theory of mixed
