It was tacitly accepted by most chemists in the eighteenth century that compounds had fixed compositions and the elements in them would combine in precise, invariable proportions – practical chemistry could hardly have proceeded otherwise. But there had been no explicit investigations of this, or declarations on the matter, until Joseph-Louis Proust, a French chemist working in Spain, embarked on a series of meticulous analyses comparing various oxides and sulphides from around the world. He was soon convinced that all genuine chemical compounds did indeed have fixed compositions – and that this was so however the compound was made, or wherever it was found. Red mercuric sulphide, for instance, always had the same proportions of mercury and sulphur, whether it was made in the lab or found as a mineral.[31]
Between pole and pole [Proust wrote] compounds are identical in composition. Their appearance may vary owing to their mode of aggregation, but their properties never… The cinnabar of Japan has the same composition as the cinnabar of Spain; silver chloride is identically the same whether obtained from Peru or from Siberia; in all the world there is but one sodium chloride; one saltpetre; one calcium sulphate; and one barium sulphate. Analysis confirms these facts at every step.
By 1799, Proust had generalized his theory into a law – the law of fixed proportions. Proust’s analyses, and his mysterious law, excited attention among chemists everywhere, not least in England, where they were to inspire profound insights in the mind of John Dalton, a modest Quaker schoolteacher in Manchester.
Gifted in mathematics, and drawn to Newton and his ‘corpuscular philosophy’ from an early age, Dalton had sought to understand the physical properties of gases – the pressures they exerted, their diffusion and solution – in corpuscular or ‘atomic’ terms. Thus he was already thinking of ‘ultimate particles’ and their weights, albeit in this purely physical context, when he first heard of Proust’s work, and by a sudden intuitive leap, saw how these ultimate particles might account for Proust’s law, and indeed the whole of chemistry.
For Newton and Boyle, though there were different forms of matter, the corpuscles or atoms of which they were composed were all identical. (Thus there was always, for them, the alchemical possibility of turning a base metal into gold, for this only entailed change of form, a transformation of the same basic matter.)[32] But now the concept of elements, thanks to Lavoisier, was clear, and for Dalton there were as many kinds of atoms as there were elements. Every one had a fixed and characteristic ‘atomic weight’, and this was what determined the relative proportions in which it combined with other elements. Thus if 23 grams of sodium invariably combined with 35.5 grams of chlorine, this was because sodium and chlorine atoms had atomic weights of 23 and 35.5. (These atomic weights were not, of course, the actual weights of atoms, but their weights relative to that of a standard – for example, that of a hydrogen atom.)
Reading Dalton, reading about atoms, put me in a sort of rapture, thinking that the mysterious proportionalities and numbers one saw on a gross scale in the lab might reflect an invisible, infinitesimal, inner world of atoms, dancing, touching, attracting, and combining. I had the sense that I was being enabled to see, using the imagination as a microscope, a tiny world, an ultimate world, billions or trillions of times smaller than our own – the actual constituents of matter.
Uncle Dave had shown me gold leaf, beaten and hammered out until it became almost transparent, so that it transmitted light, a beautiful bluish green light. This leaf, a millionth of an inch thick, he said, was only a few dozen atoms thick. My father had shown me how a very bitter substance such as strychnine could be diluted a millionfold and still be tasted. And I liked to experiment with thin films, to blow soap bubbles in the bath – a speck of soapy water could be blown, with care, into a huge bubble – and to watch oil, in iridescent films, spreading on wet roads. All these prepared me, in a way, to imagine the very small – the smallness of particles that composed the millionth-of-an-inch thickness of gold leaf, a soap bubble, or an oil film.
But what Dalton intimated was infinitely more thrilling: for it was not just atoms in the Newtonian sense, but atoms as richly individual as the elements themselves – atoms whose individuality gave elements theirs.
Dalton later made wooden models of atoms, and I saw his actual models in the Science Museum as a boy. These, crude and diagrammatic as they were, excited my imagination, helped give me a sense that atoms really existed. But not everyone felt this, and, for some chemists, Dalton’s models epitomized the absurdity, as they saw it, of an atomic hypothesis. ‘Atoms’, the eminent chemist H.E. Roscoe was to write, eighty years later, ‘are round bits of wood invented by Mr. Dalton.’
It was indeed possible, in Dalton’s time, to regard the idea of atoms as implausible, if not outright nonsense, and it would be over a century before indisputable evidence for the existence of atoms was secured. Wilhelm Ostwald, for one, was not convinced of the reality of atoms, and in his 1902 Principles of Inorganic Chemistry he wrote:
Chemical processes occur in such a way as if the substances were composed of atoms… At best there follows from this the possibility that they are in reality so: not however, the certainty… One must not be led astray by the agreement between picture and reality, and confound the two… An hypothesis is only an aid to representation.
Now, of course, we can ‘see’ and even manipulate individual atoms, using an atomic force microscope. But it required enormous vision and courage, at the very beginning of the nineteenth century, to postulate entities so utterly beyond the bounds of any empirical demonstration possible at the time.[33]
Dalton’s theory of chemical atoms was detailed in his notebook on the 6th of September, 1803, his thirty-seventh birthday. He was at first too modest or too diffident to publish anything on his theory (he had, however, worked out the atomic weights of half a dozen elements – hydrogen, nitrogen, carbon, oxygen, phosphorus, sulphur – which he recorded in his notebook). But word was soon out that he had hatched something astonishing, and Thomas Thomson, the eminent chemist, went up to Manchester to meet him. A single short conversation with Dalton in 1804 ‘converted’ Thomson, altered his life. ‘I was enchanted’, he later wrote, ‘with the new light which immediately burst upon my mind, and I saw at a glance the immense importance of such a theory.’
Although Dalton had presented some of his thoughts to the Literary and Philosophical Society in Manchester, they did not become known to a wider public until Thomson wrote of them. Thomson’s presentation was brilliant and persuasive, much more so than Dalton’s own exposition, which was crammed, awkwardly, into the final pages of his 1808 New System.
But Dalton himself realized that there were fundamental problems with his theory. For to pass from a combining or equivalent weight to an atomic weight required that one know the exact formula of a compound, for the same elements, in some cases, might combine in more than one way (as in the three oxides of nitrogen). So Dalton assumed that if two elements formed only a single compound (as hydrogen and oxygen appeared to do in water, or nitrogen and hydrogen in ammonia), they would do so in the simplest possible ratio: one to one. This ratio, he felt, would surely be the most stable. Thus he took the formula of water (in modern nomenclature) to be HO, and the atomic weight of oxygen to be the same as its equivalent weight, namely 8. Similarly, he took the formula of ammonia to be NH, and thus the atomic weight of nitrogen to be 5.
31
Yet Proust’s view was challenged by Claude-Louis Berthollet. A senior chemist of great eminence, an ardent supporter of Lavoisier (and a collaborator with him on the Nomenclature), Berthollet had discovered chemical bleaching and accompanied Napoleon as a scientist on his 1798 expedition to Egypt. He had observed that various alloys and glasses manifestly had quite varied chemical compositions; therefore, he maintained, compounds could have a continuously variable composition. He also remarked, when roasting lead in his laboratory, a striking, continuous color change – did this not imply a continuous absorption of oxygen with an infinite number of stages? It was true, Proust argued, that heated lead took up oxygen continuously and changed color as it did so, but this was due, he thought, to the formation of three distinctly colored oxides: a yellow monoxide, then red lead, then a chocolate-colored dioxide – admixed like paints, in varying proportions, depending on the state of oxidation. The oxides themselves might be mixed together in any proportion, he felt, but each was itself of fixed composition.
Berthollet also wondered about such compounds as ferrous sulphide, which never contained exactly the same proportions of iron and sulphur. Proust was unable to give a clear answer here (and indeed the answer only became clear with a subsequent understanding of crystal lattices and their defects and substitutions – thus sulphur can substitute for iron in the iron sulphide lattice to a variable extent, so that its effective formula varies from Fe7 S8 to Fe8 S9. Such nonstoichiometric compounds came to be called berthollides).
Thus both Proust and Berthollet were right in a way, but the vast majority of compounds were Proustian, with a fixed composition. (And it was perhaps necessary that Proust’s view became the favored one, for it was Proust’s law which was to inspire the profound insights of Dalton.)
32
Though Newton hinted, in his final
God is able to create particles of matter of several sizes and figures, and in several proportions to the space they occupy, and perhaps of different densities and forces.
33
Dalton represented the atoms of elements as circles with internal designs, sometimes reminiscent of the symbols of alchemy, or the planets; while the compound atoms (which we would now call ‘molecules’) had increasingly intricate geometric configurations – the first premonition of a structural chemistry that was not to be developed for another fifty years.
Though Dalton spoke of his atomic ‘hypothesis,’ he was convinced that atoms really existed – hence his violent objection to the terminology Berzelius was to introduce, in which an element was denoted by one or two letters of its name rather than his own iconic symbol. Dalton’s passionate opposition to Berzelius’s symbolism (which he felt concealed the actuality of atoms) lasted to the end of his life, and indeed when he died in 1844 it was from a sudden apoplexy, following a violent argument defending the realness of his atoms.