John Dalton’s atomic theory did not spring from a single dramatic experiment but from a series of investigations, most famously his nitric oxide gas experiments of 1803, in which he observed that oxygen combined with nitric oxide in fixed, whole-number volume ratios. That finding gave him the first clear experimental case of what became known as the law of multiple proportions, and it anchored his broader claim that matter is made of indivisible atoms with characteristic weights. The path from curious weather observer to father of modern atomic theory is less tidy than textbooks suggest, and the experiments themselves were surprisingly simple in their setup.
How Studying the Weather Led to Atoms
Dalton did not start out trying to prove that atoms existed. He was a self-taught natural philosopher in Manchester, England, deeply interested in meteorology. He kept daily weather records for decades, and his early scientific questions revolved around the behavior of the atmosphere: why different gases stayed mixed instead of separating into layers, how water vapor dissolved into air, and why the pressure of a gas mixture seemed to equal the sum of the pressures each gas would exert alone. These atmospheric puzzles pushed him toward thinking about the physical nature of gases at the particle level.
His meteorological work led him to see the constitution of the atmosphere as a problem that needed solving, and in pursuing it, he gradually shifted his attention toward specifically chemical questions.1Studies in History and Philosophy of Science Part A. John Dalton’s puzzles: from meteorology to chemistry By around 1801, he was conducting experiments on the absorption of gases in water, measuring how much of each gas a given volume of water would take up. He noticed that different gases dissolved in water in distinctly different amounts, and he began to wonder whether the particles of different gases might have different sizes and weights. That question was the bridge between his weather studies and his atomic theory.
The Gas Absorption Experiments
Before reaching the nitric oxide work that most historians consider his breakthrough, Dalton spent considerable time studying how gases dissolve in water. He measured the solubility of various gases and found that the amount absorbed did not simply scale with pressure in a uniform way across all gases. Each gas seemed to have its own characteristic relationship with water. Dalton reasoned that the particles of each gas must differ physically, and the most natural way they could differ was in weight.
This was a conceptual leap. Most of Dalton’s contemporaries thought of gases as essentially similar elastic fluids, distinguished only by their chemical identity. Dalton proposed instead that each type of gas consisted of particles with a specific, fixed weight. He assembled an early table of what he called “atomic weights,” assigning hydrogen a weight of 1 and estimating the relative weights of oxygen, nitrogen, carbon, and several other elements based on the proportions in which they combined. The table was rough and contained errors, but the core idea was revolutionary: if atoms of different elements have characteristic weights, then chemical combinations should follow simple numerical patterns.
The Nitric Oxide Experiments of 1803
The experiments that gave Dalton his strongest early evidence involved nitric oxide, a colorless gas that reacts visibly with oxygen. When nitric oxide meets oxygen, the two gases combine and produce reddish-brown nitrogen dioxide, which dissolves readily in water. Dalton could therefore track the reaction by watching the gas volume shrink as the product was absorbed.
Working with closed vessels over water, Dalton discovered that oxygen combined with nitric oxide in two distinct ratios: one volume of oxygen reacted with either one volume or two volumes of nitric oxide, producing two different nitrogen oxides.2PubMed. Dalton’s disputed nitric oxide experiments and the origins of his atomic theory There was no middle ground. The oxygen did not combine with, say, one and a half volumes of nitric oxide to make some intermediate compound. The ratios were whole numbers, and the two products were chemically distinct substances.
This was the first clearly documented case of what Dalton would formalize as the law of multiple proportions.3Substantia. The Reinvention of the Nitrous Gas Eudiometrical Test in the Context of Dalton’s Law on the Multiple Proportions of Combination The law states that when two elements form more than one compound, the ratios of the mass of one element that combines with a fixed mass of the other are small whole numbers. In the nitric oxide case, the amount of oxygen combining with a given amount of nitrogen was in a ratio of 1 to 2 between the two compounds. That pattern made perfect sense if matter consisted of discrete, indivisible particles that combined in simple counts, and it made very little sense otherwise.
Why Whole-Number Ratios Mattered
The significance of those whole-number ratios is easy to underestimate from a modern perspective, where atoms feel obvious. In Dalton’s time, the atomic idea had been around since ancient Greek philosophy, but it was a speculative concept with no quantitative backbone. Chemists knew that certain substances combined in fixed proportions, a finding often credited to Joseph Proust’s earlier work on the law of definite proportions. But fixed proportions alone did not prove the existence of atoms. You could explain them with continuous matter models if you tried hard enough.
Multiple proportions were a different story. If oxygen and nitrogen can form one compound in which one part of oxygen combines with one part of nitrogen, and a second compound in which two parts of oxygen combine with one part of nitrogen, the simplest explanation is that discrete units are snapping together in different configurations. It is difficult to explain why matter would spontaneously organize into exactly doubled ratios without invoking discrete particles. Dalton saw this clearly, and the nitric oxide experiments gave him the concrete evidence to argue the point.
It is worth noting that some historians have questioned the precision of Dalton’s actual measurements and whether his data truly supported the clean ratios he reported.2PubMed. Dalton’s disputed nitric oxide experiments and the origins of his atomic theory The gases he worked with were impure by modern standards, his glassware was basic, and he was measuring volumes by eye over a water trough. Some scholars believe Dalton may have rounded or selected data that fit his emerging theory. Even so, the conceptual framework turned out to be correct, and subsequent chemists confirmed the law of multiple proportions with far more precise instruments.
The Role of Caloric Theory and Other Influences
Dalton did not develop his atomic theory in a vacuum. He was influenced by earlier thinkers, and some of his theoretical commitments now look odd from a modern standpoint. In particular, he adopted a model in which atoms were surrounded by shells of “caloric,” a hypothetical fluid of heat. In this picture, gas particles repelled each other not because of any intrinsic force but because their caloric envelopes pushed against one another. Dalton drew on a similar model proposed earlier by the Irish chemist Bryan Higgins, adapting it to fit his own experimental findings.4Cambridge University Press. John Dalton and the origin of the atomic theory: reassessing the influence of Bryan Higgins
The caloric model is long abandoned, but it played a real role in how Dalton visualized atoms and their interactions. He thought of each atom as a hard central sphere wrapped in a repulsive heat atmosphere, and this influenced how he imagined atoms packing together in mixtures and compounds. It is a useful reminder that scientific breakthroughs do not require every part of the theory to be right. Dalton’s core insight, that elements are made of atoms with characteristic weights that combine in whole-number ratios, survived the death of caloric theory without much damage.
What Dalton Published and How He Presented It
Dalton laid out his atomic theory most fully in his book A New System of Chemical Philosophy, the first part of which appeared in 1808. In it, he listed his table of atomic weights, described the rules of combination he had inferred, and introduced a set of circular symbols to represent different elements. Each element got its own distinctive circle: hydrogen was a circle with a dot in the center, oxygen was an empty circle, nitrogen had a vertical line through it, and so on. Compound particles were drawn as clusters of these circles touching one another.
This visual notation was innovative for its time, though it was eventually replaced by the letter-based chemical symbols we use today, introduced by the Swedish chemist Jöns Jacob Berzelius. Dalton’s circles were clunky for writing out complex reactions, but they drove home the physical picture he was proposing: atoms are real things with definite sizes and weights, and they stick together in specific geometric arrangements. For Dalton, drawing a compound was not a metaphor. He believed the circles on the page corresponded, however roughly, to actual objects.
His table of atomic weights contained several errors by modern standards. He assumed that the simplest compound of two elements would always be a one-to-one combination. So he treated water as one atom of hydrogen bonded to one atom of oxygen, when it is actually two hydrogens to one oxygen. That mistake led him to assign oxygen an atomic weight roughly half of its true relative weight. Similar errors cascaded through his table. These were not failings of logic but of a reasonable assumption, the “rule of greatest simplicity,” that later chemists had to abandon as more data accumulated.
The Broader Experimental Context
Dalton’s nitric oxide work was his signature contribution, but it sat within a broader landscape of chemical evidence that supported atomic thinking. Around the same time, other chemists were independently finding that elements combine in fixed and reproducible weight ratios. Proust had demonstrated the law of definite proportions through careful analyses of metal oxides and other compounds. Gay-Lussac in France showed that gases react in simple volume ratios, a finding that complemented Dalton’s weight-based approach, though the two men disagreed on interpretation.
What set Dalton apart was not just the data but the theoretical framework he wrapped around it. Others had the proportions; Dalton had the atoms. He proposed that each element consists of identical atoms, that atoms of different elements differ in weight, that atoms are neither created nor destroyed in chemical reactions, and that compounds form when atoms combine in simple numerical ratios. Those four postulates, derived from relatively modest bench experiments with gases and water troughs, became the foundation of modern chemistry.
What Dalton Got Wrong
Beyond the rule of greatest simplicity and the caloric model, Dalton held several positions that later turned out to be incorrect. He believed that atoms of the same element could not bond to each other, which made it impossible for him to accept diatomic molecules like O₂ or N₂. This blind spot caused real confusion in the early decades of atomic theory, because it meant Dalton’s atomic weights for gaseous elements were often off by a factor of two.
He also resisted the idea that equal volumes of different gases at the same temperature and pressure contain equal numbers of particles, a principle proposed by Amedeo Avogadro in 1811. Dalton rejected this partly because it contradicted his assumption that atoms of the same element repel each other. It took decades and the work of Stanislao Cannizzaro in the 1860s to sort out the confusion and put atomic weights on a consistent footing. Dalton did not live to see that resolution; he died in 1844.
None of these errors diminishes the significance of his experimental work. Science advances by being wrong in productive ways, and Dalton’s framework was productive enough to transform chemistry from a discipline organized around recipes and observations into one organized around quantitative relationships between discrete particles.
Dalton’s Other Scientific Legacy
Dalton is remembered primarily for the atomic theory, but he had a parallel and entirely unrelated claim to scientific fame: he was one of the first people to describe color blindness from personal experience. Dalton was himself color blind and published an account of his condition in 1794, years before his atomic work. The condition was long called “Daltonism” in several European languages because of his early description.
In a remarkable bit of posthumous science, DNA extracted from Dalton’s preserved eye tissue, which had been kept at the Manchester Literary and Philosophical Society since his death, confirmed that he lacked the middlewave photopigment of the retina, making him a deuteranope.5PubMed. The chemistry of John Dalton’s color blindness This was a different diagnosis from what Thomas Young had guessed in Dalton’s own lifetime, and it illustrated how long scientific questions about even well-documented individuals can remain open. The analysis was published in 1995, more than 150 years after Dalton’s death, using molecular genetics techniques he could never have imagined.
Dalton himself had hypothesized that his color blindness was caused by a blue tint in the vitreous humor of his eye. He arranged for his eyes to be examined after his death to test this, and the dissection quickly disproved his theory: the vitreous fluid was clear. But Dalton’s instinct to test his own hypothesis experimentally, even posthumously, says something about the man’s temperament. He was, at his core, someone who wanted to measure things and see whether reality matched what the theory predicted. That same temperament drove him to set up gas reactions in glass vessels and count the volumes, and it is what made the atomic theory not just a philosophical claim but a scientific one.