I have heard people say, “I’ve never seen an isolated atom, and yet I know atoms exist. Therefore, you can know something exists even if its existence has never been confirmed by sensory evidence.” But the truth is that atomic theory is indeed confirmed by sensory evidence. Chemical formulae in a compound — such as how, in water, for every one atom of oxygen, there are two atoms of hydrogen — are a model to explain the principles in Nature, and, the accuracy of that model is verified in how it is used successfully to make predictions that are confirmed, accordingly, by sensory observation to be likewise accurate. The following essay is inspired by discussions of similar points in David Harriman’s book The Logical Leap.
It was through sensory observation that Joseph-Louis Proust proved the Law of Definite Proportions. The chemical compound he used was copper carbonate — CuCO3. He found that regardless of how the copper carbonate was procured, and no matter the quantity it was in, the mass ratio of copper to carbon to oxygen will consistently be a respective 5.3 to 1 to 4. Whatever quantity of copper carbonate you have, for every 5.3 grams of copper it has, it will correspondingly have 1 gram of carbon and 4 grams of oxygen. Within any chemical compound, the separate elements only could always configure together in the same proportions to one another like that
- if each element ultimately consisted of tiny microscopic units that could not be divisible any further at the chemical level, and
- if the chemically-indivisible units of each element bonded with the others always in the same ratio to one another.
That is how we know that a chemical compound can be reduced to separate molecules, and that each molecule of the compound consists of two or more elements with each element coming in a whole-number of chemically-indivisible units (no fractions or decimals for each number of atoms in the molecule).
Proust was able to measure the quantity of the mass of copper, carbon, and oxygen each in his samples of copper carbonate as he knew how to trigger particular chemical reactions to break down the copper carbonate into its separate elements. He heated the copper carbonate to evaporate the water and carbon dioxide. This left him with copper oxide (the result of a bond between copper and oxygen). Then, using hydrogen but not adding it, Proust was able to remove the oxygen, leaving only the pure copper. As measurements of weight were a reliable proxy for measuring mass, Proust weighed the remaining sample at each stage of the process. First he weighed the complete copper carbonate. Then he weighed the separate copper oxide, water, and carbon dioxide. Finally he weighed the remaining sample of pure copper. You can see an English translation of his own description of the procedure over here. All this involved direct sensory inputs.
Joseph-Louis Proust could not see any isolated atoms. But he did use his senses directly in measuring the quantity of each element’s mass within the compound and observing first-hand how consistent was the ratio in the quantity of mass for each element as compared to the others.
Proust observed directly through his senses the consistency of his Law of Definite Proportions. And what explains the consistent applicability of the Law of Definite Proportions in the separately observed samples of copper carbonate is that all copper carbonate consists of molecules, with each molecule consisting of a particular whole-number of units of copper, a particular whole-number of units of carbon, and a particular whole-number of units orf oxygen.
Likewise, no matter the quantity of water you have, the mass ratio of hydrogen to oxygen will always be 1 to 8. For every one gram of hydrogen, there will always be eight grams of oxygen. This only could have been true if, at the chemical level, water consisted of tiny microscopic particles (molecules) in which there was an arrangement where a particular whole-number of no-further-divisible units of hydrogen existed in a constant ratio to a whole-number of likewise-no-further-divisible units of oxygen. Remembering the ancient Greek idea of Leucippus and Democritus that all matter might ultimately consist of microscopic further-irreducible units, physicist John Dalton gave these chemically no-further-indivisible units the same name that these presocratic philosophers did: atoms.
Further sensory-based experimentation allowed for other chemists and physicists to apply the Law of Definite (or Constant) Proportions to ascertain what whole-number of atoms of each element was in a molecule of a particular compound. Clearly, no one can observe the mass or weight of an isolated atom. But from experiments like Proust’s, John Dalton ascertained that the mass of an atom of a particular element would consistently be greater than the mass of an atom from another type of element. For example, an atom of oxygen will always be of greater mass than an atom of hydrogen. Hence, Dalton ascertained that the discrete masses of atoms from different elements could be found by comparing them against one another, with the consistent mass of an atom from a particular element —hydrogen — being used as the consistent standard.
Joseph Louis Gay-Lussac experimented by having many containers of the same size — the same volume — and putting different gasified elements or compounds in them. He ascertained that if two glass containers of equal volume each have a different chemical in them, with both samples being of the same temperature and under the same pressure, then both samples will each contain an equal whole-number of molecules (it is possible for a gas to consist of single-atom molecules). Through quantitative measurements of volume that he took through his senses, Gay-Lussac observed that if he has 2 milliliters of hydrogen gas, he will require 1 millileter of oxygen to produce a chemical reaction resulting in gasified water (water vapor), and the volume of the resulting water will be 2 milliliters.
Through the repetition of this sensory observation, scientists such as Gay-Lussac ascertained that in water, each molecule consists of two hydrogen atoms and a single oxygen atom. And as prior experimentations already demonstrated that in water, there are 8 grams of oxygen for every 1 gram of hydrogen, the atomic weight of oxygen could be ascertained. Using the mass of a hydrogen atom as the standard — the assigned atomic mass of hydrogen being “1” — it is the case that a single oxygen atom is of a mass sixteen times that of a hydrogen atom. Hence, the atomic mass of oxygen is 16.
This is the calculation. We are solving for the atomic mass of oxygen, y. The atomic mass of hydrogen is x. 2x/1y = 1/8. And we know x = 1. Therefore, 2/y = 1/8. And so y = 2 * 8 = 16.
Aristotle is not recorded to have conducted controlled experiments, but note that all controlled experiments rely on Aristotle’s Law of Identity. To the extent that the variables in two different samples are the same in the pertinent respect, they are of the same type. Everything in the control sample and the experimental are the same in the pertinent context except for the one variable being tested. Gay-Lussac’s experimental results were meaningful because all of the other variables (the volumes, the temperature, the pressure) were, in the pertinent context, the same. By having all variables in both the control sample and experimental sample being the same except for the one variable we are testing, we can ascertain that this one variable is what causes the differences in results between the control sample and experimental sample. Also, all equations, such as the one above, rely on the principle that if A equals B, and B equals C, then A equals C, and that itself is a rephrasing of the Law of Identity. Hence, although the man himself was doubtful that all matter ultimately consists of atoms, the application of Aristotle’s Law of Identity was crucial to the eventual validation of atomic theory.
It was through the application of the Law of Identity that Amedeo Avogadro ascertained that Gay-Lucsac’s findings could be applied to ascertaining the atomic masses of other elements. And once chemists and physicists figured out the atomic masses of copper (63.546) and carbon (12.011) respectively, they could go back to Joseph-Louis Proust’s findings about copper carbonate to ascertain that a molecule of copper carbonate consists of 1 copper atom, 1 carbon atom, and 3 oxygen atoms.
Yes, an isolated atom cannot be seen. But the model that says that all matter ultimately consists of atoms is a model that, when applied, is vindicated through its observed practical results. Sensory observations confirm the validity of atomic theory.
