2.3 CHEMISTRY AND THE ATOM

By studying and classifying chemical substances, Antoine-Laurent de Lavoisier discovered simple chemical substances which could not be broken down any further. It is possible, for instance, to transform ammoniac salt, a well defined solid substance, into a mixture of gases that can be separated by suitable fractionation into ammonia gas and hydrochloric acid gas. The ammonia gas can further be transformed into a gaseous mixture of nitrogen and hydrogen, which in turn are easily separated. The hydrochloric acid gas can in turn be broken down into chlorine and hydrogen by dissolving it in water and then removing the added water. In this way, 100 grams of ammoniac salt is decomposed into 26.16 grams of nitrogen, 7.50 grams of hydrogen, and 66.34 grams of chlorine, all of which are simple substances. In this manner, any material system whatsoever can be decomposed into masses each composed of one of a relatively small number of simple substances and the resulting masses are found to be independent of the decomposition process used.

It was also found during the chemical analyses that these simple substances, or elements, cannot be mixed together in any which way one might wish. Thus Proust showed that the proportions in which two substances combine cannot vary continuously. Three grams of carbon can combine with 8 grams of oxygen by burning but not, say, with 8.1 grams. In fact, Dalton later found

that if two definite compounds are taken at random from among the multitude of those containing the simple substances $A$ and $B$ , and if the masses of the element $B$ that are found to be combined with the same mass of element $A$ are compared, those masses are usually in a very simple ratio to each other.
Jean Perrin, Atoms (1913)

To make chemistry a science, these kind of findings which systematically summarize chemical experience must be given an explanatory construct:

So long as, for chemical actions of matters on one another, no concept which admits to be constructed, i.e., no law of the approach or withdrawal of the parts of matters can be stated according to which their motions together with the consequences of these can be intuited and presented a priori in space, chemistry can become nothing more than a systematic art or experimental doctrine, but never a science proper.
Immanuel Kant, Metaphysical Foundations of Natural Science (1786)

In 1808 Dalton realized that, just as it did in the case of thermodynamic experience, the corpuscular conception provided such a construct. He knew that by the corpuscular conception of matter each of the chemical elements are composed of molecules specific to that element. In the same spirit, Dalton then supposed that each molecule of a given element is made up in turn of a fixed species of particles, all absolutely identical. These particles, would pass through the various chemical and physical transformation that we are able to bring about with the given element without ever becoming subdivided; being indivisible, they were eventually called atoms. Since any single molecule necessarily contains a whole number of atoms of the corresponding element, this simple extension of the corpuscular conception immediately explained both Proust’s as well as his own empirical laws, and transformed the experimental doctrine of chemistry into a science.