ABSTRACT
The contemporary ‘atom’ and ‘molecule’ belong to both chemistry and physics. For much of the nineteenth century, the words ‘atom’ and ‘molecule’ were used interchangeably to describe any invisible and massy particle that was capable of expressing motion and force. The term ‘molecular physics’ was common throughout the century for describing the investigation of the mechanical properties of these particles. After the time of John Dalton (1766–1844), the word ‘atom’ increasingly came to be identified with the chemical atom as an element’s experimentally-determined combining weight relative to the standard of oxygen or hydrogen. Chemists frequently used the term ‘compound atom’ rather than ‘molecule’ for combinations of atoms into complex chemical substances.
