"No matter how properties of simple bodies may change in the free state, something remains constant, and when the element forms compounds, this something is material existence and establishes the characteristics of the compounds, which include the given element. In this respect we know only one constant peculiar to an element, namely the atomic weight. The size of the atomic weight, by the very essence of matter, is common to the simple body and all its compounds. Atomic weight belongs not to coal or diamond, but to carbon."