were principles both of the chemist's operations and of the mixts they operated upon. As instruments they were, unlike specific chemical reagents, "natural and general," always at work in every chemical operation. As constituent elements, they did not contradict the chemistry of displacement but transcended it: the chemist could never isolate or characterize an element as he characterized a body; an element was not isolable, for it could not be separated from a mixt without re-creating a new mixt in the process. In his book The Sceptical Chymist of 1661, Robert Boyle criticized the traditional understanding of the composition of materials and initiated the modern understanding of chemical elements. See also Sulfur-mercury theory of metals References History of chemistry Alchemical substances