We now know how erroneous in most points this reasoning was, but we must admit that Boyle established one point most satisfactorily, viz. that because earth, or air, or fire, or water is obtained by heating or otherwise decomposing a substance, it does not necessarily follow that the earth, or air, or fire, or water existed as such in the original substance. He overthrew the doctrine of elementary principles held by the "vulgar chymists." Defining elements as "certain primitive and simple bodies which, not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved," Boyle admitted the _possible_ existence, but thought that the facts known at his time did not warrant the assertion of the _certain_ existence, of such "elements." The work of Hooke and Mayow on combustion tended to strengthen this definition of "element" given by Boyle. Black, as we have seen, clearly proved that certain chemical substances were possessed of definite and unvarying composition and properties; and Lavoisier, indirectly by his explanation of combustion, and directly in his "Treatise on Chemistry", laid down the definition of "element" which is now universally adopted. An element is a substance from which no simpler forms of matter--that is, no forms of matter each weighing less than the original substance--have _as yet_ been obtained. In the decade 1774-1784 chemical science was thus established on a sure foundation by Lavoisier. Like most great builders, whether of physical or mental structures, he used the materials gathered by those who came before him, but the merit of arranging these materials into a well-laid foundation, on which the future building might firmly rest, is due to him alone.