There are some mixt bodies, from which it has not been yet made appear, that any degree of fire can separate either salt, or sulphur, or mercury, much less all the three. The most obvious instance of this truth is gold, which is a body so fixed, and wherein the elementary ingredients (if it have any) are so firmly united to each other, that we find not in the operations, wherein gold is exposed to the fire, how violent soever, that it does discernably so much as lose of its fixedness or weight, so far is it from being dissipated into those principles, whereof one at least is acknowledged to be fugitive enough.

Part I - The Sceptical Chymist (1661)