For 18th-century discoveries, around the time that Antoine Lavoisier first questioned the phlogiston theory, the recognition of a new "earth" has been regarded as being equivalent to the discovery of a new element (as was the general practice then). For some elements (e.g. Be, B, Na, Mg, Al, Si, K, Ca, Mn, Co, Ni, Zr, Mo), this presents further difficulties as their compounds were widely known since medieval or even ancient times, even though the elements themselves were not. Since the true nature of those compounds was sometimes only gradually discovered, it is sometimes very difficult to name one specific discoverer. In such cases the first publication on their chemistry is noted, and a longer explanation given in the notes.