The vapour density of calomel vapour corresponds to the formula HgCl or Hg + HgCl2. Odling showed, by amalgamation of gold-leaf, the presence of free mercury. Harris and V. Meyer separated mercury from the vapour by diffusion, and also showed that solid potassium hydroxide turned yellow (HgO) when plunged into the vapour. Diffusion might naturally increase the dissociation, and even a pure mercurous salt would certainly give mercuric oxide at 240–260°, since at that temperature mercurous oxide is unstable. The quantitative conclusion of these observers, namely, that the vapour is completely dissociated, therefore, was not in any way justified by their qualitative experiments, although it has been accepted by many chemists.