The published determinations of the vapour densities of ammonium chloride are not consistent. Thus the percentage dissociation calculated from the observations at 300° to 323° ranges from 87 to 100; at 350°, from 83 to 100; and at 445°, from 89 to 99. The results also lack quantitative significance because they were nearly all obtained by the Dumas or the V. Meyer method. When the former method was used, the density was that at atmospheric pressure, although the temperature corresponded to a much higher vapour pressure. When the latter method was used, in most cases, for the same reason, the vapour was highly unsaturated. In a few instances the vapour pressure was less than one atmosphere, and the vaporisation could proceed only by diffusion of the vapour into the air contained in the apparatus.