When a solution of oxyhaemoglobin is exposed to the atmosphere for a few days its color changes to a dingy brown, and it takes up more oxygen than it previously contained. The new product is called methccmoglobin. The oxygen is more firmly combined than in the oxyhaemoglobin, so that it cannot be removed by passing other gases (CO, etc.) through the liquid, or by exhaustion with an air pump. Methaemoglobin gives an absorption spectrum which differs from that of oxyhaemoglobin in having only a single band, and from that of reduced haemoglobin in that the single band is placed more to the red side of the spectrum, i. e., between the lines C and D. This substance can also be formed by the addition of potassium permanganate or alkaline nitrites to haemoglobin. A solution of methaemoglobin, though unaltered when placed in vacuo, may be reduced to haemoglobin by ammonium sulphide. It then regains its red color, shows the spectrum of reduced haemoglobin, and when shaken with air reforms oxyhaemoglobin.