Haemoglobin, the red colouring matter of vertebrate blood, C758H1203N195S3FeO218, and its derivatives haematin, C32H30N4FeO3, and haematoporphyrin, C16H18N2O3, are colouring matters about which we possess definite chemical knowledge, as they have been isolated, purified and analysed.

Thus haematin is found in the so-called bile of slugs, snails, the limpet and the crayfish.

By a dilute acid haemoglobin is decomposed into globin, and ``haematin,'' a ferri-pyrrol derivative of the probable formula C34H34N4FeO5; under certain conditions the iron-free ``haematoporphyrin'' is obtained.

But this substance, as well as the so-called haematin used by me, ought perhaps to have been classed with indigestible substances.

The secretion of Drosera and gastric juice were both able to dissolve some element or impurity out of the globulin and haematin employed by me.