There is no doubt but that its value as a vehicle of oxygen depends upon the presence of iron. In the matter of taking up and dropping oxygen, hæmatin behaves somewhat in the same manner as hæmoglobin; whereas if iron be removed from hæmatin the “iron-free hæmatin” loses its respiratory value. It is almost certain that a molecule of hæmoglobin contains a single atom of iron. On this supposition its molecular formula may be calculated. It is not quite the same for all animals, although the variations are slight. For the blood of the horse it is as follows: C₇₁₂H₁₁₃₀N₂₁₄S₂FeO₂₄₅. This means a molecular weight of 16708. We give the figures, because the properties of hæmoglobin will be better understood if its prodigious molecular weight is borne in mind. In a sense, the reason for the great size of its molecule is not far to seek. The atomic weight of iron (Fe = 56) is much greater than that of either of the other elements contained in hæmoglobin. The molecule needs to be very great to float an atom of iron. As it is, the corpuscles are heavier than the plasma which surrounds them, in the proportion of about 13 to 12. Although hæmoglobin is a crystallizable substance, its immense molecule is absolutely non-diffusible. It cannot pass through a membrane. This is of no consequence as regards the relation of hæmoglobin to the walls of the capillary bloodvessels, since it is contained in corpuscles; but it is of great importance as regards its relation to the discs which carry it. A very small quantity of enveloping substance suffices to prevent it from diffusing into the plasma of the blood. The great molecules are held together and isolated from the fluid in which they float by a minimal amount of insoluble globin.