In arterial blood it is present as oxidised hæmoglobin, and the same obtains in blood which has been exposed to the air under certain conditions and for a varying period of time. [Illustration: Fig. 14.--Photo-micrograph of crystals of hæmin, × 250. (R. J. M. Buchanan.)] In venous blood, especially when obtained under conditions preventing oxidation, as from the heart cavity of an animal newly asphyxiated, it is present as deoxidised hæmoglobin. (_b_) In dry stains, especially if they have been subjected to the action of impure air containing the products of coal-combustion, the colouring matter becomes changed into _methæmoglobin_, or hæmoglobin in which its combination with oxygen has been altered in such a way that a current of a neutral gas, such as hydrogen or nitrogen, will not dissociate it, as it does with oxyhæmoglobin. Such stains have a brownish colour, and may give an acid reaction. (_c_) In stains which have retained moisture, from having lain in damp places, the hæmoglobin becomes converted into _hæmatin_. The same change takes place in dry stains after a longer period of time. On examining the solution of the colouring matter from a blood-stain with the spectroscope, the spectrum will vary according to its condition and the nature of the solvent used. The spectra of hæmoglobin and its derivatives are characteristic, and afford conclusive evidence of the presence of blood. The spectra must be recognised, however, in more than one condition. Other substances may yield spectra very similar to that of oxyhæmoglobin, but when subjected to certain tests they do not alter in the same way. They cannot be made to give the spectra of reduced hæmoglobin and reduced hæmatin, and any colouring matter which may be made to yield the spectra of reduced hæmoglobin and reduced hæmatin is derived from blood. Blood Spectra 1. _Oxidised hæmoglobin_ (O₂Hb) is characterised by the presence in its solar spectrum of two absorption bands between the D and E lines. The first band commences at the D line and extends a short distance towards the E. The second commences at a little distance from it, and terminates at the E line; it is about twice the breadth of the first. The band at D is more defined than the other (Fig. 15, 1). 2. _Deoxidised_ or _reduced hæmoglobin_ presents one broad band occupying almost the whole of the space between D and E slightly to the left of these lines (Fig. 15, 2). 3. _Methæmoglobin_ presents two bands between D and E, in the same position as those of O₂Hb, but in addition a third band between C and D and near to the former (Fig. 15, 3). A solution of oxyhæmoglobin or methæmoglobin may be reduced by the addition of a reducing agent, such as Stokes‘ reagent, consisting of ferrous sulphate with a small quantity of tartaric acid dissolved in water and rendered alkaline at the time of using with ammonia, or, better still, by the addition of ammonium sulphide. The spectrum will change to that of reduced hæmoglobin.