4. _Acid hæmatin_ presents a spectrum with a band between D and E, commencing at a little distance from D and ending at E, also a narrower band between C and D and commencing at C. It is a difficult spectrum to obtain. 5. _Alkaline hæmatin_ presents a spectrum with a single band between C and D near to the D line. It is more difficult to obtain than the spectrum of acid hæmatin. It is not necessary, however, to obtain these spectra, viz. 5 and 6, but it is necessary to reduce solutions of either acid or alkaline hæmatin in order to obtain the spectrum of _reduced hæmatin_. To do so proceed as follows. To some of the solution of colouring matter obtained from the stain add a small quantity of a 20 per cent. solution of sodium hydrate; the solution will alter in colour, and the spectrum of O₂Hb or MetHb will disappear. On adding to this solution of alkaline hæmatin a few drops of ammonium sulphide, or Stokes‘ fluid, it becomes claret-coloured, and on examination with the spectroscope the spectrum of _reduced hæmatin_ will be seen. This is the most pronounced of all blood spectra. Its production can be hastened by gently warming the solution. If the stain be old and already changed into hæmatin, its solution will yield the spectrum of acid hæmatin, and will give the spectrum of _reduced hæmatin_ on the addition of ammonium sulphide or Stokes‘ fluid without previous alkalisation. 6. _Reduced hæmatin_ presents a spectrum with a dark band about midway between D and E, and a broad but paler band commences near the E line and extends to the _b_ line (Fig. 15, 4). [Illustration: Fig. 15.--Blood Spectra. 1. Oxyhæmoglobin. 3. Methæmoglobin. 5. CO hæmoglobin. 2. Reduced hæmoglobin. 4. Reduced hæmatin. 6. CO hæmoglobin and reduced hæmoglobin.] In cases of death by asphyxia, in which the hæmoglobin is in combination with CO₂, the blood, if removed and examined immediately after death, gives a spectrum of reduced hæmoglobin, but on exposure to the air it rapidly changes to oxyhæmoglobin. The period after death at which blood is usually submitted for medico-legal examination is sufficiently late to allow of this change, and so prevents the possibility of determining death by asphyxia by spectroscopic examination of the blood. Where death has been caused by the action of _carbon monoxide_, the blood is of a _cherry-red colour_, and it will retain this colour unchanged for a long time, in fact for years, due to a stable combination of the CO and hæmoglobin, called carboxyhæmoglobin. Such blood yields a very characteristic spectrum, with two bands similar to those of O₂Hb, but nearer to the violet end (Fig. 15, 6). Their position should be assured by accurate measurement and comparison with a spectrum of O₂Hb. The CO hæmoglobin, however, cannot be reduced; on the addition of ammonium sulphide or Stokes‘ fluid the bands remain unaltered.