(i) =Spectroscopic Appearances.=--If a solution of a recent stain be examined by the spectroscope, we get two absorption bands situated between the lines D and E, the one nearer E being doubly as broad as the other. These bands indicate _oxyhæmoglobin_.
If we now add a little ammonium sulphide to this solution, we get the spectrum of _reduced hæmoglobin_, which is a single broad absorption band situated in the interval between the preceding oxyhæmoglobin bands. By shaking the solution, oxyhæmoglobin is again reproduced, and gives its special absorption bands.
If ammonia be added to the original solution, _alkaline hæmatin_ is produced, or if acetic acid be chosen, _acid hæmatin_ is produced, and each gives its appropriate absorption bands.
_Methæmoglobin_ is formed in stains which have been exposed to the air for a few days, and _hæmatin_ is found in old stains. _Hæmochromogen_ gives a very characteristic spectrum, and is obtained by reducing alkaline hæmatin by ammonium sulphide. _Carbon monoxide hæmoglobin_ gives a spectrum which resembles that of oxyhæmoglobin, but it is not reduced by ammonium sulphide.
(j) =Precipitin Test.=--This allows us to tell whether the blood is from a human being or not. A specific serum must be obtained from a rabbit which is sensitized as follows: 10 c.c. of human blood is injected into its peritoneal cavity at intervals, until from three to five injections have been given. The serum of this animal's blood will then give a white precipitate only when brought into contact with dilute solutions of human blood, but with the blood of no other animal. This is known also as the 'biologic,' or Uhlenhuth's test.
=Rust Stains.=--These are yellowish-red in colour, and do not stiffen the cloth. The iron may be dissolved by placing the stain in a dilute solution of hydrochloric acid, when, on adding ferrocyanide of potassium, Prussian blue is produced.
=Fruit Stains= are seldom so dark as blood-stains. Solutions of these do not change colour or coagulate on boiling; ammonia changes the colour to blue or green; acid brightens the original colour, while chlorine bleaches it.
=Hairs.=--Human hairs must be identified and distinguished from those of the lower mammals. If the hair has been pulled out from the root, the microscope will show that the bulbous root has a concave surface which fitted over the hair papilla, or that the root is encased in a fatty sheath.