By shaking the solution, oxyhaemoglobin is again reproduced, and
gives its special absorption bands.
If ammonia be added to the original solution, _alkaline haematin_ is
produced, or if acetic acid be chosen, _acid haematin_ is produced, and
each gives its appropriate absorption bands.
_Methaemoglobin_ is formed in stains which have been exposed to the air
for a few days, and _haematin_ is found in old stains. _Haemochromogen_
gives a very characteristic spectrum, and is obtained by reducing
alkaline haematin by ammonium sulphide. _Carbon monoxide haemoglobin_
gives a spectrum which resembles that of oxyhaemoglobin, 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.
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