FORENSIC MICROSCOPY. 127 
size and sometimes of considerable age. Two problems 
are involved—first, as to the presence or absence of blood 
in the specimen presented, and second, as to its nature, 
whether human or animal. The presence of blood in 
general may be settled pretty easily by four methods— 
one chemical, one microchemical, one microspectro- 
scopical, and one purely microscopical. The chemical 
test consists in the application to the stain of an alcoholic 
solution of gum guaiacum, which yields a blue color 
reaction on the further <ddition of aerated turpentine. 
The microchemical test depends on the fact that hemo- 
globin, the complex albuminoid substance characteristic 
of the red blood-corpuscles, yields on treatment with 
acetic acid a crystalline iron salt, hematin, whose triple 
chloride is known as hemin. In making this test the 
blood, if in a dry condition, is placed on a slide and gently 
warmed in glacial acetic acid. This is allowed to evapo- 
rate and replaced by a 0.07% solution of sodium chloride, 
which in its turn is carefully evaporated without heating 
the slide above 60°C. Another drop of acetic acid is 
added and the slide is this time heated so that the liquid 
boils. After evaporation it is examined under the 4-in. 
objective; if blood be present, crystals of hemin (or 
hematin trichloride) will appear as minute flat rhom- 
boids often superposed in the form of a cross. They 
vary from yellow to reddish brown in color and from 
0,005-0.02 mm. in size. This test is a delicate one, serv- 
ing to detect from 0.05-0.15 mg. of dried blood, and even 
very old material gives satisfactory results, as in the classic 
case of the blood of the murdered Kotzebue, detected 
