§ 42A.] biological test for blood. 73 
To each tube is now added 10 drops of serum from rabbits which 
have been repeatedly subcutaneously injected with human blood. 
The tubes are placed in water at 37-40° C. ; after 10 minutes 
human blood will show a precipitate, which will be copious after half an 
hour. A slight precipitate may be neglected. This test may be also 
applied to old blood-stains. A precipitate is also said to be obtained 
with blood from monkeys. Care should be taken to neutralise the blood 
if it be acid, as it will, if acid, always give a precipitate. When blood 
has been dried on certain materials, such, for instance, as thick polished 
yellow leather, it has been found impossible to get the reaction. 
