DETECTION OF BLOOD. 



177 



porcelain dishes. Unreduced colouring-matters will leave a 

 coloured residue. 



A pink colour is usually due to the presence of blood ; this 

 may be detected by warming the milk to 50° C, and separating 

 it in a high speed centrifuge (Fig. 15) ; if blood is present a 

 bright red deposit is seen at the bottom of the tube. The deposit 

 may be examined microscopically, and it is usually found that 

 the blood-corpuscles have become considerably disintegrated, 



Fig. 15. — High-Speed C.-'ntrifuL:e. 



and have the appearance shown in the plate (Fig. 16). As a 

 confirmatory test the residue should be treated with a drop of 

 acetic acid on a microscope slide, a cover glass placed over the 

 mixture, and the acetic acid gently evaporated over a very 

 small rtame. When nearly dry, the slide should be examined 

 with J-inch power, and the presence of brown rhomboid crystals 

 of hoemin hydrochloride wUl indicate blood. 



Preservatives. — In order to check the growth of micro- 



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