COLOL RING-MATTERS . 



213 



ph.osph.ine (di-amino-phenyl-acridine usually mixed with di- 

 amino-toluyl-acridine) or caramel has been used. 



It is better, therefore, to use the general method : Take 

 50 c.c. (or more) of milk, make just alkaline to litmus, and 

 evaporate to a paste, and extract the fat thoroughly with ether. 

 Evaporate the ethereal solution, and shake up the fat with 

 a little hot distilled water, separate the water, and evaporate 

 to dryness in a small porcelain dish. Pure milk gives no coloured 

 residue ; if the residue is coloured, this will be due to a reduction 



Fig. 28. High-Speed Centrifuge. 



product of an azo colouring-matter, or to the unreduced colouring- 

 matter. Next, extract the fat-free residue with absolute alcohol, 

 filter the alcoholic extract, and evaporate to dryness in three or 

 four porcelain dishes. Unreduced colouring-matters will leave 

 a coloured residue. 



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

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

 it in a high speed centrifuge (Fig. 28) ; if blood is present a 



