214 THE DETECTION OF ADDED SUBSTANCES. 



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

 may be examined miscroscopically, and it is usually found that 

 the blood-corpuscles have become considerably disintegrated, 

 and have the appearance shown in the plate (Fig. 29). 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 very gently evaporated over a 

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

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

 of haemin hydrochloride will indicate blood. 



Detection of Urine. It has been found that urine is 

 occasionally added to milk, either maliciously, or accidentally as 

 by the micturition of a dog, which may occur if a milk vessel is 

 left on the ground. As milk is practically free from urea or 



f 



Fig. 29. Blood in Milk. 



other compounds liberating gas from sodium hypobromite, the 

 detection is easy. 



A rapid qualitative test consists in half-filling a small test 

 tube (2 X J is large enough) with sodium hypobromite (1 c.c. 

 of bromine dissolved in 10 c.c. of 30 per cent, caustic soda solu- 

 tion) ; carefully fill the tube with milk so that the two liquids 

 do not mix. Place the thumb over the tube, and invert once or 

 twice, and then hold it, with the thumb still over the opening, 

 upside down. Milk causes practically no pressure on the thumb, 

 and gives not more than one-fifth of its volume of gas ; if urine 

 has been added, much pressure is developed, and the liquid 

 is forced out, and milk containing 1 per cent, of urine yields 

 about two-fifths the volume of gas, while 5 per cent, causes 

 an evolution of gas equal in volume to the milk taken. 



