284 PHYSIOLOGICAL CHEMISTRY 



a few crystals of ammonium oxalate, and 1-2 drops ether. Mix 

 and cover. In a few minutes crystals of oxyhemoglobin separate 

 out. Observe under the microscope and draw. 



Chemical Tests for Recognizing the Presence of Blood. 



44. Hemin Crystals. Place a drop or two of blood on a slide. 

 Add one or two crystals of sodium chloride (no more) and rub 

 with a glass rod until the salt has dissolved. Place the slide on 

 a ring about a foot above a small flame and allow the blood to 

 evaporate slowly to complete dryness. Eub the red residue to a 

 powder with a knife blade, collect the powder in a small pile and 

 add a drop of glacial acetic acid from a glass rod. Rub to a 

 paste, place a portion on a clean slide, and add a drop of glacial 

 acetic acid. Cover with a cover slip and cautiously heat over a 

 small flame until the acid begins to boil. Let a drop more of the 

 acid run under the slide and allow to cool. If no crystals form, 

 add another drop or two of acid and repeat the boiling. Exam- 

 ine the crystals of hemin (hematin hydrochloride) under the 

 microscope and draw. The crystals are brownish-red rhomboids. 



This test is one of the best methods for detecting small quan- 

 tities of blood in blood stains. Blood stains on cloth, etc., are 

 soaked in distilled water or alkali, the solution evaporated and 

 treated as above. The formation of hemin crystals is an abso- 

 lute proof of the presence of blood; it does not, however, dis- 

 tinguish between the blood of man and that of some other ani- 

 mals. 



45. Benzidene Reaction. The benzidene reaction is also one 

 of the most delicate and reliable tests for blood. Different solu- 

 tions of benzidene vary considerably in sensitiveness, so that in 

 practice a control always should be run, using distilled water in 

 place of blood. Benzidene solutions should be kept in the dark 

 as they are easily altered by the action of light. Mix a saturated 

 solution of benzidene in alcohol or glacial acetic acid, with an 

 equal volume of 3% hydrogen peroxide and add one cubic centi- 

 meter of a dilute blood solution. If the mixture is not already 

 acid, acidify with acetic acid. Note the greenish blue color. This 



