248 PHYSIOLOGICAL CHEMISTRY 



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

 acid run under the slide and allow to cool. Examine 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. 



(b) 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 

 test may be performed by adding a small amount of solid benzi- 

 dene. and glacial acetic acid instead of the benzidene solution. 



(c) Guaiac test. This test, if properly performed is extremely 

 delicate. As it is given by substances other than blood, however, 

 conclusions from a positive result should be drawn only with 

 caution. A negative result is conclusive evidence of the ab- 

 sence of blood. 



(1) Dilute defibrinated blood by adding 2-3 drops to a test 

 tube of distilled water. Add about one-eighth volume of hydro- 

 gen peroxide (3 vols. per cent) and float on the surface a layer 

 of tincture of guaiacum (or of guaiaconic acid). Note the slow 

 appearance of a green-blue color above the junction of the 

 liquids. Boil a small volume of blood and repeat the test. It is 

 still positive. 



(2) On a slice of raw carrot put a little hydrogen peroxide and 



