PROTEINS 



119 



takes place. Thus it appears that the blood contains an enzyme 

 capable of decomposing peroxides. 



Hemin Test. One of the best tests for blood is the prepara- 

 tion of hemin crystals. Hemin is the hydrochloride of hema- 

 tin. The hemin test will not differentiate between the blood of 

 man and other animals. Although hemoglobins from different 

 animals differ, it is the globin portion which varies, and the 

 hematin from different animals is undoubtedly the same. The 

 hemin test is very delicate. The suspected stains are extracted 

 with water or weak alkali, the solution evaporated to dryness 

 and the residue used for the test. If the solution is very dilute, 

 the pigment may be precipitated with tannic acid, and the test 

 made on the precipitate. 



The test is performed by evaporating a drop of blood or sus- 

 pected material to dryness on a slide, adding sodium chloride and 

 a few drops of glacial acetic acid. The mixture is covered with a 

 cover glass and heated carefully until the acid boils. On cool- 

 ing, brown or reddish brown rhomboidal crystals of hemin form. 

 If crystals do not form the first time, the mixture should be 

 boiled again as above. It may be necessary to repeat the process 

 several times. As hematin is very stable, blood stains which 

 have putrefied, or which are even centuries old still will give 

 the test. Wood smoke and swamp water do not destroy hem- 

 atin ; it may be destroyed if certain moulds have grown on the 

 stain however. It is of interest that the excreta spots of blood 

 sucking insects will give a positive hemin test. 



Guaiac Test. Benzidine Test. Blood has the property of 

 transferring oxygen from hydrogen peroxide to easily oxidized 

 chemical substances such as guaiac, benzidine and a variety of 

 other compounds. On adding a small amount of a freshly pre- 

 pared 1% alcoholic gum guaicum solution to a solution con- 

 taining blood, and then a small volume* of 3% hydrogen peroxide, 

 a bluish green color develops. Old turpentine which has not 

 recently stood in direct sunlight may be used in place of the 

 hydrogen peroxide. In performing the benzidine test it is best 

 to dissolve in glacial acetic acid the amount of benzidine which 



