PROTEINS 285 



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

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



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. 



46. 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. 



47. On a slice of raw carrot put a little hydrogen peroxide and 

 some of the guaiacum solution. Observe the color. Kepeat with 

 boiled carrot. The reaction also is given by milk, pus, saliva, 

 and various other substances, but these substances do not give 

 the test after having been boiled, thus differing from blood. 

 They contain an oxidase which is responsible for the reaction 

 and which is destroyed by boiling. In the case of blood, the 

 reaction depends probably on the catalytic action of the iron 

 constituent of hematin. 



48. Catalase. The presence of a catalase in blood may be dem- 

 onstrated readily. To a little defibrinated blood add hydrogen 

 peroxide. Observe the bubbles of oxygen given off. This reac- 

 tion is due also in some measure to the blood pigment, but fresh 

 blood contains a catalase in addition. Repeat the experiment 

 with boiled blood. Explain. 



49. In testing a blood stain, in addition to the tests given 

 above, a small portion of the fabric should be extracted with 

 glycerol or 0.9% sodium chloride, and the extract examined with 



.the microscope for corpuscles. 



50. An aqueous extract of the stain may be tested for the 

 formation of hemochromogen. If the stain does not dissolve in 



