120 PHYSIOLOGICAL CHEMISTRY 



can be taken on a knife point, then add an equal volume of 

 hydrogen peroxide and the liquid to be tested. If blood is pres- 

 ent, the color produced will be a greenish blue. If blood has 

 been diluted 200,000 times it still will give a very definite posi- 

 tive reaction. Other substances also respond to these tests, such 

 as milk, living matter, etc. A slice of raw carrot gives a very 

 good test. Milk, and living matter, (carrot) if boiled, no longer 

 will give the test, whereas blood gives it quite as readily after 

 boiling. Evidently in the case of blood the material responsible 

 for the test is not an enzyme. As negative tests, these color 

 tests may be taken to indicate the absence of blood. As positive 

 tests, however, they are not conclusive without further con- 

 firmation from other tests. Also they do not distinguish human 

 blood from that of other animals. 



Absorption Spectra of Oxyhemoglobin and Hemoglobin. 

 Hemoglobin and many of its derivatives show characteristic ab- 

 sorption spectra. Thus a spectroscopic investigation often is 

 of the greatest value in detecting blood in feces, urine, gastric 

 contents or in stains in medico-legal work. The student is re- 

 ferred to the discussion of absorption spectra under pentoses 

 in the chapter on carbohydrates. The nature of the absorption 

 bands depends not only upon the substance present, but the con- 

 centration of the solution and thickness of the layer through 

 which the light passes. Blood diluted ten times with water, and 

 observed with a spectroscope in a flat sided cell about one centi- 

 meter in thickness allows only a little red light to pass through. 

 If the solution is diluted however, it shows two absorption bands 

 near together between the D and E Frauenhofer lines. On fur-, 

 ther dilution these lines become fainter and at sufficient dilution, 

 finally disappear. 



If Stokes' reagent is added to an oxy hemoglobin solution, thus 

 converting the oxyhemoglobin into hemoglobin the double bands 

 give place to a single continuous band in about the same loca- 

 tion. 



The amount of hemoglobin or oxyhemoglobin in blood or 

 other fluids may be estimated by various means, the most con- 



