GENERAL PROPERTIES! THE CORPUSCLES. 399 



a change in color, taking on a brownish tint. This change is due to 

 the formation of methemoglobin, and it is said that to some extent 

 the transition occurs very soon after the blood is exposed to the air, 

 and that, therefore, determinations of the quantity of hemoglobin 

 by the ordinary colorimetric methods should be made promptly to 

 avoid a deterioration in color value. Methemoglobin may be 

 obtained rapidly by the action of various reagents on the blood, 

 some of them oxidizing substances, such as permanganate of potash 

 or ferricyanid of potash, some of them reducing substances. In- 

 deed, it is known that the change may occur within the blood-vessels 

 by the action of such bodies as the nitrites, antifebrin, acetanilid, 

 etc. According to most observers, methemoglobin contains the 

 same amount of oxygen as hemoglobin; it is combined differently, 

 however, forming a more stable compound, which can not be dis- 

 sociated by the action of a vacuum. On this account, therefore, 

 methemoglobin is not capable of acting as a respiratory pigment, 

 and to the extent that it is formed in the blood this tissue suffers a 

 loss of its functional value as a carrier of oxygen. By the stronger 

 action of reducing solutions such as ammonium sulphid the 

 oxygen may be removed from the methemoglobin and reduced 

 hemoglobin be obtained. Methemoglobin crystallizes in needles, 

 and its solutions give an absorption spectrum which varies ac- 

 cording as the solution is neutral or has an alkaline reaction. In 

 neutral solutions the characteristic band is one in the orange, as 

 indicated in Fig. 170. In alkaline solution the absorption spectrum 

 has three bands, two of which are nearly identical with those of 

 oxy hemoglobin . 



Hematin (C 34 H 34 N 4 FeO 5 ) is obtained when hemoglobin is de- 

 composed by the action of acids or alkalies in the presence or oxygen. 

 It may occur in the feces if the diet contains hemoglobin or hematin, 

 or in case of hemorrhages in the stomach or small intestine, since 

 both the pancreatic and the gastric secretion break up hemoglobin, 

 with the formation of hematin. It is an amorphous substance, of a 

 dark-brown color, easily soluble in alkalies or in acid alcoholic solu- 

 tions. These solutions give a characteristic absorption spectrum 

 which is represented in Fig. 170. 



Hemin (C 34 H 33 O 4 N 4 FeCl) is regarded as the hydrochloric acid 

 ester of hematin and is obtained by the action of HC1 upon blood 

 previously treated with alcohol. The compound is obtained in the 

 form of crystals, which under the microscope appear usually as 

 small, rhombic plates of a dark-brown color. These crystals may 

 be obtained from small quantities of blood stains, etc., no matter 

 how old, and they have been relied upon, therefore, as a sure and 

 easy test for the existence of blood, that is, hemoglobin. The 

 test is one that has been much used in medicolegal cases, and may 



