196 PEACTICAL PHYSIOLOGY 



legal point of view is Haemin. Chemically it is hydrochloride of 

 haematin, and is obtained by decomposing haemoglobin by means of 

 an acid in the presence of free hydrochloric acid. It is important 

 because it forms very characteristic crystals, and can be obtained 

 with no great difficulty even from blood stains several months old. 

 It is, therefore, a useful medico-legal test for blood. 



EXPERIMENT VI. Smear a small drop of blood on a slide and allow 

 to dry. Place a cover slip over it, and then run some glacial acetic 

 acid underneath the cover slip. Warm till small bubbles begin to 

 appear, cool, examine for haemin crystals. These are small dark-brown 

 elongated rhombic crystals, usually collected into star-shaped or rosette- 

 like groups (see Fig. 143). In this preparation the sodium chloride of 

 the blood reacts with the acetic acid and hydrochloric acid is liberated, 

 which then combines with the haematin simultaneously produced from 

 the decomposition of haemoglobin. If it be desired to do the test with 

 an old blood stain it is necessary to supply a crystal of sodium 

 chloride. 



EXPERIMENT VII. A piece of rag which has some time previously 

 been soaked in blood is given round. Prepare haemin crystals from it 

 by placing a piece of it the size of a split pea on a slide, adding two or 

 three drops of glacial acetic acid and one crystal of common salt, 

 covering with a cover slip, and then warming till bubbles of gas arise. 

 On cooling, crystals of haemin will be seen. 



The other compounds and derivatives of haemoglobin will be 

 considered in connection with the spectroscope since it is by means of 

 their spectra that they are identified. 



CHAPTER VIII. 



THE SPECTROSCOPIC EXAMINATION OF HAEMOGLOBIN AND 

 ITS DERIVATIVES. 



A SPECTROSCOPE consists essentially of a screen, in which there is 

 a small slit, through which light from any desired source can pass, 

 a prism, and a series of lenses forming the telescope, through which 

 the observer looks. 



For qualitative work the small direct vision spectroscope (Fig. 144 A) 

 is serviceable. When the position of the bands, however, is required, 

 one of the larger compound forms is necessary. 



Adjustment of the Spectroscope. It is necessary to have an exact 

 focus of the image of the slit. In the small direct vision spectroscope this 



