ELEMENTAKY PHYSIOLOGICAL CHEMISTRY 



195 



It exists in two conditions, viz., as haemoglobin and as oxy- 

 haemoglobin, and these differ from one another in that oxyhaemoglobin 

 contains loosely combined oxygen, whereas haemoglobin only contains 

 the firmly combined oxygen of the proteid molecule. Unlike other 

 proteids haemoglobin can easily be obtained as crystals. 



EXPERIMENT V. Mix a drop of rat's blood with a drop of distilled 

 water on a slide : place a cover slip over it : as evaporation slowly proceeds 



FIG. 143. Haemin. x 1500. 



the haemoglobin, which has been dissolved out of the corpuscles by the 

 water, will crystallise out. These crystals will of course be first seen at 

 the edge of the preparation. The crystals may be permanently 

 mounted by allowing complete crystallisation to take place, then 

 removing the cover slip and mounting in Canada Balsam. The crystals 

 are orange-red in colour, and usually assume the v form of rhombic 

 prisms (see Fig. 142); 



Of the compounds of haemoglobin the most important from a medico- 



