90 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



In nearly all animals the crystals are rhombic prisms ; but in 

 the guinea-pig they are rhombic tetrahedra (four-sided pyramids) ; 

 in the squirrel, hexagonal plates ; and in the hamster, rhombohedra 

 and hexagonal plates. 



The crystals also contain a varying amount of water of crystallisa- 

 tion : this may in part explain their different crystalline forms 

 and solubilities. 



Different observers have analysed haemoglobin. They find carbon, 

 hydrogen, nitrogen, oxygen, sulphur, and iron. The percentage of 

 iron is 0*4. The amounts of the other elements are variously given, 

 but, roughly, they are the same as in the proteids. We know at 

 present as little of the chemical structure of haemoglobin as of the 

 proteids generally. 



Oxyhaemoglobin may be estimated in the blood (1) by the amount 

 of iron in the ash, or (2) by certain colorimetric methods which are 

 described in the Appendix. 



On adding an acid or alkali to haemoglobin, it is broken up into 

 two parts, a proteid called globin, and a brown pigment called 

 hcematin, which contains all the iron of the original substance. 



Globin is a somewhat unique proteid. It is coagulable by heat, 

 soluble in dilute acids, and precipitable from such solutions by 

 ammonia. It closely resembles a substance previously separated 

 from red corpuscles by Kossel and termed by him histone (Schulz). 



Hsematin is not crystallisable ; according to Hoppe-Seyler its 

 formula is C34H35N4FeO,5 ; other observers give different for- 

 mulae. It presents different spectroscopic appearances in acid and 

 alkaline solutions, which we shall study more fuUy in the advanced 

 c^rse. It also yields several products under the influence of certain 

 reagents, which we shall also again consider in the advanced course. 

 For the present, we will mention only two of these, haemin and 

 hsematoporphyrin. 



Haemin is of great importance, as the obtaining of this substance 

 in a crystalline form is the best chemical test for blood. Haemin 

 crystals, sometimes called, after their discoverer, Teichmann's 

 crystals, are composed of the hydrochloride of haematin. They 

 may be prepared for microscopic examination by boiling a frag- 

 ment of dried blood with a drop of glacial acetic acid on a slide ; 

 on cooling, dark brown plates and prisms belonging to the tricliuic 

 system, often in star-shaped clusters and with rounded angles 

 (fig. 31), separate out. 



In the case of an old blood-stain it is necessary to add a crystal 



