302 COLOURING MATTERS. 



it loses half its acid, and then consists of 4 atoms of haematin and 

 3 atoms of acid. In the combinations of haematin with metals it 

 appears from an experiment of Mulder's that 1 atom of haematin 

 is combined with 1 atom of base. 



The question in what condition does the iron exist in the blood, 

 or on what iron-compound is its red colour dependent ? is one that 

 has long engaged the attention of chemists and physiologists. 

 Without considering that, with an equal right, we might inquire 

 into the causes of the colour of indigo, carmine, or peroxide of iron, 

 it was universally believed that the blood's colour must depend on 

 the last named substance, and consequently, all experiments on the 

 subject were instituted with the view of ascertaining in what state 

 of combination the peroxide of iron lay concealed. It would be 

 superfluous for us to notice the different views regarding the 

 combinations in which the peroxide of iron has been supposed to 

 exist in the blood. We must not, however, omit all notice of the 

 circumstance, that a discovery of Engelhardt's showed the fallacy of 

 these views, for he ascertained that the iron of the blood might be 

 precipitated by alkalies and liver of sulphur, if chlorine gas had 

 been previously, and for some time, passed through the blood ; and 

 this led him to the somewhat illogical conclusion that the iron 

 could not be oxidised, but must exist in a metallic state in the 

 blood ; for Rose's discovery that the precipitation of peroxide of 

 iron and other metallic oxides may be prevented by all the non- 

 volatile organic acids, shows that notwithstanding Engelhardt's 

 experiment, the iron may be contained in the blood in the state of 

 peroxide. Finally, Lecanu discovered the true colouring matter of 

 the blood, the haematin ; and as almost all the iron of the blood is 

 contained in this substance, attempts were again made to refer the 

 colour of this pigment to peroxide of iron. But we know, from 

 the experiments of Scherer, San son, and Mulder, that the iron 

 must be contained in some other combination than in direct com- 

 bination with oxygen, and that the iron may be abstracted from 

 the red blood-pigment without affecting its colour. That the iron 

 is directly combined with the group of atoms constituting hsematin, 

 is not a probable view ; at present, however, we are in possession 

 of no facts throwing any additional light on the nature of the iron- 

 compound. 



The white body formed by the action of chlorine and water on 

 haematin, was found by Mulder to be devoid of iron, and to be 

 composed in accordance with the formula C 44 H 22 N 3 O 6 + 6C1O 3 . 



Preparation. We treat blood with about eight times its volume 



