36 ORGANISED FLUIDS. 



It is clear, then, that the coloured corpuscles arc the seat 

 in which these changes occur. Again, from the fact that the 

 blood becomes bright red or arterial on exposure to oxygen, 

 as in the lungs, and dark red or venous on being submitted 

 to the action of carbonic acid, as in the capillaries, it has 

 been inferred that they are first carriers of oxygen from the 

 lungs to all parts of the system, and second, vehicles for the 

 conveyance of carbon back again to the lungs. 



This inference is correct as far as it goes, but it fails to 

 explain why the imbibition of oxygen or carbonic acid gases 

 should be accompanied by changes in the colour of the blood ; 

 and it also fails to show why those gases themselves should 

 be imbibed. 



From the constant presence of iron in the coloured blood 

 corpuscles, it has been inferred that this is the base with 

 which the oxygen and the carbonic acid gases combine, but 

 the exact nature of the combinations thus formed it was re- 

 served for the illustrious Liebig to make known. 



Liebig declares, that in arterial blood the iron is in the state 

 of a peroxide, and in venous blood in the condition of a car- 

 bonate of the protoxide. 



To this conclusion Liebig has arrived by observing the 

 manner in which the above-mentioned compounds of iron 

 comport themselves when not in connexion with the blood, 

 but when exposed to the same influences as the blood itself is 

 subjected to. 



Thus he says, " The compounds of the protoxide of iron 

 possess the property of depriving other oxidised compounds of 

 oxygen, while the compounds of peroxide of iron under other 

 circumstances give up oxygen with the greatest facility." 



Again, " Hydrated peroxide of iron, in contact with organic 

 matters destitute of sulphur, is converted into carbonate of 

 the protoxide." 



Lastly, (( Carbonate of protoxide of iron in contact with 

 water and oxygen is decomposed, all the carbonic acid is 

 given off, and by absorption of oxygen it passes into the 

 hydrated peroxide, and which may again be converted into 

 a compound of the protoxide." 



