LECT. VII. AGENCY OF IRON. 137 



of oxygen which remains with it, or if it be the effect of 

 these two circumstances combined. To explain this matter 

 clearly accurate experiments are wanting. 



Magnus has proved that venous blood, in losing the 

 greatest possible quantity of carbonic acid, becomes less 

 deeply coloured, but without ever acquiring a vermilion tint. 

 This fact leads us to assume that these two causes have a 

 simultaneous influence on the change of colour which the 

 blood undergoes during respiration. 



I ought to add, that if we carefully remove all the serum 

 which surrounds the coagulum, and afterwards wash the 

 latter with distilled water to deprive it of every trace of 

 serum, it in this state no longer acquires, by contact with 

 oxygen, that beautiful vermilion colour which it assumes 

 when it is immersed in the serum. Here is a saturated 

 solution of common salt, which I pour, drop by drop, on 

 the blood clot; and you perceive that those parts on which 

 the drops fall, acquire a vermilion colour, whilst the other 

 parts of the surface undergo no change. 



It appears, then, that the salts of the serum are not pas- 

 sive in the modification which the colour of the blood under- 

 goes in the presence of oxygen. We now know that the 

 serum absorbs a much greater quantity of carbonic acid 

 than can be dissolved by water. We may, therefore, say 

 that the serum influences the change of colour of the blood, 

 in consequence of containing a portion of carbonic acid, of 

 which it is afterwards deprived by the oxygen. 



Agency of Iron. But how can we account chemically for 

 the change in the colour of the blood globules? On this 

 point science has not hitherto enlightened us. The large 

 quantity of iron (5 or 6 for 100) which invariably exists in 

 these globules, and is met with, in this proportion, in no 

 other animal substance, has given rise to the idea that this 

 metal, found in the blood, sometimes in the condition of 



