86 THE BLOOD. 



the scarlet haemoglobin, during the blood's passage through 

 the capillaries. The changes of colour are more probably 

 due to this cause, namely, a varying quantity of oxygen 

 chemically combined with the haemoglobin, than to any 

 mechanical effect of this gas, or to the influence of carbonic 

 acid, either chemically, on the colouring matter, or me- 

 chanically, on the corpuscles which contain it. We are 

 not, perhaps, in a position to deny altogether the possible 

 influence of mechanical conditions of the red corpuscles on 

 the colour of arterial and venous blood respectively ; but it 

 is probable that this cause alone would be quite insufficient 

 to explain the differences in the colour of the two kinds of 

 blood, and therefore if it be an element at all in the change, 

 it must be allowed to take only a subordinate position. 



The distinction between the two kinds of haemoglobin 

 naturally present in the blood, or, in other words, the 

 proof that the addition or subtraction of oxygen involves 

 the production respectively of two substances having funda- 

 mental differences of chemical constitution, has been made 

 out chiefly by spectrum- analysis, the effects produced by 

 placing oxidised and de-oxidised solutions of haemoglobin 

 in the path of a ray of light traversing a spectroscope being 

 different. For while theSoxidised solution causes the ap- 

 pearance of two absorption bands in the yellow and the 

 green part of the spectrum, these are replaced by a single band 

 intermediate in position, when the oxidised or scarlet solution 

 is darkened by de-oxidising agencies, or, in other words, 

 when the change which naturally ensues in the conversion 

 of arterial into venous blood is artificially produced.* 4 



The greater part of the haemoglobin in both arterial and 

 venous blood probably exists in the scarlet or more highly 

 oxidised condition, and only a small part is de -oxidised and 

 made purple in its passage from the arteries into the veins. 



* The student to whom the terms employed in connection with 

 spectrum analysis are not familiar, is advised to consult, with reference 

 to the preceding paragraph, an elementary treatise on Physics. 



