78 THE BLOOD. 



let haemoglobin and deoxidized or purple haemoglobin. The 

 change of color produced by the passage of the blood through 

 the lungs, and its consequent exposure to oxygen, is due, prob- 

 ably, to the oxidation of purple, and its conversion into scarlet 

 haemoglobin; while the readiness with which the latter is de- 

 oxidized offers a reasonable explanation of the change, in re- 

 gard to tint, of arterial into venous blood, the transformation 

 being effected by the delivering up of oxygen to the tissues, by 

 the scarlet haemoglobin, during the blood's passage through 

 the capillaries. The changes of color 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 chemi- 

 cally, on the coloring matter, or mechanically, 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 color of arterial and venous blood 

 respectively ; but it is probable that this cause alone would be 

 quite insufficient to explain the differences in the color 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 nat- 

 urally 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 fundamental differences 

 of chemical constitution, has been made out chiefly by spectrum- 

 analysis, the effects produced by placing oxidized and de- 

 oxidized solutions of haemoglobin in the path of a ray of light 

 traversing a spectroscope being different. For while the oxi- 

 dized solution causes the appearance of two absorption bands 

 in the yellow and the green part of the spectrum, these are re- 

 placed by a single band intermediate in position, when the ox- 

 idized or scarlet solution is darkened by deoxidizing agencies, 

 or, in other words, when the change which naturally ensues in 

 the conversion of arterial into venous blood is artificially pro- 

 duced. 1 



The greater part of the haemoglobin in both arterial and 

 venous blood probably exists in the scarlet or more highly ox- 

 idized condition, and only a small part is deoxidized and made 

 purple in its passage from the arteries into the veins. 



The differences in regard to color between arterial and 



1 The student to whom the terms employed in connection with 

 spectrum analysis are not familiar, is advised to consult, with ref- 

 erence to the preceding paragraph, an elementary treatise on Physics. 



