THE CHEMISTRY OF RESPIRATION. 405 



gests itself as the cause of the change in color of the blood is 

 that its red corpuscles have shrunk in the pulmonary circula- 

 tion, and so reflect more light and give the blood a brighter 

 look. This idea gains some support from the fact that, as 

 seen under the microscope, the red blood corpuscles of some 

 animals, as the frog, do expand somewhat when exposed to 

 carbon dioxide gas and shrink up a little in oxygen. But 

 that this is not the chief cause of the color change is readily 

 proved. By diluting blood with water the coloring matter of 

 the red corpuscles can be made to pass out of them and go 

 into solution in the plasma, and it is found that such a 

 solution, in which there can be no question as to the reflect- 

 ing powers of colored solid bodies suspended in it, is brighter 

 red when supplied with oxygen than when deprived of that 

 gas. This suggests that the coloring matter or hcemoglobin 

 of the red corpuscles combines with oxygen to form a scarlet 

 compound, and when deprived of that gas has a darker and 

 more purple color; and other experiments confirm this. 

 Haemoglobin combined with oxygen is known as oxyhcemo- 

 globin, and it is on its predominance that the color of arterial 

 blood depends. Haemoglobin uncombined with oxygen, 

 sometimes named reduced hmmoglolin, predominates in 

 venous blood, and is the only kind found in the blood of a 

 suffocated mammal. 



The Laws Governing the Absorption of Gases by a 

 Liquid. In order to understand the condition of the gases 

 in the blood liquid it is necessary to recall the general laws 

 in accordance with which liquids absorb gases. They are as 

 follows : 



1. A given volume of a liquid at a definite temperature if 

 it absorbs any of a gas to which it is exposed, and yet does 

 not combine chemically with it, takes up a definite volume 

 of the gas. If the gas be compressed the liquid will still, at 

 the same temperature, take up the same volume as before, 

 but now it takes up a greater weight; and a weight exactly 

 as much greater as the pressure is greater, since one volume 

 of a gas under any pressure contains exactly twice as much 

 of the gas by weight as the same volume under half the pres- 

 sure; and so on. A liter or a quart of water, for example, 

 exposed to the air will dissolve a certain amount of oxygen. 

 If the air (and therefore the oxygen in it) be compressed to 

 one fourth its bulk then the water will dissolve exactly the 



