336 THE CHEMISTRY OF CREATION. 



withdraw variable quantities of the blood circu- 

 lating- in the system. The fluid thus derived is 

 always of a dark colour, and sometimes is almost 

 black. But, occasionally, disease calls for the 

 opening of an artery, and then the most striking 

 difference is perceptible in the appearance of the 

 blood ; for it is of a vivid bright-scarlet hue. If 

 the dark venous blood is exposed for some little 

 time to the fresh air, it loses its dark colour, and 

 assumes the lighter aspect of arterial blood : 

 but it still differs from arterial blood in many 

 important particulars. This change is directly 

 attributable to the influence of air, for it would 

 not take place in a vacuum. If a moist piece 

 of bladder were laid over the fluid, it would 

 not prevent the change from dark to red ; and 

 it is known to physiologists, that when dark 

 blood becomes circulated in an organised living 

 structure over a large surface, upon which al- 

 ternate currents of fresh air play, the mere cir- 

 cumstance that air is not brought into direct 

 contact with blood does not interfere with its 

 chemical effects on that fluid. Direct con- 

 tact with air is therefore not necessary to 

 effect the change, since it will take place very 

 readily through the medium of an interposed 

 animal membrane. This is, in part, due to the 

 laws of the interpenetration or diffusion of 

 gases, and in part to the remarkable forces called 



