290 PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



air, as it penetrates more and more deeply into the 

 bronchial tubes, loses its O and receives more and 

 more CO 2 , its rate of diffusion is not impaired, since 

 with its advance it is meeting a continually increasing 

 density of mixed gases. Thus, from the upper air- 

 passages down to the air cells, a gaseous exchange 

 is constantly going on between the less dense mixture 

 of inspired air and the denser mixture of the air 

 occupying the lungs, fresh inspirations maintaining 

 the lower density of the air in the upper parts ; and 

 the exchange going on between the blood circulating 

 in the walls of the air cells, and the air occupying 

 the cells themselves constantly maintaining the density 

 in the deeper parts. The application of physical laws 

 to this exchange between blood and air will be dis- 

 cussed later. 



Diffusion of gases through porous septa. 

 Gases have been found able to pass through porous 

 septa. Elaborate experiments have been made both 

 by Bunsen and Graham as to the laws regulating the 

 diffusion. A glass tube, filled with the gas to be 

 experimented with, closed at one end with a plug of 

 gypsum, the other end being immersed in mercury, 

 was employed. It is called a DIFFUSIOMETER. The 

 diffusion took place through the septum, but not at 

 the same rate as it would have taken place without it. 

 The septum was not found to affect the exchange by 

 any absorption of the separated gases. But it was 

 necessary to take into account the nature of the gas 

 and of the porous diaphragm, determining the co- 

 efficient of friction, as it is called, between the gas and 

 the diaphragm. Where the tube was filled with 

 hydrogen and air was on the other side of the septum, 

 both being at the same pressure, the hydrogen passed 

 out faster than the air entered, and so the mercury 

 rose in the tube. On the other hand, if the tube 

 were filled with C0 2 the air entered faster than the 



