118 RESPIRATION 



gases which do not chemically interact mix into a homogeneous mass 

 when brought together. Thus, if a large bottle be half-filled with 

 carbon dioxide and then the upper half filled with hydrogen, in a short 

 time the bottle will contain a homogeneous mixture, although the 

 carbon dioxide is many times heavier than the hydrogen. Again, if 

 one bottle be filled with oxygen, a tube 1 meter long and only 1 cm. in 

 diameter connected with it and inserted into a similar bottle above 

 containing hydrogen, in a few hours both bottles will contain a homo- 

 geneous mixture of the hydrogen and oxygen,- although the light hydro- 

 gen has to pass downward through a narrow tube in doing its part 

 of the diffusion. This phenomenon is brought about by the fact that 

 the molecules of gases are continually and rapidly moving in straight 

 lines as far as they can go namely, until they meet with obstacles, 

 whether other molecules or the walls of the vessel containing the gas. 

 As the rate of the molecular movements of a gas increases with its 

 temperature, diffusion takes place faster in a warm environment, e. g., 

 in the lung, than in cool surroundings. The rate varies also with the 

 density of the gases, in exact terms inversely as the square root of the 

 density (Graham's law). If a vessel be filled with oxygen and hydrogen 

 each on one side of a porous and thin earthenware partition dividing 

 the vessel equally, the two gases will pass through this diaphragm at 

 very different rates; 4 c.c. of hydrogen will work its way through the 

 pores of the earthenware and into the other half of the vessel while 

 1 c.c. of oxygen is passing in the opposite direction. This process 

 of admixture, whether or not through a dry partition, is diffusion, and 

 is the process in part which obtains in the small bronchi of the lungs. 

 It must be discriminated carefully from the phenomena which occur 

 when gases mix through an organic membrane, such as one .made of 

 skin, rubber, or epithelium; the process then is osmosis. In diffusion 

 there is immediate mixture or else the passage of the gases through 

 minute tubes such as those of the earthenware partition or the smallest 

 bronchi, the conditions being relatively simple. In gaseous osmosis, on 

 the other hand, the rate of interchange depends not alone on the natures 

 of the passing gases and their passageways, but more on the nature of 

 the dividing membrane. Among the determining factors in osmosis are 

 the relative diffusibilities of the two gases: their respective densities; 

 the different degrees exerted by the membrane on the different gases by 

 virtue of which the gas which adheres the most strongly penetrates the 

 diaphragm most easily; and the degree of actual liquefaction of the two 

 gases sometimes, which may thus penetrate the membrane and evap- 

 orate to gases again on the other side. Osmosis is the process which 

 obtains in external and internal respiration. The "membrane" in 

 this case is, of course, the complex and protoplasmic epithelial layers 

 of the alveolar wall and that of the capillaries, plus the layers of inter- 

 vening organic liquid, already described. 



THE COURSE AND THE KINETICS OF THE CARBON DIOXIDE OUTWARD. 

 This can be described much more briefly than was the corresponding 



