10 ANATOMY FOR NURSES [Chap. I 



up to mix with the hydrogen molecules, so that the two will eventu- 

 ally be mixed uniformly. 



(6) If a layer of water is placed carefully over a layer of sulphuric 

 acid, in such a way that the two do not mix, two distinct layers 

 will be formed with the heavier sulphuric acid at the bottom. 

 The acid molecules will begin to move up and mix with the water 

 molecules, while the water molecules will move down to mix with 

 the sulphuric acid molecules. The action is much slower than 

 with the gases. 



2. When the gases or liquids are separated by certain membranes. 



(c) In the illustration given in (a) if a membrane, permeable 

 to gases, be stretched over the mouth of the bottle, the gases 

 wull mix evenly through it. Also, if a membranous sac of carbon 

 dioxide is placed in a vessel containing oxygen, the two gases will 

 diffuse through the membrane, until the mixture of gases inside 

 and outside is uniform. 



(d) When a bladder of alcohol is immersed in water, the two 

 liquids will diffuse through the membrane; the water diffusing 

 more rapidly than the alcohol, the bladder will become distended. 



This subject of diffusion is an important one, as it has a great 

 deal to do with life and with physiological processes. Because 

 of diffusion the heavier carbon dioxide in the atmosphere is pre- 

 vented from settling to the bottom of the atmosphere, thereby 

 forming a layer of the same next to the earth, a condition which 

 would seem to make life impossible. Diffusion of gases through 

 membranes makes possible the exchange of carbon dioxide and oxy- 

 gen through the walls of the lungs. (See page 246.) 



The explanation of the process is found in the suppositions of 

 the Jcinetic theory that : — 



1. There are spaces between the molecules making up all bodies. 



2. Molecules are in rapid motion in straight lines, the motion of 

 gas molecules being much more rapid and unrestrained than in the 

 case of liquid molecules. 



Two forms of diffusion are spoken of distinctively as osmosis 

 and dialysis. 



Osmosis. — If a solution is separated from the clear solvent 

 by a membrane, which is permeable to the liquid but not to the 

 substance in solution, the liquid will pass through the membrane 

 and the volume on the side of the solution will be increased. Also, 



