THE STORY OF THE LIVING MACHINE. 



to pass through the membrane, each passing 

 through the membrane into the other compart- 

 ment. The force which drives these liquids through 



the membrane is 

 considerable, and 

 may sometimes be 

 exerted against 

 considerable pres- 

 sure. A simple ex- 

 periment will illus- 

 trate this force. In 

 Fig. 2 is represent- 

 ed a membranous 

 bag tightly fast- 



FIG. i. To illustrate osmosis. In the 

 vessel A is a solution of sugar ; in B, 

 is pure water. The two are sepa- 

 rated by the membrane C. The 

 sugar passes through the membrane 

 into B. 



ened to a glass 

 tube. The bag is 

 filled with a strong 

 solution of sugar, 

 and is immersed in a vessel containing pure water. 

 Under these conditions some of the sugar solution 

 passes through the bag into the water, and some 

 of the water passes from the vessel into the bag. 

 But if the solution of sugar is inside the bag and 

 the pure water outside, the amount of liquid pass- 

 ing into the bag is greater than the amount passing 

 out; the bag soon becomes distended and the water 

 even rises in the tube to a considerable height at 

 a (Fig. 2). The force here concerned is a force 

 known as osmosis or dialysis, and is always exerted 

 when two different solutions of certain substances 

 are separated from each other by a membrane. 

 The substances in solution will, under these con- 

 ditions, pass from the dense to the weaker solu- 

 tion. The process is a purely physical one. 



This process of osmosis lies at the basis of 

 the absorption of food from the alimentary canal. 



