THE MECHANICS OF THE LIVING MACHINE 



307 



gradually absorbed, soaking through into the blood vessels 

 that lie within the walls. This process of food absorption in- 

 volves another set of forces, which are, at least to a considerable 

 extent, either chemical or physical. The primary force con- 

 cerned is what physicists call osmosis or dialysis, a 

 force which has no special connection with life. If 

 a membrane separates two liquids of different con- 

 sistency (Fig. 134), a force is exerted on the liquids 

 that causes each to pass through the membrane in 

 an opposite direction, until the constitution of the 

 liquids on the two sides of the membrane is the 

 same. The force that drives these liquids through 

 the membrane is a powerful one, since it is exerted 

 against a high pressure. In Figure 134 a mem- 

 branous bladder is attached to the lower end of a 

 glass tube. If a solution of sugar is placed inside 

 of this bladder and pure water outside of it, the 

 sugar and the water will both pass through the 

 membrane in opposite directions. Under these cir- 

 cumstances, however, more water passes from the 

 outside into the bladder than passes from the blad- 

 der outward. The result is that the bladder be- 

 comes more and more filled with liquid, and enough 

 pressure is produced in the bladder to force the 

 water up the tube, in which it may rise to quite a 

 height. This force is known as osmosis, and it is 

 always exerted whenever two solutions of unequal 

 concentration are separated from each other by a 

 membrane. Some substances, like the white of an 

 egg, are not capable of passing through a membrane, 

 and we refer to them by the term colloidal or non- 

 dialyzable. Other substances, like salt and sugar, will readily 

 pass through membranes, and we speak of them as crystalline 

 or dialyzable. 



Osmosis is the fundamental force concerned in the absorption 



FIG. 134. 

 A DIAGRAM 



ILLUSTRAT- 

 ING THE 

 FORCE OF 

 OSMOSIS 



