42 THE STORY OF LIFE'S MECHANISM. 



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 passing 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 con- 

 siderable 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 

 conditions, pass from the 

 dense to the weaker solution. 

 :^ The process is a purely physi- 

 cal one. 



This process of osmosis 



FIG. 2.-In the bladder A lies at the basis f the 



is & sugar solution, absorption of food from the 

 pure th wate? el sllgar alimentary canal. In the 



passes out and water fi rst pl ace , most of the food 



into the bladder until . * ' , . 



it rises in the tube when swallowed IS not SOlll- 



ble, and therefore not capable 

 of osmosis. But the process of digestion, as we 

 have seen, changes the chemical nature of the 

 food. The food, as the result of chemical 

 change, has become soluble, and after being dis- 

 solved it is dialyzable, i.e. capable of osmosis. 



