34 Introduction 



Energy. — Brownian movement increases with higher temperatures 

 and nearly ceases at extremely low temperatures, thus reflecting the 

 movements of the molecules of the solution. This motion is an ex- 

 pression of energy, which is defined as work or the ability to do work. 

 Energy causes the movement of a body against resisting forces such as 

 gravity; its expenditure may result in heat as when coal is burned. 

 Within the animal body, fats and carbohydrates are burned with oxygen 

 to produce energy. 



Energy may exist in several forms which can be converted from 

 one to another. Kinetic energy is the energy of motion ; potential 

 energy is the energy of a body position in relationship to other bodies. 

 Thus the energy of water behind a dam is potential, easily converted 

 to kinetic when the gates are opened. 



All matter is but a form of energy, and three forms are recognized : 

 gas, liquid, and solid. The number of molecules per unit of space is 

 usually greatest in the solid and least in the gas. Contrariwise, molecu- 

 lar motion is greatest in the gas and least in the solid. 



Diffusion. — The kinetic energy of molecules accounts for the 

 fact that molecules rapidly spread through a medium. This is more 

 easily demonstrated in liquids and gases than in solids. If a can of ether 

 is opened in a room, the odor will soon spread throughout the closed 

 area. In liquids this same phenomenon can be demonstrated by drop- 

 ping a crystal of potassium dichromate in water. Soon the entire con- 

 tainer of water will be colored a uniform purple. This tendency of 

 molecules to spread from areas of greater concentration to those of 

 less is known as diffusion. 



Osmosis. — Application of this tendency of material in solution to 

 equalize is very important in biological systems. In them, however, 

 the movement is often through a membrane. Most living structures 

 are surrounded by membranes that are semipermeable; that is, they 

 will permit only particles of a certain size to pass through. The passage 

 of materials in solution through a semipermeable membrane is known 

 as osmosis. 



This can be demonstrated with a simple experiment. A semi- 

 permeable sac of celloidin is filled with a heavy sugar solution such as 

 molasses, and a glass tube is fastened to the open end. This sac is then 

 placed in a container of distilled water ; the level of the solution in the 

 tube will rise rapidly. The smaller water molecules move in and out of 

 the celloidin sac, with more entering than leaving. The sugar mole- 



