22 PHYSIOLOGY 



area than before. This expansion may continue until a 

 solid becomes a liquid and a liquid becomes a gas. As 

 a liquid evaporates its particles get into the spaces 

 between the particles of other substances. If this diffu- 

 sion takes place through a membrane, the process is 

 called osmosis. 



A physical change does not disturb the nature of a 

 substance, but in a chemical change new substances are 

 formed either by the union of simple into complex, or 

 the disintegration of complex into simple substances. 

 The oxidation of carbon and hydrogen in the burning of 

 wood is similar to the oxidation of those substances in 

 the human body ; in both cases carbon dioxide and water 

 are formed. The passage of water and carbon dioxide 

 through a membrane is typical of the way in which most 

 substances pass from cell to cell in the human body. 



