174 GENERAL SCIENCE 



different atomic weights. When like atoms unite to make 

 molecules the substance is known as an element in chemistry, 

 and when the molecules of any substance are made up of 

 different kinds of atoms the substance is a chemical com- 

 pound. The molecules of some compounds, small as mole- 

 cules are conceived to be, contain large numbers of atoms 

 of different kinds. Other molecules, as in common salt 

 (sodium chloride) , are believed to contain but two atoms each. 



Many chemical changes occur when a sufficiently high 

 temperature is reached, as when a little gun-powder is heated. 

 In the chemical laboratory it is a common practice to dis- 

 solve substances and use their solutions in bringing about 

 chemical changes. In such cases heat is often employed to 

 hasten and make complete the changes. In the natural 

 world about us heat and light from the sun, and the presence 

 of moisture in the soil and in the air, are all agencies in aiding 

 chemical changes. 



In the decay of organic matter the micro-organisms known 

 as bacteria are active as agents in the chemical changes that 

 occur. In the processes of digestion not only is chemical 

 change in the foodstuffs promoted by solution and warmth, 

 but the digestive secretions contain substances known as 

 enzymes whose uses are to bring about chemical changes in 

 the food materials. These enzymes seem to cause chemical 

 changes by reason of their presence, and without themselves 

 undergoing change. Diastase (dl'-a-stas) produced in plants, 

 ptyalin (tl'-a-lin) in the saliva, pepsin in the gastric juice of 

 the stomach, and pancreatin are examples of enzymes. 



The term metabolism (me -tab'-o-lizm) is used to include all 

 chemical changes occurring in the body whether for its 

 nourishment or for removal of the waste due to life activi- 

 ties. The maintenance of bodily conditions favorable to 

 these chemical changes constitutes to a large degree the 

 problem of health and length of life. Impaired metabolism 



