6 APPLIED BIOLOGY 



but not a change in composition. Such changes of state 

 which do not affect the composition of substances are called 

 physical changes. 



That branch of science which treats of the form and physi- 

 cal changes of matter produced by heat, light, sound, elec- 

 tricity, gravitation, etc., was formerly called natural phi- 

 losophy, but is now usually known as physics. 



8. Chemical Change. All matter is subject to another 

 kind of change in which the composition is affected and new 

 substances are formed. Ordinary burning of wood or gas, 

 and dissolving baking soda in vinegar or other acid, are com- 

 mon examples. The substances burned, or dissolved in acids, 

 are changed to other substances. Such transformations 

 which affect the composition of matter are chemical changes. 



9. Elements. The nature of chemical change will be 

 clear after some further consideration of the composition 

 of matter. In chemistry, the science which treats of the 

 composition and chemical changes of substances, we learn 

 that all forms of matter all living and lifeless substances 

 in land, air, and water are composed of about 80 elements, 

 of which about 20 are very common. Some of these ele- 

 ments exist naturally in the solid state ; for example, iron, 

 copper, lead, sulphur, gold, nickel, silver, platinum, carbon, 

 magnesium, aluminum, tin, and zinc. Some others, like 

 mercury (quicksilver), are liquid; and still others are gases, 

 of which the two known as oxygen and nitrogen constitute 

 the greater part of the air. 



Chemical Symbols. For convenience in writing the names 

 of the elements, chemists have adopted certain symbols or 

 abbreviations. The ones which will be most needed for ref- 

 erence in this book are : H for hydrogen ; N, nitrogen ; 

 O, oxygen; C, carbon; S, sulphur; P, phosphorus; Na, 

 sodium (or natrium) ; K, potassium (or kalium) ; Fe, iron 

 (or ferrum) ; Ca, calcium (lime). A table giving the full list 

 may be found in any elementary textbook of chemistry. 



