COMPOSITION OF MATTER 27 



ters in its symbol, and the second letter is always a small letter. 

 Co therefore, stands for cobalt, an element, but CO stands for 

 carbon monoxide, a chemical compound of carbon and oxygen. 

 A few elements, such as sodium (Na), silver (Ag), and potas- 

 sium (K), have symbols which do not include their initials. 

 In such cases, it will be found that the symbols were given to 

 these substances when they were called by other names. The 

 names have since been changed, but the symbols have not. A 

 group of chemical symbols indicating the number and kind of 

 atoms in a molecule of a given substance is called a chemical 

 formula. Thus sulphuric acid has the formula H 2 S0 4 , lime- 

 stone, CaCOs, and orthoclase a common mineral, K Al Si 3 8 . 

 The chemical symbol when standing alone is understood to 

 indicate a single atom of the element; when there is more 

 than one of a kind of atom in a molecule, the number is in- 

 dicated by writing the number of atoms below the line. Thus, 

 in the formula for orthoclase, we see that there are three atoms 

 of silicon and eight of oxygen in its molecules. . 



32. Chemical Compounds. A chemical compound is not a 

 mere mechanical mixture of two or more elements, but is a 

 different substance that often has properties not found in any 

 of the elements from which it was made. When sulphur and 

 iron are mixed together they remain a mere mechanical mixture 

 until heat is applied. When heated they soon form a new 

 substance known as sulphide of iron. This has a different 

 color from the substances composing it, and is not affected by 

 a magnet, though the iron before it was combined was strongly 

 attracted. It is not always possible, however, to tell in 

 advance what sort of a substance will be formed by the union 

 of two or more elements. The union of two gases does not 

 necessarily produce a gaseous compound, nor does the union of 

 two solids always produce a solid. Water, which is a liquid 

 at ordinary temperatures, is composed of two gases, oxygen 

 and hydrogen. When these gases are free and uncombined, 



