exist as a solid and have a metallic luster, must be capable 

 of conducting heat and electricity, must be opaque, hard, 

 malleable, ductile, and capable of forming compounds with 

 oxygen. Probably no single metal has all these properties, 

 but no substance would be accepted as a metal that did not 

 possess many of them. Iron, nickel, copper, and mercury are 

 among the more familiar metals. Carbon, sulphur, and phos- 

 phorus may be named as examples of the nonmetals. Theo- 

 retically, at least, each chemical element may exist as a solid, 

 a liquid, or a gas, but many have not yet been produced in all 

 three of these conditions. Increasing the temperature will 

 make many of the ordinary solids liquid, and the reverse of 

 this process, combined with pressure, serves to liquefy even 

 the lightest gases. Water, while not a chemical element, will 

 serve to illustrate this change of state. In its more familiar 

 form it is a liquid, but if heated to 21 2 F. it becomes a 

 gas, and if cooled below 32 F. it becomes a solid. 



Atoms and molecules. An atom is the smallest part of a 

 chemical element that can enter into combination with other 

 parts. Atoms may therefore be said to be the units of which 

 more complex compounds are built. Not very much is known 

 regarding the size of atoms, but they are estimated to be 

 about one hundred-millionth of an inch in diameter and to 

 bear about the same relation to the size of a tennis ball that 

 the latter bears to the earth. Atoms usually do not long 

 exist as such, but combine with other atoms to form molecules, 

 which are the smallest enduring particles of a compound, just 

 as the atoms are the smallest part of a chemical element. All 

 the atoms of a single chemical element are exactly alike, 

 otherwise it would not be a chemical element. 



Chemical formulas. Atoms have the same relation to chemi- 

 cal compounds that letters have to words, and the chemist is 

 therefore able to write a definite formula for the molecule of 

 every substance. Each element has its own chemical symbol, 



