mixture of the elements; but-placed as close as the 

 printer's types will conveniently and consistently 

 allow, they must be considered to stand for a com- 

 pound of the elements,that is to say, hydrochloric acid 

 gas (HC1). The collection of symbols representing 

 a molecule is termed a formula. 



A Chemical Formula represents a molecule either of 

 an element or of a compound. It has four other 

 functions: 



1. It indicates at a glance the names of the 

 elements in a molecule. 



2. Its symbol or symbols, together with a small 

 figure attached to the foot of any symbol, show the 

 number of atoms in the molecule. 



3. It stands for a constant weight of a com- 

 pound the molecular weight the sum of the 

 combining weights or of the weights of the atoms in 

 the molecule. 



4. It represents two volumes of the substance, if 

 volatilizable, in the state of gas or vapor, and the 

 number of volumes of gaseous elements from which 

 two volumes of any gaseous compound were 

 obtained. 



Hz, C1 2 and HC1 are the fotmulce of hydrogen, 

 chlorine, and hydrochloric acid gas. 

 H f +Cl t =2HCl. 



Such a set of letters, figures, and marks as that on 

 the preceding line is collectively termed an equation, 

 because it indicates the equality of the number and 

 nature of the atoms before and after chemical 

 action. On the left hand of the sign of equality are 

 shown two molecules, and on the right hand two 

 molecules; but, of the molcules on the left one 

 contains two atoms of hydrogen and the other two 

 atoms of chlorine, while of the molecules on the 

 right each contain one atom of hydrogen and one of 



-26- 



