32 CHEMISTRY. 



for one atom of the element it represents. IT, then, means 

 one atom of hydrogen no more and no less. If we want to 

 express two, three, or more atoms, we place a little 2 or 3 to 

 the right hand of the symbol ; thus Na 2 stands for two 

 atoms of sodium, and O 3 means three atoms of oxygen. 

 Sometimes large figures are placed in front, as 5N, which 

 means five atoms of nitrogen. Now unlike atoms unite to 

 form molecules of new bodies, and to represent this we write 

 the symbols alongside of each other. Thus HC1 will signify 

 the body having one atom of hydrogen united to one atom 

 of chlorine ; you will learn farther on that this body is call- 

 ed hydrochloric acid. 



29. The Symbols in Formulae. The example given in the 

 last paragraph, HC1, is called the formula of the body. In 

 place of writing "one atom of hydrogen united to one atom 

 of chlorine," we say "HC1," which is certainly much short- 

 er. In the same way CO stands for a gas called carbonic 

 oxide, containing as you see one atom of carbon and one 

 atom of oxygen. ISTaCl signifies chloride of sodium; CaO, 

 oxide of calcium. These examples anticipate of necessity 

 the method of forming the names of compounds, which you 

 will learn more fully in the section on nomenclature. In 

 the examples we have taken so far, only one atom unites 

 with one atom of another substance; it frequently happens, 

 however, that several atoms of an element unite with one 

 of another element, and to show this we use the little figure 

 referred to in 28, and which is called a coefficient. Thus 

 water is made up of two atoms of hydrogen and one of oxy- 

 gen, hence it is written H 2 O ; marsh gas is CH 4 , sulphuric 

 acid is H 2 SO 4 , etc., etc. Interpreting the last formula, we 

 would say that two atoms of hydrogen, one of sulphur, and 

 four of oxygen, held together by chemical attraction, make 

 one molecule of sulphuric acid. Potassium nitrate is KNO 3 , 

 gypsum is CaSO 4 , sodium carbonate is N"a 2 CO 3 . If a snb- 



