78 SMITH'S INTERMEDIATE CHEMISTRY 



changes showed the name of each substance and also weights to 

 indicate the combining proportions. Having now found a unit 

 weight for each element, we can condense the statement still 

 further, by using a letter or pair of letters, called a symbol, to stand 

 for one chemical unit weight of each element. Thus O stands for one 

 unit or 16 parts of oxygen, H for one unit or 1.008 parts of hydro- 

 gen, Cl for one unit or 35.46 parts of chlorine. A pair of letters is 

 required when the names of several elements begin with the same 

 letter. Thus, C stands for 12 parts of carbon, Ca for 40 parts of 

 calcium, Cr for 52 parts of chromium, each of these amounts being 

 one chemical unit of the element. These symbols are interna- 

 tional, and are alike in all languages. In some instances they are 

 based upon the Latin name for the element. Thus, Fe stands for 

 55.84 parts of iron (ferrum), Sn for 119 parts of tin (stannum), Ag 

 for 107.88 parts of silver (argentum) and Hg for 200.6 parts of 

 mercury (hydrargyrum). Again K stands for 39.1 parts of potas- 

 sium (German, kalium) and Na for 23 parts of sodium (German, 

 natrium). 



Formulae. The composition* of a substance can be shown 

 briefly by putting together the symbols of the constituent elements, 

 and using numbers for the multiples of the unit weights, where 

 such are required. The resulting expression is called a formula. 

 In the last column of the table on p. 76 the formulae of the various 

 substances therein considered are indicated. Thus water contains 

 oxygen 16 parts (0) and hydrogen 2.016 parts (= 2 X 1.008 = 

 H 2 ), and receives the formula H 2 O. Hydrogen chloride contains 

 hydrogen 1.008 parts (H) and chlorine 35.46 parts (Cl), and its 

 formula is HC1. 



It must be noted particularly, that the formula, to be consistent, 

 must represent also the total weight of the unit quantity of the sub- 

 stance (wt. of 22.4 1.) in the gaseous state. Thus, HC1 (1.008 



* The composition means the names of the elements contained in the 

 substance, and also the proportion by weight of each element (see p. 38). 



