272 ELEMENTARY GENERAL SCIENCE CHAP. 



(2) H 2 + C1 2 = 2HC1. 



1 mol. hydrogen with 1 mol. chlorine form 2 mols. 

 hydrochloric acid gas. 



Atomic Weights. We have, however, found that any volume 

 of oxygen is 16 times as heavy as the same volume of hydrogen, 

 so that each molecule of oxygen is 16 times as heavy as a 

 molecule of hydrogen. Consequently the ratio of the weight 

 of the atoms must also be 16. So that if we regard the weight 

 of an atom of hydrogen as our unit, the atomic weight of 

 oxygen must be 16. The atomic weight of an element is, 

 therefore, the ratio of the weight of its atom to the weight 

 of the atom of hydrogen. 



By similar reasoning the value 35 '5 may be obtained as the 

 atomic weight of chlorine (see page 266), and hence the symbols 

 01, O, and H will represent respectively 35*5, 16 and 1 parts 

 by weight of chlorine, oxygen, and hydrogen. A list of the 

 symbols of the elements with their atomic weights is given on 

 page 275, but we shall in general use simplified numbers, as 

 16 in place of (15 '87) for oxygen. 



We may, therefore, extend the meaning of the chemical 

 equations given above and read thus : 



4 ( = 2x2) parts by weight of hydrogen with 32 ( = 2x16) 

 parts by weight of oxygen unite to form 36 ( = 2 x 2 + 2 x 16) parts 

 by weight of water. 



Formulae for Sulphur Dioxide and Sulphuretted Hydro- 

 gen. It may be easily proved experimentally that when 

 sulphur burns in oxygen there is no alteration of volume. 

 From this, by an application of Avogrado's Law, it follows that 

 from a certain number of molecules of oxygen we obtain the 

 same number of molecules of sulphur dioxide. Hence each 

 molecule of this compound is obtained from 1 molecule, i.e., 

 2 atoms, of oxygen. This does not show how many atoms of 

 sulphur are present, but if we assume the presence of 1 atom 

 the formula would be SO 2 . As we have previously found that 

 sulphur gains 100 per cent, in weight on burning, it is clear 

 that the 1 atom of sulphur has the same weight as 2 atoms of 

 oxygen, i. e. , the atomic weight is 2 x 16 = 32. This number 

 agrees with that obtained for the atomic weight of sulphur by 

 other methods, and hence indicates that the formula adopted is 



