SUBSTANCES AND PROPERTIES 9 



Each name stands for a substance. Two substances in contact 

 with one another (mixed), but not united chemically, are con- 

 nected by the + sign. The arrow shows where the chemical 

 change comes in, and the direction of the change. We read the 

 statement thus: Iron and oxygen brought together under suit- 

 able conditions undergo chemical change into oxide of iron, called 

 also ferric oxide. Similarly we may write: 



Lead + Oxygen Oxide of lead. 

 Tin + Oxygen > Oxide of tin. 



Explanation of Rusting. Experiment shows that the 

 process of rusting is accompanied by a slow increase in the 

 weight of the solid, due to the gradual addition of oxygen to the 

 metal. Now, this increase in weight ceases of its own accord, 

 when a certain maximum has been reached. This occurs when 

 the last particles exhibiting the properties of the metal have dis- 

 appeared. Thus, lead gains in weight until every 100 parts of the 

 metal have gained 7.72 parts of oxygen, and tin until every 100 

 parts have gained 26.9 parts of oxygen. When these increases 

 have occurred, the metal is found to have been all used up, and 

 prolonged heating and stirring cause no further union with oxy- 

 gen and no further change in weight. This fact, that each sub- 

 stance limits itself of its own accord to combining with a fixed 

 proportion of the other substance, in forming a given compound, 

 is one of the most striking facts about chemical combination. In 

 mixtures, any proportions chosen by the experimenter may be 

 used. In chemical union, the experimenter has no choice; the 

 proportions are determined by the substances themselves. Thus, 

 100 parts of iron when turning into ordinary red rust take up 43 

 parts of oxygen, no more and no less. 



This fact enables us to make our condensed statements more 

 specific and complete by including in them the proportions by 

 weight used in the chemical change: 



