CHEMICAL DIVISIBILITY. 39 



Chemical affinity. There must be some cause which enables or 

 even forces the different elements to unite with each other so as to 

 Form compound bodies. There must be, for instance, a cause which 

 enables oxygen and mercury to combine and form a red powder. 



This cause is to be found in the existence of another form of the 

 general attraction which causes the smallest particles of different 

 elements to unite to form new substances with new properties. This 

 kind of attractive power is called chemical force or chemical affinity, 

 and bodies possessing this capacity of uniting with each other are said 

 to have an affinity for each other. 



Thore is a great difference between chemical attraction and the 

 various forms of attraction spoken of heretofore. Cohesion simply 

 holds together the molecules* of the same substance, adhesion_acts 

 chiefly between the molecules of solid and liquid substances, gravita- 

 tion acts between masses. But all these forces do not change the 

 nature, the external and internal properties of matter ; this is done 

 when chemical force or affinity is operating, when a chemical change 

 takes place. 



For instance : In a piece of yellow sulphur the molecules are held 

 together by cohesion, and we can counteract this cohesion by mechan- 

 ical subdivision, reducing the sulphur to a fine powder ; or by the 

 application of heat we can further subdivide the sulphur, melt, and 

 finally volatize it ; or we can throw a piece of sulphur into the air, 

 when it will fall back upon the earth in consequence of gravitation ; 

 or we can dip it into water, when it becomes moist in consequence of 

 surface-action. Yet in all these cases sulphur remains sulphur. 



It is entirely different when sulphur enters into chemical combina- 

 tion exerting chemical attraction, for instance, when it burns ; this 

 means when it combines with the oxygen of the atmospheric air. In 

 this case a new substance, a disagreeably smelling gas, a. compound of 

 oxygen and sulphur, is formed. 



It is consequently a complete change in the properties of matter 

 which follows the action of true chemical attraction ;flwe might define 

 affinity to be a force by which elements unite and new substances are 

 generated. | 



Atoms. Molecules, as stated heretofore, are the smallest particles 

 of matter which can exist. All matter consists of molecules, conse- 

 quently the red oxide of mercury must also consist of molecules. 



By heating the oxide of mercury, oxygen and mercury are obtained, 

 each of which also must consist of molecules. As the oxide of mercury 



