92 PRINCIPLES OF CHEMISTRY. 



to bring about chemical change. Indeed, it has been found that all 

 chemical changes maybe divided into two classes: (1) those which 

 take place spontaneously and in which a certain amount of the chem- 

 ical energy of the substances acting is liberated and converted into 

 some other forms of energy, which thus become available for use ; (2) 

 those which do not take place spontaneous! v, but which must be sus- 

 tained by the addition of energy from without to the substances act- 

 ing. The energy set free in spontaneous chemical actions usually 

 appears as heat, and all actions proceeding with liberation of heat 

 are called exothermic, while those actions which absorb heat are called 

 endothermic. The study of the energy changes in chemical actions is 

 very important for a full understanding of such actions. The study 

 of the heat produced or absorbed in chemical changes constitutes a 

 subdivision known as Thermochemistry. 



Chemical reaction, in its broadest sense, refers to any chemical 

 change, but is used more especially when the intention is to study 

 the nature of the substances decomposed or formed. The expression 

 reagent is applied to those substances used for bringing about such 

 changes. 



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 

 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. 



There 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 



