ELEMENT, COMPOUND, CHEMICAL AFFINITY, ETC. 91 



Physical phenomena accompanying- chemical action. By a 

 careful observation of all the details in a great variety of chemical 

 actions, an intimate connection between physics and chemistry be- 

 comes apparent. A noteworthy fact that stands out prominently is 

 that in every change in composition, energy in some form is produced 

 or consumed. In the decomposition of red oxide of mercury, heat 

 energy is constantly being absorbed and rendered latent as the oxide 

 separates into the free elements, oxygen and mercury. As soon as 

 the heat-supply is withdrawn the decomposition ceases. When mag- 

 nesium burns, that is, unites with oxygen, a great quantity of heat 

 and intense light is produced, and the action after starting is self-sus- 

 taining. "We have seen that electric energy is consumed in bringing 

 about chemical change. On the other hand, chemical action under 

 proper control can be made to produce electric energy, that is, an 

 electric current. In some cases the energy of light rays is consumed 

 in producing chemical change, for example, in photography. De- 

 composition can be accomplished in certain instances by mechanical 

 energy, as violent trituration, while, on the other hand, the production 

 of mechanical energy by chemical action is illustrated by the explo- 

 sion of gunpowder and movement of muscles. 



Chemical or internal energy. Exothermic and endothermic 

 actions. From the previous discussion, we learn that there must be 

 another kind of energy stored up in latent form in matter which 

 under proper conditions is converted into forms of energy with which 

 we have already become familiar in Section I. on Physics ; namely, 

 heat, electricity, light, mechanical energy. This form of energy, 

 which is made manifest during chemical change, is called chemical 

 or internal energy. Every element and compound should be looked 

 upon not merely as consisting of certain kinds of matter, but also as 

 a storehouse of chemical energy. In many chemical changes part of 

 the internal energy is liberated in forms which can be measured, such 

 as heat, electricity, etc. This portion is known as free or available 

 internal energy, and is different in amount according to the nature of 

 the substances concerned in the chemical change. For example, the 

 same weight of various articles of food, when undergoing combustion 

 in the animal body, produces very different amounts of heat, and 

 therefore these foods have different values as fuels for keeping up the 

 temperature of the animal body. 



Sometimes it is necessary to augment the internal energy of sub- 

 stances from a supply of energy such as heat, electricity, etc., in order 



