ENERGY AND CHEMICAL CHANGE 155 



is that the gas appears to arise from the surface of the platinum. 

 It is easy to show, however, that the platinum by itself is not 

 acted upon by dilute acids and, in this case, undergoes no change 

 whatever; it serves simply as a suitable conductor for the elec- 

 tricity. Here, then, in place of the heat which the first plan 

 produced, we get an electric current. The arrangement is, in 

 fact, a battery-cell, for a battery is a system in which a chemical 

 action which would otherwise give heat furnishes electricity in- 

 stead. Thus, electrical energy may be consumed or produced 

 in connection with a change in composition. 



Even violent rubbing in a mortar, in the case of some substances, 

 can effect an appreciable amount of decomposition in a few min- 

 utes. In this way silver chloride can be separated into silver and 

 chlorine, just as by light. It is the mechanical energy which is the 

 agent, and part of it is consumed in producing the change, and 

 only the balance appears as heat. Conversely, the production of 

 mechanical energy, as the result of chemical change, is seen in 

 the behavior of explosives and in the working of our muscles. 

 Thus, mechanical energy may be used up or produced in chemica 

 changes. 



Summing up our experience, we may state that no change in 

 composition occurs without some accompaniments, such as the 

 production or consumption of heat, light, electrical energy, or, in 

 some cases, mechanical energy. 



Classification of the Accompaniments of Change in 

 Composition: Energy. The problem of classifying (i.e., 

 placing in a suitable category) things like heat, light, and elec- 

 tricity has occupied much attention. In all changes in com- 

 position, one of these natural accompaniments is given out or 

 absorbed, sometimes in great amount, yet in none is any alteration 

 in weight observed.* There are many things which are real, 



* Electrons (see p. 195) do possess mass, but it is very small compared with 

 that of the materials concerned. 



