156 SMITH'S INTERMEDIATE CHEMISTRY 



however, even if they are not affected by gravitation. In the 

 present instance we reason as follows : 



A brick in motion is different from a brick at rest. The former 

 can do some things that the latter cannot. Furthermore, we can 

 easily make a distinction in our minds. The brick can be deprived 

 of the motion and be endowed with it again. Thus, we can get 

 the idea of motion as a separate conception. Similarly, we ob- 

 serve that a piece of iron behaves differently when hot, and when 

 cold, when bearing a current of electricity, and when bearing 

 none. We conceive then of the brick or the iron as having a 

 certain amount and kind of matter which is unalterable, and as 

 having motion, heat, or electricity added to this or removed. 

 Thus, we describe our observations, by using two categories, one 

 of which includes the various kinds of matter, and the other, 

 various things whose association with matter seems to be in- 

 variable and is often so conspicuous. The latter we call the forms 

 of energy. 



The Practical Importance of Energy in Chemistry. 



The absorption or liberation of energy accompanying a chemical 

 transformation of matter is often, of the two, the more important 

 feature. We do not burn coal in order to manufacture carbon 

 dioxide gas. We are glad to get rid of the material product 

 through the chimney. It is the heat we want. We do not buy 

 gasoline (petrol) for an automobile in order to obtain various gases 

 to expel through the muffler. We really pay for the mechanical 

 energy. It is the same with burning illuminating-gas or magnesium 

 powder when we want light, and with eating food, which we do, 

 chiefly, to get energy to sustain our activity. We do not run 

 electricity for hours into a storage battery, in order to make a 

 particular compound (lead dioxide, for example), but in order to 

 save and store the energy for future use. In industry and life 

 fully half the total amount of chemical change involved is set in 

 motion by us, solely on account of the energy changes it involves. 



