294 BIOLOGY 



A different type of potential energy is illustrated by a 

 bit of ordinary coal. The coal that is put into a furnace 

 contains, stored within itself, a large amount of energy in a 

 dormant form. That it contains the energy is perfectly evi- 

 dent from the fact that we need only put it under proper con- 

 ditions, by kindling it, and the energy will be liberated from 

 the coal in the form of heat, which may be converted into 

 ^notion by an engine. We can get no motion out of the steam 

 engine unless we put the energy into the furnace in the form 

 of coal, wood, or other fuel. Evidently fuels may be looked 

 upon as containing a store of dormant energy. These types of 

 passive energy, which exhibit no action, but which are capable 

 of being brought into activity when placed in the right condi- 

 tions, are spoken of as potential energy or energy of position. 



THE CONSERVATION OF ENERGY 



Energy can neither be created nor destroyed. Just as we 

 cannot destroy nor create matter, so we cannot destroy nor 

 create energy, the amount of energy present in our universe 

 to-day being the same as it has been in all previous time. This 

 statement does not seem quite so self-evident as the statement 

 that matter cannot be created or destroyed, for many ex- 

 amples occur that, at first sight, seem to be instances of the 

 destruction of energy. A stone which has been dislodged from 

 its position upon the roof falls rapidly to the ground and de- 

 velops energy in falling, but on reaching the ground it stops 

 suddenly and its energy seems to have disappeared. When a 

 cannon ball strikes a ledge of rock it suddenly stops. Any 

 examples of the stopping of motion would seem to be illus- 

 trations of the destruction of energy. 



A careful examination, however, shows that in these cases 

 there is in reality, no destruction of energy, but simply the con- 

 version of one form of energy into another. In the case of the 

 stone lodged on the roof, it is evident that at one time a cer- 

 tain amount of energy must have been used to lift this stone 



