I.] MATTER AND ENERGY. 19 



mere concomitant of the mechanical action, as was for- 

 merly believed ; it is energy of motion, which, as we have 

 every reason to believe, has been transferred from the mass 

 to the ixLolecules. 



The conservation of energy was formerly, and is fre- 

 quently still, called the conservation of force. But, as I 

 shall have to explain, the word force has another meaning ; 

 and Professor Eankine has ^Droposed the word energy in 

 this sense. 



In order to explain what is meant by the conservation 

 of energy, I must begin by defining the words momentum, 

 force, energy, and work. 



The momentum of a moving tody is the product of its Defiui- 



. , ., 7 ., tions of 



mass %nto its vetocity. Momen- 



Force is that ivhich produces motion. Equal forces are t^^™- 



Force 



those Avhich, when acting through equal times, produce 

 equal momenta. It follows from this that equal forces, 

 acting in opposite directions, neutralize each other ; as, for 

 instance, two weights on the opposite ends of a lever 

 which balance each other, so as to prevent either from 

 descending. 



Energy is that tvhieh does loorh. Equal quantities of Energy, 

 energy are those which are capable of doing equal quan- 

 tities of work : or, in other words, equal quantities of 

 energy are those which are capable of overcoming equal 

 resistances through equal spaces. When for instance a 

 weight is raised, a quantity of energy is expended and 

 transformed, proportionate to the product of the weight 

 into the height througli which it has been raised. 



Work is thus correlative with energy, and v>^ork may be and "Work, 

 defined as resistance overcome. 



Energy is not the same as Force. All energy has its 

 origin in force, but force cannot pass into energy unless it 

 is at liberty to act. Thus, gravity is a force, but no energy 

 is due to it unless it has space through which it can act by 

 causing bodies to fall : and the quantity of energy is pro- Measure of 

 portionate to the gravitative force, multiplied into the space eiiergy. 

 through which that force acts. For instance : the weight, or 

 aravitative force, of the. water of the ocean causes it to 



c2 



