ON THE CONSERVATION OF FORCE. 359 



A raised weight can produce work, but in doing so it 

 must necessarily sink from its height, and, when it has 

 fallen as deep as it can fall, its gravity remains as before, 

 but it can no longer do work. 



A stretched spring can do work, but in so doing it 

 becomes loose. The velocity of a moving mass can do 

 work, but in doing so it comes to rest. Heat can perform 

 work ; it is destroyed in the operation. Chemical forces 

 can perform wc-rk, but they exhaust themselves in the 

 effort. 



Electrical currents can perform work, but to keep them 

 up we must consume either chemical or mechanical forces, 

 or heat. 



We may express this generally. It is a universal 

 character of all knoiun natural forces that their capacity 

 for work is exhausted in the degree in which they actu- 

 ally perform work. 



We have seen, further, that when a weight fell without 

 performing any work, it either acquired velocity or pro- 

 duced heat. We might also drive a magneto-electrical 

 machine by a falling weight ; it would then furnish elec- 

 trical currents. 



We have seen that chemical forces, when they come 

 into play, produce either heat or electrical currents or 

 mechanical work. 



We have seen that heat may be changed into work ; 

 there are apparatus (thermo-electric batteries) in which 

 electrical currents are produced by it. Heat can directly 

 separate chemical compounds ; thus, when we burn lime- 

 stone, it separates carbonic acid from lime. 



Thus, whenever the capacity for work of one natural 

 force is destroyed, it is transformed into another kind of 

 activity. Even within the circuit of inorganic natural 

 forces, we can transform each of them into an active 

 condition by the aid of any other natural force which is 



