218 HELMHOLTZ 



we must consume either chemical or mechanical forces, or 

 heat. 



We may express this generally. It is a universal character 

 of all known natural forces that their capacity for work is 

 exhausted in the degree in which they actually 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 ma- 

 chine by a falling weight; it would then furnish electrical 

 currents. 



We have seen that chemical forces, when they come into 

 play, produce either heat or electrical currents or mechani- 

 cal 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 limestone, it sep- 

 arates 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 capable of work. The 

 connections between the various natural forces which mod- 

 ern physics has revealed, are so extraordinarily numerous 

 that several entirely different methods may be discovered for 

 each of these problems. 



I have stated how we are accustomed to measure mechani- 

 cal work, and how the equivalent in work of heat may be 

 found. The equivalent in work of chemical processes is 

 again measured by the heat which they produce. By simi- 

 lar relations, the equivalent in work of the other natural 

 forces may be expressed in terms of mechanical work. 



If, now, a certain quantity of mechanical work is lost, 

 there is obtained, as experiments made with the object of 

 determining this point show, an equivalent quantity of heat, 

 or, instead of this, of chemical force; and, conversely, when 

 heat is lost, we gain an equivalent quantity of chemical or 

 mechanical force; and, again, when chemical force disap- 

 pears, an equivalent of heat or work; so that in all these 



