ON THE INTERACTION OF NATURAL FORCES. 153 



first expressed by Carnot, we can give it the following move 

 general expression : ' Only when heat passes from a warmer to 

 a colder body, and even then only partially, can it be converted 

 into mechanical work.' 



The heat of a body which we cannot cool further, cannot be 

 changed into another form of force into electric or chemical 

 force for example. Thus in our steam-engines we convert a 

 portion of the heat of the glowing coal into work, by permitting it 

 to pass to the less warm water of the boiler. If, however, all the 

 bodies in Nature had the same temperature, it would be impos- 

 sible to convert any portion of their heat into mechanical work. 

 According to this we can divide the total force store of the 

 universe into two parts, one of which is heat, and must continue 

 to be such ; the other, to which a portion of the heat of the 

 warmer bodies, and the total supply of chemical, mechanical, 

 electrical, and magnetical forces belong, is capable of the most 

 varied changes of form, and constitutes the whole wealth of 

 change which takes place in Nature. 



But the heat of the warmer bodies strives perpetually to pass 

 to bodies less warm by radiation and conduction, and thus to 

 establish an equilibrium of temperature. At each motion of a 

 terrestrial body a portion of mechanical force passes by friction 

 or collision into heat, of which only a part can be converted 

 back again into mechanical force. This is also generally the case 

 in every electrical and chemical process. From this it follows 

 that the first portion of the store of force, the unchangeable heat, 

 is augmented by every natural process, while the second portion, 

 mechanical, electrical, and chemical force, must be diminished ; 

 so that if the universe be delivered over to the undisturbed 

 action of its physical processes, all force will finally pass into 

 the form of heat, and all heat come into a state of equilibrium. 

 Then all possibility of a further change would be at an end, and 

 the complete cessation of all natural processes must set in. The 

 life of men, animals, and plants could not of course continue if 

 the sun had lost his high temperature, and with it his light, if 

 all the components of the earth's surface had closed those com- 

 binations which their affinities demand. In short, the universe 



