94 I.IFE AND PEATH. 



the latter. This is due to a condition of thermal 

 energy which is called temperature. The same 

 quantity of thermal energy, of heat, may be stored 

 in the same thermal body at different temperatures. 

 If this quantity of thermal energy is in a very hot 

 body we can utilize a large portion of it ; if it is in a 

 relatively cold body we can only convert a small 

 portion of it into mechanical work. Thus the value 

 of energy, — i.e., its capacity of being converted into 

 a higher and more useful form, — depends on tem- 

 perature. 



The Capacity of Conversion depends on Temperature. 

 — The conversion of heat into work assumes two 

 bodies of different temperatures, the one warm and 

 the other cold ; a boiler and a condenser. Every 

 thermal machine conveys a certain amount of heat 

 from the boiler to the condenser, and what is not thus 

 carried is changed into work. This residue is only a 

 small fraction, a quarter, or at most a third of the 

 heat employed, and that, too, in the theoretically 

 perfect machine, in the ideal machine. 



This output, this utilizable fraction depends on the 

 fall of temperature from the higher to the lower level, 

 just as the work of a turbine depends on the height 

 of the waterfall which passes through it. But it also 

 depends on the conditions of this fall, on the 

 accessory losses from radiation and conduction. 

 However, Carnot has shown that the output is the 

 same, and a maximum for the same fall of temperature, 

 whatever be the working agent (steam, hot air, etc.), 

 and whatever be the machine — provided that this 

 agent, this substance which works is not exposed to 

 accessory losses, that it is never in contact with a 

 body having temperature different to its own — or 



