THE MOTIVE POWER OF HEAT. 125 



accomplished by causing the working substance (a gas in Carnot's 

 case) to pass through two isothermal changes, the first at a higher and 

 the second at a lower temperature, alternated with two adiabatic* 

 changes by which the temperature of the working substance is allowed 

 to fall and then raised again. Each separate step was itself reversible 

 and so the whole cycle was reversible. The great virtue in this is that 

 at the close of the cycle of operations the intrinsic energy of the body 

 is exactly the same that it was at the beginning; and so we make no 

 mistake in saying that the difference between the quantity of heat- 

 energy given out by the body during the isothermal change at the 

 higher temperature and that absorbed by it during the isothermal 

 change at the lower temperature is exactly equal to the amount of 

 external work done by the body in the course of the cycle. 



By applying this principle Carnot showed that the production of 

 motive power is possible wherever there is a difference of temperature, 

 the motive power being due to a transfer of heat-energy from the 

 hotter to the colder body, its quantity being independent of the agents 

 employed to develop it, but depending solely upon the temperatures of 

 the bodies between which the transfer occurs, provided the process is 

 reversible. 



The most striking fact concerning this memoir is that Carnot used 

 hardly any mathematics at all, but arrived at his conclusions by sheer 

 logical exercise of his mind, expressing the different processes entirely 

 in words and using only such terms as would be clear to one not a 

 scientist. Some of his conclusions are incorrect on account of the 

 erroneous assumption of the materiality of heat, but sometimes he is 

 led to conclusions correct in form, although the deduction is erroneous. 

 Instinct seems to have led him in the right direction. 



It may be of interest to go through Carnot's memoir and pick out 

 the various important statements as he himself italicized them. They 

 are in part as follows : 



The production of motive power in the steam-engine is not due to a real 

 consumption of the caloric, but to its transfer from a hotter to a colder body. 



Wherever there is a difference in temperature the production of motive 

 power is possible, and conversely. 



The maximum motive power resulting from the use of steam is also the 

 maximum motive power which can be obtained by any other means. 



The motive power of heat is independent of the agents employed to develop 

 it; its quantity is determined solely by the temperature of the bodies between 



* By isothermal changes are meant changes in volume and pressure in- 

 volving changes in the heat-energy of the working substance, but unaccom- 

 panied by any changes in temperature. 



By adiabatic changes are meant changes in volume and pressure involv- 

 ing changes in temperature, but unaccompanied by any gain or loss of heat- 

 energy of the working substance. 



