MOTIVE POWER OF HEAT. 143 



height EE^ , to a position E^F l , performing work 

 by the pressure of the vapor below it during its 

 ascent. 



[During this operation a certain quantity, H, of heat, 

 the amount of latent heat in the fresh vapor which is 

 formed, is abstracted from the body A] 



(2) The cylinder being removed, and placed on 

 the impermeable stand K, let the piston rise grad- 

 ually, till, when it reaches a position E^, the 

 temperature of the water and vapor is T, the same 

 as that of the body B. 



[During this operation the fresh vapor continually 

 formed requires heat to become latent ; and, therefore, as 

 the contents of the cylinder are protected from any acces- 

 sion of heat, their temperature sinks.] 



(3) The cylinder being removed from K, and 

 placed on B, let the piston be pushed down, till, 

 when it reaches the position E Z F Z , the quantity of 

 heat evolved and abstracted by B amounts to that 

 which, during the first operation, was taken from A. 



[Note of Nov. 5, 1881. The specification of this 

 operation, with a view to the return to the primi- 

 tive condition, intended as the conclusion to the 

 four operations, is the only item in which Carnot's 

 temporary and provisional assumption of the mate- 

 riality of heat has effect. To exclude this hypothe- 

 sis, Prof. James Thomson has suggested the fol* 



