778 REPOKT— 1891. 



used in the generation of steam under constant pressure than under constant 



FV 

 J 



PV 



■volume, the extra quantity of heat being U = -^=r-, J being Joule's equivalent, and 



U the heat units. 



The author suggests the following explanation of the way in which the heat 

 equivalent of the external work of evaporation is used. If heat flows out of steam 

 when mechanical work is done by it during its formation, it must be supposed that 

 steam is cooled by the outflow. If heat flows out and if cooling follows, the cor- 

 responding condensation or liquefaction takes place, and a further supply of heat is 

 demanded to re-evaporate steam so liquefied; or, what is the same thing, the 

 further supply of heat is used in continuously preventing the liquefaction from 

 reaching moi-e than the incipient stage. The action here sketched is readily con- 

 ceived if for the purpose of explanation the evaporation and the external work be 

 supposed to take place jjcr snltuni. Suppose a piston, immediately over the water 

 in a simple evaporating vessel, to have been moved by the steam through a small 

 distance A. Then heat corresponding to the work done in moving the piston 

 through A will have flowed out of the steam, and this quantity of heat Q being 



gone, condensation-^ must have taken place in order that the temperature T and 



pressure P of the remaining steam may be unaffected (L being latent heat of 

 evaporation). Now before the piston can be again moved through a further 

 similar distance A', that quantity Q must be restored by a further demand on the 

 source of heat, and if Q' be the quantity of heat required to produce the volume of 

 steam V, then the total quantity of heat Q^ required to move the pistoa through 

 distance A' will be Q" = Q' + Q, in order that volume V may be produced and the 

 condensed steam Q be re-evaporated. 



Now if A be taken as less than any assignable distance or the process assumed 

 continuous, then evaporation and incipient liquefaction may be supposed to be con- 

 temporaneous, and Q and Q,' will be supplied contemporaneously. 



In this way it appears to the author that an explanation can be found of the 

 mode of conversion of heat into the external work of evaporation under constant 

 pressure, or of conversion of heat into the work performed by a steam-engine during 

 the admission part of the stroke, or, more correctly speaking, the work done by 

 the steam on the piston during admission. If this be a true statement of the 

 actual mode of employment of the heat converted into the mechanical work of a 

 steam-engine during the admission part of the stroke, then it follows that liquefac- 

 tion takes place during adrai;sion, which must be sufficient to represent the 

 mechanical work done. This being so, the question arises. To what extent will 

 this liquefaction result in water or suspended moisture in the cyfinder of a steam- 

 engine ? The outflow of heat and corresponding liquefaction may be supposed to 

 take place at the moving wall or piston, and in the hypothetic case supposed the 

 liquefied steam may be assumed to be re-evaporated by the steam or water imme- 

 diately below, which in its turn demands and receives more heat for its resuscita- 

 tion from the source. In the case of the steam-engine cjdinder, however, it is open 

 to question whether the killed molecules in the cylinder or next the piston are 

 resuscitated by the incoming steam, which follows up the movement of the piston. 

 If they are not, then liquefaction will take place in the steam-engine cylinder during 

 admission as a result of the performance of work, although the work is the external 

 work of the evaporation which is performed in the boiler. The heat required for 

 evaporation is that of Regnault's tables, but under the assumption here explained 

 (when the liquefaction takes place in the cylinder and the resulting water does not 

 return without loss of heat to the boiler), the heat required to raise the tempera- 

 ture of the quantity - of feed water to the temperature of evaporation must be 

 added, because in order that one pound of steam may be supplied to the cylinder 



as steam at cut off, the extra quantity — of feed water must be supplied to the 



Li 



boiler. The quantity of water actually evaporated in the production of one pound 



