MOTIVE POWER OF HEAT. 161 



28. The preceding investigations, being founded 

 on the approximate laws of compressibility and ex- 

 pansion (known as the law of Mariotte and Boyle, 

 and the law of Dalton and Gay-Lussac), would re- 

 quire some slight modifications to adapt them to 

 cases in which the gaseous medium employed is such 

 as to present sensible deviations from those laws. 

 Regnault's very accurate experiments show that 

 the deviations are insensible, or very nearly so, for 

 the ordinary gases at ordinary pressures ; although 

 they may be considerable for a medium, such as 

 sulphurous acid, or carbonic acid under high pres- 

 sure, which approaches the physical condition of a 

 vapor at saturation ; and therefore, in general, and 

 especially in practical applications to real air-engines, 

 it will be unnecessary to make any modification in 

 the expressions. In cases where it may be necessary, 

 there is no difficulty in making the modifications, 

 when the requisite data are supplied by experiment. 



29.* Either the steam-engine or the air-engine, 

 according to the arrangements described above, 

 gives all the mechanical effect that can possibly be 

 obtained from the thermal agency employed. For 



* This paragraph is the demonstration, referred to above, 

 of the proposition stated in 13, as it is readily seen that 

 it is applicable to any conceivable kind of therinodynamic 

 engine. 



