122 MOTIVE POWER OF HEAT. 



it would doubtless offer a notable advantage over 

 vapor of water.* 



As to the other permanent gases, they should be 

 absolutely rejected. They have all the inconven- 

 iences of atmospheric air, with none of its advan- 

 tages. The same can be said of other vapors than 

 that of water, as compared with the latter. 



If we could find an abundant liquid body which 

 would vaporize at a higher temperature than water, 

 of which the vapor would have, for the same vol- 

 ume, a less specific heat, which would not attack 

 the metals employed in the construction of ma- 

 chines, it would undoubtedly merit the preference. 

 But nature provides no such body. 



The use of the vapor of alcohol has been pro- 

 posed. Machines have even been constructed for the 

 purpose of using it, by avoiding the mixture of ita 

 vapor with the water of condensation, that is, by 

 applying the cold body externally instead of intro- 

 ducing it into the machine. It has been thought 

 that a remarkable advantage might be secured by 

 using the vapor of alcohol in that it possesses a 

 stronger tension than the vapor of water at the 

 same temperature. We can see in this only a fresh 

 obstacle to be overcome. The principal defect of 



* Note I, Appendix B, 



