120 MOTIVE POWER OF HEAT. 



(This fact is proved by what has already been 

 stated.) Now we have seen how important it is to 

 produce by change of volume the greatest possible 

 changes of temperature. 



(2) Vapors of water can be formed only through 

 the intervention of a boiler, while atmospheric air 

 could be heated directly by combustion carried on 

 within its own mass. Considerable loss could thus 

 be prevented, not only in the quantity of heat, but 

 also in its temperature. This advantage belongs 

 exclusively to atmospheric air. Other gases do 

 not possess it. They would be even more difficult 

 to heat than vapor of water. 



(3) In order to give to air great increase of 

 volume, and by that expansion to produce a great 

 change of temperature, it must first be taken under 

 a sufficiently high pressure; then it must be com- 

 pressed with a pump or by some other means be- 

 fore heating it. This operation would require a 

 special apparatus, an apparatus not found in steam- 

 engines. In the latter, water is in a liquid state 

 when injected into the boiler, and to introduce it 

 requires but a small pump. 



(4) The condensing of the vapor by contact with 

 the refrigerant body is much more prompt and 

 much easier than is the cooling of air. There 

 might, of course, be the expedient of throwing the 



