18 



Prime Movers. 



regard to any whatever : much more being lost, by the addi- 

 tional quantity required to be evaporated, than is gained by the 

 lower temperature at which the evaporation takes place. In 

 reality, the effect produced by any vapour depends on the 

 amount of heat which is necessary for its production : and this, 

 for water, alcohol, and ether, are as follows . — 





Specific Gravity. 



Boiling Point. 



Heat of Conversion 

 into Vapour. 





1-000 

 0-825 

 0-700 



212° 

 175° 

 112° 



942° 



425-5° 



302-6° 





Ether 





Cagniard de la Tour reduced alcohol, spec. grav. 0*837 at a 

 temperature of 497°, to a vapour having a calculated pressure 

 of one hundred and nineteen atmospheres ; but it occupied a 

 space not quite three times its original volume. He converted 

 sulphuric ether, at 392°, into a vapour having a pressure of 

 nearly thirty-eight atmospheres ; but it occupied a space not 

 twice its original volume. At 497°, water exerts a pressure of 

 about forty-four atmospheres ; and occupies a space about ninety- 

 nine times its original volume. At 392 , it exerts a pressure of 

 about fifteen atmospheres ; and occupies a space about one 

 hundred and forty-one times its original volume. Sulphuret of 

 carbon has an elastic force equal to about four atmospheres, at 

 212°; and to nearly twenty-nine atmospheres, at 392°; but it 

 is liable to the same objections as alcohol and ether. Oil gas 

 vapour, which is produced from the liquid that is separated 

 from oil gas by the pressure used to render it portable, was 

 suggested, by Tredgold, as perhaps very suitable to supply the 

 place of the vapour of water in a steam-engine ; it boils at 1 70°, 

 and remains liquid at common temperatures. But there can be 

 no doubt that the mechanical effect of its vapour, also, is de- 

 pendent on the quantity of heat absorbed in passing from the 

 liquid to the vaporous state — this being most probably a general 

 law. 



The remarkable expansion of carbonic acid and other gases, 

 when liquified, very soon attracted notice. Sir H. Davy ex- 

 pected that it would afford a mechanical agent, on account of 

 the immense difference between the increase of elastic force in 

 gases under high and low temperatures, by similar increments 

 of temperature. The force of carbonic acid at 12°, was found 

 equal to that of air compressed to one-twentieth of its bulk ; 

 and at 32°, to that of air compressed to one thirty-sixth ; 

 making an increase of pressure equal to thirteen atmospheres. 

 It was ascertained by Thilorier, that the pressure of the vapour 



