NATURAL PHILOSOPHY. 163 



other without losing any of its own motion, as to suppose that the piston of 

 a steam-engine can be set in motion without a corresponding quantity of 

 energy being lost by some other body. 



In expanding air spontaneously to a double volume, delivering it, say, into 

 a vacuous space, it has been proved repeatedly that the air does not fall 

 appreciably in temperature, no external work being performed; but, on the 

 contrary, if the air, at a temperature, say, of two hundred and thirty degrees 

 Fahrenheit, be expanded under pressure or resistance, as against the piston 

 of a cylinder, giving motion to it, raising a weight, or otherwise doing work 

 by giving motion to some other body, the temperature will fall nearly one 

 hundred and seventy degrees when the volume is doubled, that is, from two 

 hundred and thirty degrees to about sixty degrees ; and, taking the initial 

 pressure of forty pounds, the final pressure would be fifteen pounds per 

 square inch. 



When a pound weight of air in expanding, at any temperature or pressure, 

 raises one hundred and thirty pounds one foot high, it loses one degree in 

 temperature ; in other words, this pound of air would lose as much molecular 

 energy as would equal the energy acquired by a weight of one pound falling 

 through a height of one hundred and thirty feet. It must, however, be 

 remarked, that but a small portion of this work, one hundred and thirty foot- 

 pounds, can be had as available work, as the heat which disappears does not 

 depend on the amount of work or duty realized, but upon the total of the 

 opposing forces, including all resistance from any external source whatever. 

 "When air is compressed the atmosphere descends and follows the piston, as- 

 sisting in the operation with its whole weight ; and when air is expanded, 

 the motion of the piston is, on the contrary, opposed by the whole weight of 

 the atmosphere, which is again elevated. Although, therefore, in expanding 

 air the heat which disappears is in proportion to the total opposing force, it 

 is much in excess of what can be rendered available ; and, commonly, where 

 air is compressed, the heat generated is much greater than that which is due 

 to the work which is required to be expended, the weight of the atmosphere 

 assisting in the operation. 



Let a pound of water, at a temperature of two hundred and twelve degrees 

 Fahrenheit, be injected into a vacuous space or^vessel, having 26.36 cubic 

 feet of capacity, the volume of one pound of saturated steam at that tem- 

 perature, and let it be evaporated into such steam, then 893.8 units of heat 

 would be expended in the process. But if a second pound of water, at two 

 hundred and twelve degrees, be injected and evaporated at the same temper- 

 ature, under a uniform pressure of 14.7 pounds per square inch due to the 

 temperature, the second pound must dislodge the first, by repelling that pres- 

 sure, involving an amount of labor equal to 55,800 foot-pounds (that is, 14.7 

 pounds X 144 square inches X 26.36 cubic feet), and an additional expendi- 

 of 72.3 units of heat (that is, 55,800 -r- 772), making a total for the second 

 pound of 965.1 units. 



Similarly, when one thousand four hundred and eight units of heat are 

 expended in raising the temperature of air at constant pressure, one thousand 

 of the units increase the velocity of the molecules, or produce a sensible 

 increment of temperature ; while the remaining four hundred and eight parts, 

 which disappear as the air expands, are directly expended in repelling the 

 external pressure. 



Again : If steam be permitted to flow from a boiler into a comparatively 

 vacuous space, without giving motion to another body, the temperature of 



