FOKCE DEVELOPED IN EVAPORATION, 



its presence is not indicated by the thermometer, is incontestably 

 established by the result of the following process : 



Let the steam, at 212, which has been evolved from a cubic 

 inch of water at 32, be mixed with 5| cubic inches of water at 

 the temperature of 32. The steam will be at once reconverted 

 into water, and the mixture will be 6| cubic inches of water, the 

 temperature of which will be 212. Thus it appears that the steam 

 at 212, when reconverted into a cubic inch of water at 212, 

 parts with as much heat as suffices to raise 5| cubic inches 

 of water from 32 to 212, which is exactly the quantity of heat 

 which disappeared while the water was converted into steam. 



The heat which is thus contained in steam, without affecting 

 the thermometer, is said to be LATENT, and the latent heat of steam 

 is therefore stated to be about 1000, the meaning of which is, 

 that to convert boiling water into steam as much heat must be 

 imparted to it as would raise it 1000 higher in temperature if it 

 did not undergo that change of state. 



12. In the preceding explanation we have supposed the piston r 

 to carry a weight of 1 5 Ib. Let us now consider in what manner 

 the phenomena would be modified if it were loaded with a greater 

 or less weight. 



If it were loaded with 301b., the conversion of the water under 

 it into steam would not commence until the temperature is raised 

 to 251|, and when the whole of the water is evaporated, the piston 

 would be raised to the height of only 883 inches, being a very 

 little more than half the height to which it was raised when the 

 evaporation took place under half the pressure. For all practical 

 purposes, then, we shall be sufficiently accurate in stating, that 

 when the weight on the piston p is doubled, it will be raised by 

 the evaporation of a given quantity of water to half the height. 



In general, in whatever proportion the weight on the piston is 

 increased, the height to which it is raised by the evaporation of a 

 given quantity of water will be decreased, and in whatever pro- 

 portion the weight is diminished, the height will be increased. 



13. It follows, therefore, that in all cases, whatever be the 

 pressure under which the evaporation takes place, the same mecha- 

 nical force is developed by the evaporation of the same quantity of 

 water. Strictly speaking, there is a little more force with greater 

 pressures, but the difference is so small, and so nearly balanced by 

 certain practical disadvantages attending high pressures, that it 

 may be wholly disregarded. 



Since the amount of force developed by each cubic inch of water 

 evaporated is equivalent to 2100 Ib. raised one foot, we shall be 

 sufficiently near the truth in stating in round numbers that such 

 a force is equivalent to a ton weight raised a foot high. 



201 



