286 



being maintained always at the temperature of saturation for its 

 density, until it is restored to its original temperature, at which 

 temperature let it be liquefied : then the excess of the heat absorbed 

 by the fluid above the heat given out will be equal to the expansive 

 power generated. 



From this principle it is deduced that when a vapour is sensibly 

 in the state of perfect gas, and of very small density as compared 

 with its liquid, the total heat of evaporation increases uniformly with 

 the temperature, and the rate of increase is sensibly equal to the ap- 

 parent specific heat of the vapour at constant pressure. This con- 

 clusion is verified by the experiments of M. Regnault upon the eva- 

 poration of water. As an additional verification of the theory, the 

 real specific heat of steam is calculated from the total heat of evapo- 

 ration, and also from the specific heat of atmospheric air ; and the 

 results of these two processes are found to agree exactly, being 

 equal to 0'183 of the apparent specific heat of liquid water. 



The fourth and last section of the second part is an investigation 

 of the mechanical action of steam, treated as a perfect gas, and the 

 power of the steam-engine. 



The density of steam of saturation at 100 centigrade, is calcu- 

 lated from its chemical composition on the assumption of its being a 

 perfect gas, and found to agree with the result of experiment, being 

 TeVe of the maximum density of water; and thence it is inferred 

 that, in the absence of more precise data, steam at ordinary pres- 

 sures may be treated in practice as a perfect gas, without material 

 eiTor. 



The mechanical action of unity of weight of steam while entering 

 a cylinder, and before it has begun to expand, is found by multiply- 

 ing its pressure by its volume. The expansive action is next inves- 

 tigated, taking into account the liquefaction of a portion of the steam 

 in supplying the heat required to expand the rest. The exact ex- 

 pression of this action is extremely complicated ; but approximate 

 formulsB of a more simple kind are given, suitable for calculating its 

 amount with accuracy sufficient for practice, in different portions of 

 the scale of pressures. From the sum of those two portions of power, 

 deductions are made for the loss of power arising from clearance, and 

 for the effect of the counter-pressure of the escaping steam. Thus is 

 obtained the complete expression for the gross effect of unity of weight 

 of steam, which, being multiplied by the weight of water effectively 



