MECHANICAL ACTION OF HEAT. 171 
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(16.) For the vapours of which the properties are known, the negative terms 
of this expression exceed the positive at all ordinary temperatures, so that the 
kind of apparent specific heat now under consideration is a negative quantity :— 
that is to say, that if a given weight of vapour at saturation is increased in tem- 
perature, and at the same time maintained by compression at the maximum elas- 
ticity, the heat generated by the compression is greater than that which is required 
to produce the elevation of temperature, and a surplus of heat is given out; and 
on the other hand, if vapour at saturation is allowed to expand, and at the same 
time maintained at the temperature of saturation, the heat which disappears in 
producing the expansion is greater than that set free by the fall of temperature, 
and the deficiency of heat must be supplied from without, otherwise a portion of 
the vapour will be liquefied, in order to supply the heat necessary for the eapansion 
of the rest. 
This circumstance is obviously of great importance in meteorology, and in the 
theory of the steam-engine. There is as yet no experimental proof of it. It is 
true, that, in the working of non-condensing engines, it has been found that the 
steam which escapes is always at the temperature of saturation corresponding to 
its pressure, and carries along with it a portion of water in the liquid state; but 
it is impossible to distinguish between the water which has been liquefied by the 
expansion of the steam, and that which has been carried over mechanically from 
the boiler. 
The calculation of the proportion of vapour liquefied by a given expansion, 
requires the knowledge of the latent heat of evaporation, which forms the subject 
of the next section. 
Section IIJ.—Or tHe Latent anp Toran HEAT or Evaporation, ESPECIALLY 
FOR WATER. 
(17.) The latent heat of evaporation of a given substance at a given tempe- 
rature, is the amount of heat which disappears in transforming unity of weight 
of the substance from the liquid state, to that of vapour of the maximum density 
for the given temperature, being consumed in producing an increase of volume, 
and an unknown change of molecular arrangement. 
It is obvious, that if the vapour thus produced is reconverted into the liquid 
state at the same temperature, the heat given out during the liquefaction must be 
VOL. XX. PART I. 22 

