
DYNAMICAL THEORY OF HEAT. 275 
(1.) The rate of variation with the temperature, of the pressure of saturated 
steam. 
(2.) The latent heat of a given weight of saturated steam. 
(3.) The volume of a given weight of saturated steam. 
(4.) The volume of a given weight of water. 
The last mentioned of these elements may, on account of the manner in which 
it enters the formula, be taken as constant, without producing any appreciable 
effect on the probable accuracy of the result. 
32. ReaNAULT’s observations have supplied the first of the data with very 
great accuracy for all temperatures between — 32° cent. and 230°. 
33. As regards the second of the data, it must be remarked that all experi- 
menters, from War?, who first made experiments on the subject, to REGNAULT, 
whose determinations are the most accurate and extensive that have yet been 
made, appear to have either explicitly or tacitly assumed the same principle as 
that of Carnot, which is overturned by the dynamical theory of heat; inas- 
much as they have defined the “ total heat of steam” as the quantity of heat 
required, to convert a unit of weight of water at 0°, into steam in the particular 
state considered. Thus ReGNnavut, setting out with this definition for “ the 
total heat of saturated steam,’ gives experimental determinations of it for the 
entire range of temperatures from 0° to 230°; and he deduces the “ latent heat 
_ of saturated steam” at any temperature, from the “ total heat,” so determined, 
by subtracting from it the quantity of heat necessary to raise the liquid to that 
temperature. Now, according to the dynamical theory, the quantity of heat 
expressed by the preceding definition depends on the manner (which may be 
infinitely varied) in which the change of state defined is effected; differing in 
different cases by the thermal equivalents of the differences of the external mecha- 
nical effect produced in the expansion. For instance, the final quantity of heat 
required to evaporate a quantity of water at 0°, and then, keeping it always in 
the state of saturated vapour,* bring it to the temperature 100°, cannot be so 
much as three-fourths of the quantity required, first, to raise the temperature of 
the liquid to 100°, and then evaporate it at that temperature; and yet either 
quantity is expressed by what is generally received as a definition of the “ total 
heat” of the saturated vapour. To find what it is that is really determined as 
“total heat” of saturated steam in REGNAULT’s researches, it is only necessary to 
* See below (Part III. § 58), where the ‘“ negative’ specific heat of saturated steam is 
investigated. If the mean value of this quantity between 0° and 100° were — 1:5 (and it cannot 
differ much from this) there would be 150 units of heat emitted by a pound of saturated vapour in 
having its"temperature raised (by compression) from 0° to 100°. The latent heat of the vapour at 
0° being 606-5, the final quantity of heat required to convert a pound of water at 0° into saturated . 
steam at 100°, in the first of the ways mentioned in the text, would consequently be 456-5, which is 
only about 5 of the quantity 637 found as “ the total heat’ of the saturated vapour at 100°, by 
REGNAULT. 
