276 PROFESSOR WILLIAM THOMSON ON THE 
remark, that the measurement actually made is of the quantity of heat emitted by 
a certain weight of water in passing through a calorimetrical apparatus, which it 
enters as saturated steam, and leaves in the liquid state, the result being reduced 
to what would have been found, if the final temperature of the water had been 
exactly 0°. For there being no external mechanical effect produced (other than 
that of sound, which itis to be presumed is quite inappreciable), the only external 
effect is the emission of heat. This must, therefore, according to the fundamen- 
tal proposition of the dynamical theory, be independent of the intermediate 
agencies. It follows that, however the steam may rush through the calorimeter, 
and at whatever reduced pressure it may actually be condensed,* the heat 
emitted externally must be exactly the same as if the condensation took place 
under the full pressure of the entering saturated steam, and we conclude that 
the total heat as actually determined from his experiments by REGNAULT, is the 
quantity of heat that would be required, first to raise the liquid to the specified 
temperature, and then to evaporate it at that temperature; and that the prin- 
ciple on which he determines the latent heat is correct. Hence, through the 
range of his experiments, that is from 0° to 230°, we may consider the second of 
the data required for the calculation of as being supplied in a complete and 
satisfactory manner. 
34. There remains only the third of the data, or the volume of a given weight 
of saturated steam, for which accurate experiments through an extensive range 
are wanting; and no experimental researches bearing on the subject having been 
made since the time when my former paper was written, I see no reason for 
supposing that the values of » which I then gave are not the most probable 
that can be obtained in the present state of science; and, on the understanding 
stated in § 33 of that paper, that accurate experimental determinations of the den- 
sities of saturated steam at different temperatures may indicate considerable errors 
in the densities which have been assumed according to the “ gaseous laws,” and may 
* Tf the steam have to rush through a long fine tube, or through a small aperture within the 
calorimetrical apparatus, its pressure will be diminished before it is condensed, and there will, there- 
fore, in two parts of the calorimeter be saturated steam at different temperatures (as, for instance, 
would be the case if steam from a high pressure boiler were distilled into the open air) ; yet, on 
account of the heat developed by the fluid friction, which would be precisely the equivalent of the 
mechanical effect of the expansion wasted in the rushing, the heat measured by the calorimeter would 
be precisely the same as if the condensation took place at a pressure not appreciably lower than that 
of the entering steam. The circumstances of such a case have been overlooked by Crausrus 
(Pocernnorrr’s Annalen, 1850, No. 4, p. 510), when he expresses with some doubt the opinion that 
the latent heat of saturated steam will be truly found from Reenauxt’s “ total heat,” by deducting 
the sensible heat ; and gives as a reason that, in the actual experiments, the condensation must have 
taken place “ under the same pressure, or nearly under the same pressure,” as the evaporation. The 
question is not, Did the condensation take place at a lower pressure than that of the entering steam ? 
but, Did Reenavtr make the steam work an engine in passing through the calorimeter, or was there 
so much noise of steam rushing through it as to convert an appreciable portion of the total heat into 
external mechanical effect? And a negative answer to this is a sufficient reason for adopting with 
certainty the opinion that the principle of his determination of the latent heat is correct. 

