110 Prof. Thomson on the Dynamical Theory of Heat. 



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 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 w^ater had been exactly 0°. 

 For there being no external mechanical efiect produced (other 

 than that of sound, which it is to be presumed is quite inappre- 

 ciable), the only external efiect is the emission of heat. This 

 must, therefore, according to the fundamental proposition of the 

 dynamical theory, be independent of the intermediate agencies. 

 It follows that, however the steam may rush through the calori- 

 meter, and at whatever reduced pressure it may actually be con- 

 densed*, 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 principle on which he determines the 

 latent heat is correct. Hence, through the range of his experi- 

 ments, that is from 0° to 230°, we may consider the second of 



* If 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, therefore, in two parts of the calori- 

 meter 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 pre- 

 cisely the same as if the condensation took place at a pressure not appre- 

 ciably lower than that of the entering steam. The circumstances of such 

 a case have been overlooked by Clausius (PoggendorfF's Annalen, 1850, 

 No. 4, p. 510), when he expresses with some doubt the opinion that the 

 latent heat of saturated steam will be trvdy found from Regnault's " total 

 heat," by deducting " the sensible heat " ; and gives as a reason that, in the 

 actual experiments, the condensation must have taken ])lace " under the 

 same pressure, or nearly under the same pressure," as the evaporation. 

 The question is not. Did the condensation take jylace at a lower pressure 

 than that of the entering steam ? but. Did Regnault make the steam work 

 an engine in passing through the calorimeter, or was there so much noise of 

 steam rvshing through it as to convert an appreciable portion of the total 

 heat into external mechanical effect? And a negative answer to this is a suf- 

 ficient reason for adopting with certainty the opinion that the principle of 

 his determination of the latent heat is correct. 



