160 
MR. MACQUORN RANKINE ON THERMO DYNAMICS. 
heat of evaporation of water, as deduced from M. Regnault’s experiments, may be 
somewhat smaller than that which corresponds to the theoretical definition, especially 
at high pressures ; and a doubt arises as to the precise applicability of the formulae 
(59.) and (60.) to those experimental results, which cannot be solved except by direct 
experiments on the density of steam. 
Notwithstanding this doubt, however, the preceding table must be regarded as 
adding a reason to those already known, for believing that saturated steam of high 
density deviates considerably from the laws of the perfectly gaseous condition*. 
(39.) Proposition XVII. — Problem. The isothermal lines for a liquid and its 
vapour , and the apparent specific heat of the liquid at all temperatures being given, and 
the expansion of the liquid by heat being treated as inappreciably small : — to determine 
a curve of no transmission for the aggregate, passing through a given point on the 
ordinate whose distance from the origin approximately represents the volume of the 
liquid. 
(Solution.) In fig. 21, let Gy represent the volume of the liquid, assumed to be ap- 
proximately constant for all temperatures under consideration ; letyA be an ordinate 
Fig. 21. 
AT 
parallel to OY, and let the heat consumed by the liquid in passing from the tempera- 
ture corresponding to any point on this ordinate to that corresponding to any other 
point, be known ; let the isothermal lines for the aggregate of liquid and vapour, all of 
which are straight lines of equal pressure parallel to OX, such as AT 15 aBT 2 , be known. 
Then to draw a curve of no transmission through any point A on the ordinate vA, 
the same process must be followed as in Proposition VIII. 
To apply to this case the symbolical representation of Proposition VIII., viz. equa- 
tion (21.), let r, be the absolute temperature corresponding to the point A (that is, to 
the isothermal line AT,) ; r 2 that corresponding to any lower isothermal line aBT 2 ; 
V B the volume of the aggregate of liquid and vapour, corresponding to the point B 
* Evidence in favour of this opinion is afforded by the experiments recorded by Mr. C. W. Siemens (Civil 
Engineer and Architect’s Journal). A remarkable cause, however, of uncertainty in all such experiments, has 
lately been investigated by Professor Magnus (Poggendorff’s Annalen, 1853, No. 8), viz. a power which solid 
bodies have of condensing, by attraction on their surfaces, appreciable quantities of gases. 
