Transformation from the Liquid to the Gaseous State. 239 



collected into one portion of the chamber, and all the liquid 

 into the other, whereas in the other case the vapour and 

 liquid are not distinctly separated from each other, but inter- 

 mixed in some way so as to occupy the whole space as an 

 apparently homogeneous mass. Hence the point C represents 

 two distinct conditions of the mass in which the pressure, 

 volume, and temperature are the same, but in which the 

 internal energies may differ very considerably. Thus, although 

 less external work is done in passing from B to C along the 

 curve BMC than in passing along the right line BC, yet in 

 virtue of the arrangement of the mass, the internal energy at 

 in the former case may be considerably greater than in 

 the latter. 



This, indeed, must be the case if the arrangement of the 

 mass be of the bubble and drop nature here suggested. For 

 if a given mass, existing partly as liquid and partly as 

 vapour, be arranged in such a way that the liquid is collected 

 together in one part of the containing vessel, while the 

 vapour is all collected in the remainder (as ordinarily occurs) , 

 and if we desire to change it from this arrangement into one 

 like that described above, in which the vapour is disseminated 

 through the liquid in bubbles, or in which the whole vessel is 

 filled with vapour and drops, a certain amount of work must 

 be done in order to effect the transformation — namely, the 

 equivalent of the surface energy possessed by the enormously 

 increased surface area of the bubbles and drops in the new 

 condition. Thus, although less external work is done in 

 passing along the isothermal BMC than along the rectilinear 

 path BC, yet the mass in the former case possesses more 

 surface energy than in the latter, and the excess of external 

 work in the latter transformation is represented in the former 

 by an excess of internal work spent in generating the excess 

 of surface film. 



Similar remarks apply to the portion CND, for in passing 

 along this curve the external work done is greater than that 

 performed in passing along CD, but this is compensated by 

 the destruction of the surface film. Thus, along BMC there 

 is on the whole a creation of surface film with less external 

 work, and along CND there is destruction of surface film 

 accompanied by increased external work — the excess in the 

 former being equal to the defect in the latter. 



In conclusion, it may be remarked that the views here put 

 forward seem to have an important bearing on many interest- 

 ing questions connected with the boiling-points of liquids, and 

 the manner in which they are affected by the presence of 

 dissolved salts. It is sufficient to merely point out, at present, 



