274 M. V. Regnault on the Elastic Forces of Vapours 



solution must boil at higher temperatures than the volatile liquid 

 alone. Thus it may be conceived, that when the pure volatile 

 liquid is submitted to the action of heat, its molecules, in order 

 to assume the state of vapour, have only to overcome the external 

 pressure to which they are subjected, and the adhesion or special 

 affinity which they possess for the similar molecules which retain 

 the liquid form. In the case of a saline solution, on the other 

 hand, the molecules which assume the form of vapour have also 

 to overcome the attraction exerted upon them by the particles of 

 the dissolved substance, an attraction which is generally greater 

 than that arising from similar particles. It is consequently 

 necessary for the development of vapour that the fluid medium 

 should assume a higher temperature than if it consisted solely of 

 the volatile substance. 



But I do not so clearly perceive how the vapour, at the moment 

 of its disengagement from the liquid, should possess a much 

 lower temperature than that of the last strata of liquid which it 

 has just traversed. I admit that the vapour, at the moment of 

 its formation in the midst of the solution, possesses an elastic 

 force greater than that which gives an equilibrium to the external 

 pressure, since it must also overcome the attractive force of the 

 saline particles. But as soon as this vapour has collected in a 

 bubble rising through the liquid, it must expand, and only pre- 

 serve the elastic force which is necessary to place it in equili- 

 brium with the hydrostatic pressure which it experiences in the 

 stratum of liquid in which it is at the moment, and with the 

 capillary action of the liquid walls of the bubble, an action which 

 diminishes in proportion as the bubble acquires greater develop- 

 ment. I admit that, in consequence of this successive expansion, 

 the temperature of the vapour must be lowered ; but as the 

 bubble is surrounded by a hotter liquid, this ought constantly 

 to furnish the heat which is lost by expansion ; and the bubble, on 

 issuing from the liquid, ought to present an equilibrium of tem- 

 perature with this. 



To explain Rudberg^s law, it is necessary to assume that the 

 vapour, as long as it remains in the midst of the boiling liquid, 

 possesses, in consequence of the attraction of the saline particles, 

 a greater density than corresponds under the same temperature 

 with the hydrostatic pressure exerted upon it ; and that it only 

 assumes its normal density at the moment when, escaping from 

 the liquid, it is relieved from this action. The vapour would 

 then experience a sudden dilatation, which would cause the 

 excess of heat to become latent, and bring it exactly to the tem- 

 perature at which its elastic force is in equilibrium with the 

 atmospheric pressure. 



But, on the one hand, it is necessary to admit that this excess 



