PHYSICAL PROPERTIES CF WATER, ETC. 71 



" Second. Aitken has asserted, as a conclusion from the results of direct experiment, 

 that even immensely supersaturated aqueous vapour will not condense without the 

 presence of a nucleus. This may be a solid body of finite size, a drop of water, or 

 fine dust particles. 



" Both of these facts fit perfectly in to the hypothesis, that the isothermal in question 

 has an asymptote parallel to the axis of pressure ; the vapour requiring (in the absence 

 of a nucleus) practically infinite pressure to reduce it, without change of state or of 

 temperature, to a certain finite volume ; while the liquid, also without change of state 

 or temperature, may by sufficient hydrostatic tension be made to expand almost to 

 the same limit of volume. 



" This limiting volume depends, of course, on the temperature of the isothermal ; 

 rising with it up to the critical point. 



" The physical, not geometrical, discontinuity is of course to be attributed to the 

 latent heat of vaporisation. The study of the adiabatics, as modified by this hypo- 

 thesis, gives rise to some curious results. 



" It is clear that the experimental realisation of the parts of the here suggested 

 curve hear to the asymptote, on either side, will be a matter of great difficulty for. any 

 substance. But valuable information may perhaps be obtained from the indications of 

 a sensitive thermo-electric junction immersed in mercury at the top of a column which 

 does not descend in a barometer tube of considerably more than 30 inches long, when 

 the tube is suddenly placed at a large angle with the vertical ; or from those of a 

 similar junction immersed in water, when it has a concave surface of great curvature 

 from which the atmospheric pressure is removed. 



" Nothing of what is said above will necessarily apply when we have vapour and 

 liquid in presence of one another, or when we consider a small portion of either in the 

 immediate neighbourhood of another body. For then we are dealing with a state of 

 stress which cannot, like hydrostatic pressure or tension, be characterized (so far as we 

 know) by a single number. The stress in these molecular films is probably one of 

 tension in all directions parallel to the film, and of pressure in a direction perpendicular 

 to it. Thus it is impossible to represent such a state properly on the ordinary indicator 

 diagram. This question is still further complicated by the possibility that the differ- 

 ence between the internal pressures, in a liquid and its vapour in thermal equilibrium, 

 may be a very large quantity." 



As soon as I heard of Berthelot's experiment, I had it successfully repeated in 

 my laboratory; and I considered that it afforded very strong confirmation of the 

 hypothesis advanced in the last preceding extract. 



But since I have been led to believe that there is probably truth in Laplace's 

 statement as to the very great molecular pressure in liquids, I have still further 

 modified the speculation. I now propose to take away the new asymptote, and make 



