Evaporation and Dissociation. 451 



pressure. If the diagram on plate iii. in our memoir on 

 alcohol (Phil. Trans. 1886, part i. p. 156) be referred to, it 

 will be seen that our actual measurement of such reduced 

 pressure was made on the isothermal 181 0, 4. Mr. John Aitken, 

 in an extended series of experiments on this subject (Trans. 

 Royal Scott. Soc. of Arts, vol. ix.), has shown that such 

 " superheating " can take place only in absence of a free 

 surface, i. e. the existence of gaseous nuclei in the liquid, into 

 which evaporation may take place. And Mr. Aitken has also 

 shown that a gas may be compressed to a volume smaller 

 than that at which liquefaction usually occurs, at any given 

 temperature, without formation of liquid. The space, again, 

 if no nuclei be present on which condensation may take place, 

 remains " supersaturated with vapour." It is evidently, there- 

 fore, only the instability of such conditions which prevents 

 their complete realization*. 



The formula? of Clausius and Van der Waals are based on 

 the assumption that two causes are in operation — those 

 referred to in the beginning of this paper — viz. the actual 

 size of the molecules, and their mutual attraction. It is 

 possible, by help of these assumptions, to realize the nature of 

 the continuous change from the gaseous to the liquid state of 

 matter. When a gas at a given temperature is reduced in 

 volume its molecules necessarily approach each other, and 

 their attraction for one another increases. This attraction 

 aids the increase of pressure in reducing volume. When a 

 certain volume is reached, the attraction has become so marked 

 that further reduction of volume is accompanied by fall of 

 pressure. If a certain volume be chosen on the descending 

 portion of an isothermal, a state of balance may be imagined 

 where pressure and cohesion unite in maintaining the volume 

 constant against the kinetic energy of the molecules, tending 

 to cause expansion. 



The conception of negative pressure, or tension, is that at 

 low temperatures and small volumes the cohesion is such that, 

 in order to overcome [it and increase volume, it would be 

 necessary to apply tension to each molecule. But after the 

 lowest pressure or greatest tension has been attained, the 

 actual size of the molecules presents a bar to closer approach; 

 and to cause further decrease of volume pressure must again 



* The reasoning of a recent paper by Wroblewski (Monatsheft der 

 Chemie, Wien, July 1886, p. 383) rests on the assumption that such con- 

 ditions are inconceivable. He supposes lines of equal density to be curves, 

 and on their close approach to the vapour-pressure curve to run parallel 

 with it. His conclusions are therefore not borne out by experimental 

 facts. 



2 H2 



