MATTER AND ITS STATES 



vessel will completely fill it, no matter if the vessel be many 

 times the size of the original volume of gas. This is an 

 observed fact. 



If such a gas is gradually cooled (i. e., allowed to do work 

 on some other body and thus to part with some of the kinetic 

 energy of its particles), a condition will be reached wherein 

 the cohesive force is greater than the repellent, and the par- 

 ticles will remain together in a definite volume. As long as 

 the two forces involved are nearly equal, the average free 

 path will still be relatively great, and although the particles 

 cling together, yet they will move very freely upon one 

 another a condition imperfectly simulated by the component 

 grains in a mass of sand. In this condition the substance 

 is said to be a liquid. Here the particles move so readily 

 upon one another that a mass of liquid still takes the form 

 of the containing vessel, as far as that is possible without 

 increase in volume. In this regard liquids are very differ- 

 ent from gases. Also, on account of the freedom of motion 

 on the part of the particles making it up, and on account of 

 the downward pull of gravity, the free surface of a liquid is 

 usually approximately level. There are, indeed, certain 

 phenomena of surface tension and adhesion which make it 

 possible for free liquid surfaces to exist in other positions 

 than the horizontal, but the present subject does not lead to 

 a discussion of these. It is necessary to call attention, how- 

 ever, to the fact that, on account of the action of the cohesive 

 force, a peculiar surface layer of particles is formed about a 

 liquid mass, a sort of thin skin or film, which possesses con- 

 siderable tensile strength, and which is much less easily 

 penetrated than the internal mass. 



By a continuation of the process of cooling (which must 

 ever be thought of as a process of causing the body to give 

 up kinetic energy by doing work, such as warming another 

 cooler body) the liquid particles may be brought still closer 



