1881.] 



On the States of Matter. 



409 



As MM. Cailletet and Hautefeuille have recently come to the con- 

 clusion that the continuity claimed by Andrews does not exist, and have 

 thus corroborated my work, I wish to place on record more fully the 

 conclusions to which this work has led me. 



In examining a subject of this kind it is important that we should 

 first arrive at a clear understanding of the meaning of the language 

 we use; second, of the means we have of proving our knowledge of 

 the state of matter ; and, third, of the value of such proof. 



First, then, as to the meaning to be attached to the words solid, 

 liquid, and gaseous. The solid state is that state of matter which in an 

 isolated portion free to move displays sufficient rigidity of its sub- 

 stance to retain permanent irregularity of figure. The liquid state 

 denotes that state of matter which in an isolated portion free to move 

 displays sufficient freedom of motion in its substance to assume a form 

 in accordance with the forces acting upon it, but which when partially 

 filling an inclosed space retains a permanent limiting surface exhi- 

 biting capillarity. The gaseous state denotes that state of matter 

 in which it assumes no surface nor definite figure, and which is so 

 extensible that any quantity will distribute itself throughout a 

 space. 



Having defined the nomenclature, let us see what means we have of 

 proving our knowledge of any state. Let us conceive that we are 

 working with what we rarely or never have — pure substances. When 

 water is nearly pure it freezes perfectly transparently ; in fact, except 

 for rigidity and difference in density it could not be distinguished 

 from liquid water. Now consider a vessel frozen quite full of pure ice ; 

 there is no free surface to examine, therefore one test of its state is 

 gone. Now let enormous pressure be brought to bear upon the ice till 

 its density is reduced to that of water at the same temperature, and 

 let the temperature be the new freezing-point ; then there is no way 

 of knowing whether the inclosed water is liquid or solid, without 

 applying special tests. Either the vessel must be broken to test the 

 rigidity of the contents or motion must be given to the vessel, and 

 dynamic tests made as to the behaviour of the contents ; but what I 

 wish to impress is, that sight can teach us nothing directly as to the 

 condition of the contents. A free surface is required to indicate the 

 state of matter. So it is with the liquid and gaseous states, but just 

 as they are more subtle, so are the modes of experimenting with them 

 more difficult. Let us consider the case of a liquid inclosed in a space 

 bounded by transparent sides having a piston free to move. When the 

 pressure of the piston is less than that of the vapour of the liquid, 

 there will be a space occupied by vapour over the liquid, and the sur- 

 face of the latter will be visible and its state recognisable at a glance ; 

 but let there be more pressure upon the piston than equals that of the 

 vapour, then no vapour will be formed, and the liquid will fill the whole 



