412 



On the States of Hatter. 



[June 16, 



therefore the gaseous state proper cannot be altered by pressure alone. 

 The liquid state is thus bounded by an isotherm which marks the 

 thermal activity equivalent to the attraction of the molecules, and the 

 continuity of the liquid and gaseous states enunciated by Andrews is 

 only apparent. 



The passage from the liquid to the gaseous state might be repre- 

 sented by a curve where the line representing decrease of capillarity 

 passes through zero to a negative side, which in reality represents re- 

 pulsion. In other words, the curve for liquid represents excess of 

 attraction over repulsion, that for gas excess of repulsion over 

 attraction. 



The existence of this cohesion in liquids has been too much over- 

 looked by the statistical method of looking at the states of matter. It 

 is something very great compared with the amount of energy required 

 to give the molecular velocity required to sustain the gaseous state. 

 Let us by Dr. Joule's method make a calculation. Suppose the mole- 

 cules in a gram of steam to be at rest it would require 92 gram- 

 degrees or, in absolute measure, 383,161 — M— — — — j to give them 

 ' sec. 2 



velocity enough to exert a pressure of one atmosphere. Now the 

 energy added to cause a gram of water to become steam at one atmo- 



mtr q'ttti 



sphere pressure is 540 gram-degrees (or 2,244,780 — -), or 



sec." 



about six times as much work to be done. And this is not all. We 

 started from rest in the other calculation, here we start from water 

 whose molecules are in motion. Five-sixths of the energy has gone to 

 overcome the cohesion of the liquid. As we know, that to bring 

 matter to the same state we must expend the same amount of energy, 

 and as we may bring matter to the gaseous state either by boiling or 

 by Andrews' apparently continuous method ; in the first case we would 

 say the heat used up represents the latent heat of steam, and in the 

 other the specific heat of water, but they are both really measures of 

 the thermal value of the energy required to overcome the attraction of 

 the molecules. When, therefore, this energy has been given to the 

 water, the cohesion has been overcome and the liquid state passed, 

 whether or not it has been visible to our eyes. Thus from all evi- 

 dence, experimental and theoretical, the liquid state has a limit, and 

 the liquid and gaseous states are not really continuous. 



The definition of the gaseous state as a state of matter not alterable 

 by pressure alone leads us to a clear division of aeriform matter into 

 two states, the vaporous and gaseous, the .first alterable, the second 

 unalterable by pressure alone. Another distinction between vapour 

 and gas is this : gases are solvents of solids ; vapours are not. Let a 

 liquid be coloured by having some non- volatile coloured solid dissolved 

 in it, and let it be heated under pressure, the liquid will remain 



