CHAPTER XVIII. 



MOLECULAR AGGREGATION AND DISSOCIATION. 



392. THROUGH all our investigations we have relied upon the supposition 

 that a gas can be regarded as a collection of separate dynamical systems, namely 

 molecules, each of which retains its identity through all time. We must now 

 discuss what changes are required when this supposition is regarded only as 

 an approximation to the truth, and not as expressing the complete truth. 



MOLECULAR AGGREGATION. 



393. We have already seen that there must be a small attractive force 

 between those molecules in a gas which are sufficiently near to one another, 

 or, more precisely, that the potential energy of the total intermolecular forces 

 in a gas is negative. 



This result, it is worth noticing, is intelligible without assuming that 

 there is any definitely attractive force inherent in a single molecule. In 85 

 we obtained as the laws of distribution for those molecules which were free 

 from intermolecular force and for those molecules which were under the 

 influence of intermolecular force, equations of the forms 



(768), 



etc. 



it being sufficient for our present purpose to consider a gas in which only one 

 kind of molecule is present, i.e. a gas which is chemically pure. In the above 

 equations W aa is the potential of the intermolecular forces between the two 

 molecules. If we denote the potential of the intermolecular forces between 

 three molecules by W aaai) and so on, we obtain as the total intermolecular 

 potential energy of the gas, 



= A* 



&... + etc ....... (769), 



