Chemical Combination of Gases, 241 



then the presence of the neutral gas would increase the 

 number of atoms, and therefore the number of shocks, and so 

 would diminish the paired time, while, as we can hardly con- 

 ceive that the presence of a neutral gas can make the atoms 

 combine more quickly, the free time will not be diminished. 

 Thus if the collision theory were correct, the dissociation 

 would be increased by the presence of a neutral gas. The fact 

 that this is not the case shows that the mere increase in the 

 number of collisions, if the increase be produced by the collision 

 of different bodies, will not produce increased dissociation. 

 We must remember that, as was remarked before, two atoms 

 will not remain together indefinitely, even if there are no 

 collisions. In the following table I have calculated by means 

 of equation (4) the quantity of iodine-vapour which would be 

 dissociated if the paired time were independent of N, and if 

 the collisions were not the cause of the dissociation, and com- 

 pared them with the experimental results obtained by Crafts 

 and Meier (Comptes Rendus, 3 Janvier, 1881, p. 41, quoted 

 by M. Lemoine, Annates de Chimie et de Physique, 5 me serie, 

 t. xxvi. p. 347) ; the theoretical and the experimental results 

 agree so closely that the difference between them might be 



accounted for by errors of experiment. The constant — which 



occurs in equation (4) is determined by making the theo- 

 retical and the experimental results agree at the pressure *4 

 of an atmosphere. I have taken D, the vapour-density of 

 normal (i.e. not dissociated) iodine-vapour, to be 8*78. 



Yapour Density of Iodine at 1250° C. 



Density. 



Pressure in r A N 



atmospheres. Observed. Calculated. 



1 . . . . 5-8 6-12 



•4 ..... . 5-54 5-54 



•3 . . . . 5-30 5-36 



•2 . . . . 5-07 5-15 



•1 . . . . 4*72 4-84 



From equation (4) we find, by substituting the observed value 

 of A at the pressure of *4 of an atmosphere, that 



tN p' 



where p is the pressure expressed in atmospheres. So that at 

 the pressure of one atmosphere 



