Change of Oxygen into Ozone.. 207 



faction, although, as we have just seen, the mechanism for 

 keeping pressure constant is very different. But the simi- 

 larity extends a little farther than to the one point of constant 

 pressure, for Bohr found that on starting at a pressure below 

 •7 millim. and suddenly diminishing the volume the pressure 

 could be raised to "8 millim., although in the course of some 

 hours it would fall back about ten per cent., a case which 

 corresponds to that of supersaturation of a vapour with slow 

 deposition of liquid ; on the other hand, when the pressure 

 was at *7 millim. and the volume was increased the pressure 

 remained at *7 millim. for a while and then began to fall off. 

 These facts prove that dissociation goes on with great 

 rapidity, while combination is slow, in agreement with our 

 theory, because dissociation being produced directly by the 

 collisions must be a very rapid process, whereas combination, 

 as it must depend on collisions of the 2 molecules under 

 very favourable circumstances, being compelled to wait for 

 the accidental occurrence of these rare conditions, must go 

 on very slowly. 



We have now to see how the theory explains the ano- 

 malous deflecting force discovered by Crookes, and also the 

 slightness of the disturbance of the viscosity. Suppose that 

 we have two chambers at temperatures 6 a and b connected by 

 a tube and containing a mixture of 3 and 2 in the cooler 

 chamber, there will be equilibrium between the 3 and the 



2 when N 2 /B = SaJ/2mv 2 a , where d a is the value of a! appro- 

 priate to the temperature 6 a ; thermal transpiration will carry 

 the mixture of N 2 /B molecules of 3 and N ly /B of 2 per 

 unit volume along the tube to the hotter chamber, where the 

 resulting increase of pressure, as well as the increase of tem- 

 perature, will dissociate some of the N 2 , with still further 

 tendency to increase the pressure ; now, when equilibrium is 

 reached between the 3 and the 2 in the hotter chamber, 

 ~N 2 /B = 3<z' b /2mvl in it ; and the final steady state of the whole 

 system is determined by the conditions that N 2 /B in the cool 

 chamber and in the hot chamber has the values given ; 

 therefore, the circulation which was proved to accompany 

 thermal transpiration will tend to carry fewer molecules of 



3 from the hot chamber to the cold than from the cold to 

 the hot, in the proportion of a b /mvl to <x.' a /mvl, and therefore, 

 to preserve the state of equilibrium, the circulation will have 

 to cany more 2 molecules from the hot chamber to the 

 cold than from cold to hot, the gain of 3 in the hot chamber 

 being dissociated to keep up the supply of 2 , and the gain 

 of 2 in the cold chamber being combined to keep up the 

 supply of 3 there, and thus the excess of N,/B, the number 



