PHODUCKD I!V HOT PLATINUM IN DIFFKRKNT CASKS. 33 



k being the dissociation constant of free oxygen. Tlic rate of formation of the Pt, O 

 systems is evidently proportional to p (ax\ whilst their rate of decomposition is 

 proportional to r. The total rate of increase is therefore 



A and B l>eing constants. In the steady state djr/dt = 0, and 



Apq a/(P) 

 = ~ ' 



if we write b = B/A and p = y*(P). Assuming that the positive ionisation is 

 proportional to the amount of adsorbed oxygen, its variation with the pressure will 

 lie given by the above expression. 



The assumption that the ionisation is proportional to the amount ot adsorbed 

 oxygen does not commit us to the view that the ions are liberated by the 

 decomposition of an oxide of platinum, though it is not inconsistent with that view. 

 The phenomena of electrolysis show that, when molecular disruption is accompanied 

 by electrical separation, the metal tends to receive a positive, and the non-metallic 

 element a negative, charge. We might therefore expect the oxygen to be liberated 

 from hot metals in the form of negative ions, whereas it is found that the oxygen 

 receives a positive charge. This seems to indicate that the positive ions are not 

 charged oxygen atoms when they are expelled from the system platinum-oxygen, 

 which possibly Incomes unstable owing to continued corpuscular bombardment. 

 However, since we know so little of the material constitution of the positive ions, it 

 seems unprofitable to dwell any longer on the question of their precise origin. 



Returning to the consideration of the formula that has lx?en obtained for the leak 

 in equation (2), we see that when the amount of dissociation is small p = &'P*. This 

 will be the case at low temperatures, if the pressure is not too small, so that under 

 these circumstances the ionisation will vary with the pressure as aP'/(/8 + I 1 '), a and y3 

 taing constants. This expression is evidently of the right form, for when P is small 

 it varies as P', whilst when P is great it becomes independent of P, and this is the 

 kind of variation with pressure that was found experimentally. The numbers in the 

 following table, which refer to an experiment at 730 C., furnish a means of examining 

 the agreement with the formula in more detail. The unit of current is 10~ 1J ampere. 

 The constants in the formula are a = 12, /? = 3'9, the pressures being reckoned in 

 millimetres of mercury. 



The agreement between the observed and calculated values of the ionisation is as 

 close as the experimental results warrant. In general, the observed leak tends to be 

 smaller than the calculated at the very lowest pressures, owing to the concentration 

 of the dissociated oxygen beginning to vary with the pressure more rapidly than as 



VOL. CX'VH. A. F 



