﻿114 Messrs. H. T. Tizard and D. R. Pye on the 



theory and experiment is in this case only moderate. In 

 particular, it will be noticed that although the ignition 

 temperature with the fan running is calculated, from the 

 results without the fan, to be 295°*5 C, actually no ignition 

 was observed at 300°C, and even in one experiment at 307° C, 

 although this is extremely doubtful, since in two other 

 experiments ignition was observed to take place at 307° C. 

 and 304° C, with the comparatively short delays of O'll and 

 0'14 second respectively. It is possible that when the loss 

 of heat is considerable, and the temperature coefficient small, 

 there is an appreciable error introduced in neglecting 

 changes of concentration when calculating the time of ignition 

 at temperatures near the ignition temperature. This would 

 account for no ignition being observed when a long delay 

 was expected. The value of 6/T given above cannot, bow- 

 ever, be very far wrong. For suppose we take the ignition 

 temperature T ' of the mixture when, the fan is running as 

 300° C. instead of 295 0, 5, and calculate the temperature co- 

 efficient solely from the difference in T and T ' without 

 regard to the " delay " curve when the fan is stationary ; 

 then we shall have 



Ti=||6 =0 .9i8 



lo 573 



and Qo'_ 1-34( 300-42) _ 



Qo ~ 0-46 (253 - 32) ~ 6 W ' 



log3-40 = -531=A(l-g,) 

 = |- x-082; 



J-O 



— =6*5 instead of 7*0. 



J-o 



This value for ~r would, in fact, fit the lower part of the 



J-o 



delay curve without the fan rather better than the value 7*0, 

 but the calculated delay curve when the fan is running- 

 would then be some way from the experimental points. If 

 we take a mean value 



6/T =6-7, 

 we shall be very unlikely to be as much as 10 per cent, from 



