158 PROFESSOR H. E. DIXOR" ON THE RATE OF EXPLOSION IN GASES. 
(2.) Mixture H 3 + O + Ng. 
Pressure . 
1 At. 
2 At. 
Rate of explosion . 
1815 
1830 
(3.) Mixture Hg + 0. 
Pressure . 
1 At. 
2 At. 
Rate of explosion . 
3532 
3538 
No doubt the ]lumbers obtained point to a slight increase in the rate on doubling 
the pressure, but the differences are so small as to be confounded with the experi¬ 
mental error. I have not, therefore, given the higher rates as the true rates of the 
explosion, but have taken the mean of the two in each case. 
The difference in the effect of pressure on electrolytic gas when pure, and when 
diluted, may be due to one of two causes. In the case of the pure gas increase of 
pressure may check dissociation, and thus increase the rate of explosion ; whereas the 
diluted gas does not dissociate, and is consequently unaffected by changes of pressure. 
Or, more probably, since the diluted gas suffers less contraction in the chemical 
change, its crucial pressure is lower than that of the pure gas, and the change from one 
to two atmospheres being wholly, or in large part, above the crucial pressure, the effect 
on the rate is nil or very slight. In all the mixtures I have tried the rate of the 
explosion-wave is diminished by a sufficient reduction of pressure, even when the 
products of combustion are carbonic oxide and nitrogen alone. Changes of pressure 
appear, therefore, to affect the rate apart from any dissociation ; and consequently 
may influence the rate of explosion of electrolytic gas independently of any effect on 
dissociation. Why changes of pressure affect the rate of explosion has yet received 
no satisfactory explanation. 
As the initial pressure of a gaseous mixture is reduced, it is found that the flame 
travels further from the ignition-point before its rate becomes constant; it requires a 
longer run to attain its maximum velocity. By sufficiently reducing the pressure it 
is possible to stop the explosion of gases altogether; but the flame is still propagated 
slowly at pressures below that at which the ex plosion-wave dies out. For instance, 
with electrolytic gas, the explosion-wave is propagated at 200 mm. pressure, but not 
at 150 mm. At 200 mm. pressure the flame travels about 2 metres before attaining 
its maximum rate. According to my experiments'" the flame is still propagated in 
* ‘Phil. Traus.,’ 1884, p. 031. 
