148 PROFESSOR H. B. DIXON ON THE RATE OF EXPLOSION IN GASES. 
The fact that the rate of explosion of hydrogen and chlorine exceeds the calculated 
velocity of tiie sound-wave, as calculated from Thomsen’s Heat of Combination 
(22,000 cals.), is possibly due to the behaviour of the unburnt chlorine in the wave- 
front. The rates agree fairly with the sound-wave calculated from the mean heat of 
combination, which I have given in the table. 
Ca.p. X.— The Instantaneous Pressures Produced in the Explosion-Wave. 
There is, lastly, one important phenomenon in the propagation of explosions in gases 
which I have not touched on—the pressure produced in the reacting gases. Bunsen 
was the first who attempted to measure this pressure. In his well-known work* he 
describes the explosion of mixtures of hydrogen and of carbonic oxide with oxygen in 
a strong vessel closed with a loaded lid. When the pressure produced in the vessel 
exceeded the pressure on the lid, the latter was raised in the manner of a safety valve, 
and some of the heated gas escaped ; when the pressure was less than that on the lid, 
no gas escaped and but little noise was heard. 
By successive trials he thus found the pressure produced by the explosion of carbonic 
oxide and oxygen to be about 10 atmospheres, and the pressure produced by the 
explosion of hydrogen and oxygen to be about 9'5 atmospheres. The comparison of 
these pressures with the numliers calculated from the heat of combination and the 
specific heats of the products led Bunsen to the conclusion that, at the highest 
temperature reached in the explosion, only one-third of the gases entered into com¬ 
bination. Berthelot and Vieille, working in a manner essentially the same, only 
replacing Bunsen’s loaded lid by a light piston moving against a spring, found the 
pressures produced in the explosion of earbonic oxide and hydrogen to be lO'l and 
9'8 atmospheres respectively. Berthelot rightly calls these pressures “ effective pres- 
sures.”f On the other hand. Mallard and Le Chatelier, using the more delicate 
indicator of Deprez, found pressures produced in the explosion of hydrogen and oxygen, 
of far greater amount, but lasting an exceedingly short time. These fugitive pressures 
of great intensity were only obtained with mixtures which burnt very rapidly :—J 
“ Cette pression tres fugitive est d’autant plus grande que la vitesse de propagation 
est plus considerable., Elle pent devenir enorme avec I’onde explosive. C’est a ces 
pressions, si instantanees qu’elles soient, que sent dus les sur])renants effets des sub¬ 
stances explosives telles que la nitroglycerine. Ce sont precisement ces pressions 
passageres, sans relation directe avec le phenomene que nous voulions mesurer, qu’en- 
registrait notre appareil.” 
On substituting for the Deprez indicator a less sensitive Bourdon manometer, 
* ‘ Gasometrisclie ]\IetIioden,’ 1857. 
t ‘Ann. Chim. et Phys.’ [vi.] 4, ji. 14, 1885. 
t ‘ Combustions des Melanges gazenx,’ j). 1‘2;3. 
