OF THE FLAHE IN THE EXPLOSION OF GASES. 
317 
the pressure of the spring the metal tongue moved; it not, it remained stationary. 
By successive trials the pressures produced in the ])omb could thus be obtained. 
With this apparatus Mallard and Le Chatelier found that, with all rapidly 
exploding mixtures, very liigli pressures were often produced during very small 
intervals of time. These pressures were not always the same for the same mixture. 
Tliese fugitive pressures are due, according to Mallard and Le Chatelier, to the 
compression-wave which is propagated as the inflammation spreads from layer to 
layer, and may become of enormous intensity in the detonation-wave itself As their 
object was to measure the mean pressure in the whole mass of gas, they abandoned 
the delicate Deprez indicator and used a Bourdon gauge. From the curves of 
pressure so registered they obtained expressions for the rate of cooling of the products 
of combustion, and so calculated the maximum pressures and temperatures ot the 
explosions. Their results may be summarised in the statement that the maximum 
temperature of explosion of moist electrolytic gas is 3350^ and the mean specific heat 
of steam between this temperature and 0 ° is 16'6, dissociation l)eing very alight, if 
any, between these limits; on the other hand, the mean specific heat of carbonic acid 
rises to 13'6 at 2000 °, and above this temperature dissociation begins. The diatomic 
gases (Og, No, CO, &c.) also show a rise of specific heat, though far less marked than 
steam or carbonic acid exhibits. 
In 1S85 Berthelot and Vieille"'^ published their researches on the pressures 
produced in the explosion of gases. They measured the pressui'es by determining 
the maximum acceleration of a piston of known weight moved against gravity, 
making a correction for the cooling effect of the walls when a small explosion vessel 
was employed. From the maximum pressures they calculated the maximum tem¬ 
peratures, arriving at results for hydrogen and carbonic oxide mixtures similar to 
those obtained by Bunsex and by Mallard and Le Chatelier. But whereas 
Bunsex attributed the defect of pressure observed to the inability of two-thirds 
of the gases to combine at the temperature reached, the French chemists attribute 
this defect of pressure to the great increase of the specific heats of the gaseous 
products of combustion. By determining the pressure produced in the explosion 
of cyanogen with its own volume of oxygen— 
CgNg + 0.2= 2CO -f Ng 
—they calculate the maximum temperature as 4394° C., and the mean specific heat 
of CO and Ng at constant volume between 0 ° and 4400° C. is found to be 9’ 6 , 
which is just double the value found at ordinary temperatures. Similar determina¬ 
tions were made with cyanogen and the oxides of nitrogen. It is to be observed 
that Berthelot and Vieille assume that at the moment of maximum pressure the 
combustion is complete. In the same way Berthelot and Vieille, by determining 
the pressures produced in the explosion of hydrogen and carboiiic oxide, obtain 
* ‘ Aiinales de Chim. efc Phys.’ [VI.], vol. 4, p. 13 (1885). 
