330 Mixter — Partial non-explosive Combination of 



tern to the point of explosion? Let us suppose the mixed 

 gases be heated from 500° to 501°. This increase in tempera- 

 ture will not cause them to explode. Assume further, that the 

 heat of combustion during one minute is sufficient to raise the 

 temperature of the gases 1°, provided no heat is lost by radia- 

 tion. Experiments have shown that this rise in temperature 

 will not cause explosion. Bat the gases heated by their own 

 slow combustion continually lose heat by radiation, and hence 

 are only slightly hotter than the surrounding medium. Were 

 it possible to prevent loss of heat, the temperature would rise 

 and the change proceed with accelerating speed. 



The observations of Freyer and Meyer (loc. cit.), that explo- 

 sive mixtures gradually heated have a higher ignition point 

 than when suddenly heated, are difficult to explain, and further 

 experiments may be useful. Their results also indicate that 

 pressure is without influence, while A. Mitscherlich* found 

 that at pressures less than one atmosphere the ignition point 

 falls with the pressure. As pressure in so many cases influ- 

 ences explosion, it is remarkable in Freyer and Meyer's experi- 

 ments that detonating gas in an open vessel, when quickly 

 heated, exploded at the same temperature as it did under a 

 pressure of two atmospheres or more. Mitscherlich's results 

 also are the opposite of what we might expect. 



Let us first consider the phenomenon of explosion of deto- 

 nating gas at a low pressure when the combination is not 

 complete. An electric spark or hot wire ignites the gas, the 

 water molecules about the source of heat collide with the 

 neighboring ones of hydrogen and oxygen, molecules of these 

 elements being shaken up by many impacts and acquiring 

 energy adequate for combination, and thus uniting and adding 

 energy to the system. In this way the change propagates 

 itself. A part of the molecules of hydrogen and oxygen have 

 not combined. Why? When a denser gas is ignited, the 

 combination is complete, and as the temperature of burning 

 hydrogen is sufficient to dissociate water, we must assume that 

 some of the water molecules are broken up, and as the tem- 

 perature falls they reform, all of the hydrogen and oxygen 

 uniting before the temperature is too low for combination. In 

 the case of the rarer gas, the impacts to which a molecule of 

 hydrogen or oxygen is subject from the nascent water mole- 

 cules are less frequent than in the denser gas, and hence it is 

 not as likely to acquire the energy needed for chemical union. 

 Moreover, molecules possessing velocities adequate for combi- 

 nation encounter each other less frequently in the rare gas ; 

 hence the chemical change will proceed more slowly, and there 

 will be more time for the system to lose heat by radiation. A 



*Ber. d. deutsch. Chem. Gesell., xxvi, 399. 



