Explosive Gases and Gaseous Mixtures. 329 



They also found that detonating gas suddenly heated in un- 

 sealed bulbs exploded at the same temperature as in sealed. 



Before discussing chemical changes let us recall the condi- 

 tion of matter in gases — the molecules are moving very rapidly, 

 some faster, some slower, an individual molecule moving in a 

 straight line for a very short distance, until it strikes another. 

 These collisions of one molecule with others are very frequent. 

 The atoms composing a molecule are also moving in respect to 

 each Other, this motion constituting a portion of the internal 

 energy of the molecules, which is continually diminishing 

 through radiation. When the gas is heated the mean velocity 

 of the molecules is increased, and at the same time the atomic 

 motion and internal energy are also increased. At a sufficiently 

 high temperature some of the collisions are so violent as to dis- 

 rupt the molecules and their atoms part company. 



When a mixture of gases is heated, chemical combination 

 results from the encounters of different kinds of molecules 

 having motion of translation, or of the atoms, or of both. At 

 any given temperature the mean velocity of molecules may be 

 calculated, but we have no data for determining at what 

 greater velocity the molecular collisions are followed by chem- 

 ical union. Polyatomic molecules are dissociated by heat, but 

 since there is no measurable dissociation of hydrogen or oxygen 

 at 1600° it does not appear that dissociation by heat plays 

 much part in the change. Whatever the chemical changes 

 caused by heat in gases, whether combination or dissociation, 

 the condition necessary for these changes results from mole- 

 cular impacts. If, for example, detonating gas be heated, a 

 part of the molecules acquire a velocity and internal energy 

 adequate for combination. Some of these encounter each 

 other and combine to form water, the free atoms of oxygen 

 uniting with each other or with hydrogen. The nascent water 

 molecules have in general a high velocity, and collide with 

 those of oxygen and hydrogen ; but it is not probable that a 

 single impact of a new water molecule with one of hydrogen 

 or oxygen imparts to the latter energy adequate for combina- 

 tion. If it were so, the change would proceed with accelerat- 

 ing speed at a temperature of 300° to 500°. This view, that a 

 nascent water molecule does not necessarily cause hydrogen 

 and oxygen to unite, accords with that expressed by the writer,* 

 that the heat of combination caused by the glow discharge of 

 electricity does not produce further combination. 



Why is it that chemical union proceeds slowly at 500°, and 

 why does not the energy resulting from combination cause 

 further combination, and thus raise the temperature of the sys- 



* This Journal, vi, 218, 1898. 



