110 THE EXPLOSIVE WAVE. 



which it develops, helps the gaseous column to take up the 

 detonation at once, which result it would attain later with less 

 regularity by the ordinary inflammation. 



4. Pressures. These are deduced from the path traced by the 

 piston. 



For explosive gas (H 2 + 0), the piston, placed at 2 cms. from 

 the point of ignition, is projected forward at first by a pressure 

 of 500 to 600 grms. per sq. cm., but this pressure falls very 

 quickly, until it becomes nil and even negative (on account of 

 the condensation of the aqueous vapour) at the end of '0005 of 

 a second. 



At -5 of a metre from the beginning, a pressure of 1-2 kgms. 

 was found. 



At 5*25 metres from the point of inflammation the first dis- 

 placement of the piston took place under a pressure of about 

 5 kgms. per sq. cm. ; and this pressure at the end of '00125 of 

 a second, was still more than 3 kgms. 



Now, at this moment, the inflammation progressed 2 '7 metres 

 in a similar tube, according to the velocities mentioned above. 



It will be seen, then, that in this part of the tube a con- 

 siderable gaseous column, formed of aqueous vapour, is main- 

 tained at a high pressure, whereas at the beginning the pressure 

 produced in one section by the combustion of the mixture is 

 almost instantaneously annulled by the condensation of the 

 sections in front of it; otherwise, the increase in pressures 

 corresponds to the increase in velocity. It was found that the 

 maximum of pressure developed by the mixture H 2 + 0, burn- 

 ing in a closed vessel, is about 7 kgms. In this case the cooling 

 influence of the sides of the vessel may be disregarded. In 

 abnormal cases, when the rate of propagation falls below 2000 

 metres the pressure falls at the same time, which shows plainly 

 the correlation of the two kinds of phenomena. 



5. Limits of Detonation. It is possibly due to similar causes 

 that certain explosions of firedamp attain an exceptional rate 

 of propagation and unusual violence. When the explosive wave 

 is not propagated, combustion may still take place to a certain 

 extent. 



The limit of detonation in oxyhydrogen compounds is at 

 about 22 per cent, of hydrogen, whereas the ordinary limit of 

 combustion in mixtures of hydrogen and oxygen is at about 6 

 per cent, of hydrogen. 



As the lower limit of detonation is approached the velocity 

 of the wave falls considerably below the theoretical velocity 

 (see above). The mixtures of cyanogen and nitric oxide such 

 as CN -f 2 NO show some points of interest. This compound, 

 contained in an eudiometer, is exploded violently by a powerful 

 spark. When ignited with a match it burns progressively. 

 But, on the other hand, we did not succeed in propagating the 



