LIMITS OF EXPLOSIONS. 393 



than ordinary sparks. All this can be easily understood, since 

 the igniting agent propagates the combustion around itself in a 

 sphere which is more or less extended, according to the quantity 

 of heat it supplies itself. 1 



Hence variations and strange phenomena result in a mixture 

 limit, the mixture becoming filled with small disseminated 

 flames, which are propagated hither and thither, and whose 

 production precedes the state of general combustion. These 

 curious effects have been the object of special study by Schlce- 

 sing and Demondesir. 



The singular phenomena witnessed in the Solfatara at Pozzuoli 

 could also be cited. Towards certain points, especially in a 

 depression, vaporous wreaths are liberated, irregular jets of 

 water vapour mixed with a trace of sulphuretted ^hydrogen. 

 It suffices to bring near them some ignited body, such as 

 tinder, when the sulphuretted hydrogen burns in contact with 

 the air in which it is disseminated, with the production of a 

 cloud which gradually extends and propagates itself all round 

 to a considerable distance. 2 



The easy ignition of sulphur and its compounds is a great 

 factor in this circumstance, but has nothing to do with explo- 

 sive phenomena. 



From the point of view of mechanical effects produced by 

 a detonating mixture, the rapidity with which the ignition is 

 propagated is very essential, the latter taking place sometimes 

 by ordinary combustion, and sometimes owing to a real ex- 

 plosive wave, which proceeds with incomparably greater rapidity 

 (pp. 49, 55, 88, 90). Now, the limits of composition at which 

 the explosive wave ceases to be produced are far higher than 

 those which correspond to simple ignition. This is a very 

 important result as regards applications (see p. 110). The limit 

 of inflammability, and especially the more or less easy propaga- 

 tion of the inflammation, is influenced by the pressure, which 

 increases the mass of heated matter in a given time and extent, 

 and consequently checks the influence of cooling. 



The limit is also influenced by the initial temperature of the 

 mixture. That is, the excess of temperature of the body which 

 produces ignition above that of the inflammable mixture ought 

 to be less according as the latter mixture is raised beforehand to 

 a higher temperature (see p. 64). 



Generally speaking, in order that the propagation of the com- 

 bustion may take place, it is necessary that the heat liberated 

 by the ignition of the first parts should be sufficient to repro- 

 duce in the adjoining portions the initial temperature at which 

 the combustion commenced. 



1 " Essai de M^canique Chimique," torn. ii. pp. 338, 343 et 346. 



2 Melloni et Piria, " Annales de Chimie et de Physique," 2" se'rie, torn. 

 Ixxiv. p. 331. 



