360 Prof. Taylor Jones, Mr. Morgan, and Prof. Wheeler on 



between the ionizing powers possessed by various igniting 

 agents (or by various types of electric, sparks), do these 

 differences effect changes in the rationale of ignition of a 

 given series of inflammable mixtures ? 



The object of the present paper is to show that, given 

 gaseous mixtures or series of mixtures of constant com- 

 position, a variation in the energy of the electric spark 

 required to ignite them, dependent on the character of the 

 spark, is to be anticipated on thermal considerations alone. 



The ignition of a gaseous mixture depends primarily on 

 the attainment of a sufficiently rapid rate of reaction within 

 a sufficient mass, or, expressed on a thermal basis, on the 

 heating of a sufficient volume to a sufficient temperature. 



If whilst the requisite volume of a given mixture was 

 being raised to the required temperature it could be isolated, 

 so that no heat could escape to the surrounding gas, then 

 (presuming that no complicating effect were introduced by 

 the mode of imparting the heat) it might reasonably be 

 expected that the quantity of heat required to ignite that 

 mixture would be constant. An unavoidable condition in 

 practice, however, is that whilst that portion of the mixture 

 adjacent to the source of ignition is receiving heat, it is also 

 communicating heat to the surrounding gas. The distri- 

 bution of temperature at any instant throughout the volume 

 of the mixture will vary with the character of. the source, 

 even though the total quantity of heat delivered by each 

 source may be the same. 



A starting-point in an examination of the manner in which 

 thermal energy is conveyed to a gaseous mixture by an 

 electric spark, is given by an inquiry into the manner in 

 which the distribution of temperature varies with time when 

 a source of heat corresponding in general character with a 

 spark is introduced into a gas. Electric sparks can be 

 divided into two main classes : (i.) those of exceedingly short 

 duration (such as single capacity sparks), and (ii.) those of 

 relatively long duration (such as inductance sparks). It will 

 be advantageous, therefore^ to consider the thermal distri- 

 bution in a gas afforded by hypothetical sources of heat of 

 (i.) instantaneous, and (ii.) continued character. In order, 

 also, to obtain some idea of the effect of the volume of an 

 electric spark on its igniting power, or " incendivity," point 

 and spherical sources of heat will be considered. 



The subject is here treated mainly as a problem in thermal 

 conduction in a uniform medium, and, to avoid complication, 

 the medium chosen is air. The numerical results obtained 

 may not represent at all closely the manner in which heat 



