24 Dr. Karl Heumann's Contributions to 



be eliminated and which may be looked on as immaterial. 

 But even without this, parallel experiments might lead to the 

 discovery of interesting relations existing between the veloci- 

 ties of ignition and the combustion-temperatures of different 

 combustible bodies. 



For solid bodies (magnesium for instance) the velocity of 

 propagation of ignition is equal to the velocity with which a 

 wire of the substance must be moved forwards in order that 

 the position of the flame may remain constant. The time re- 

 quired for the flame to travel to the end of a wire of known 

 length might also be determined. 



Easily combustible liquids might be placed in a hollow, and 

 the time which expired between the ignition of one end of the 

 liquid and the arrival of the flame at the other end noted. 

 Liquids which burn only when absorbed by wick, might be so 

 absorbed by wicks of known length, and the time required for 

 the flame to travel throughout the length of the wick placed 

 horizontally might be determined. 



By the aid of such experiments a comparative quantitative 

 expression for the liability to ignition of various combustibles 

 might be gained. 



For gases, the velocity with which the gas must issue in 

 order to maintain a constant distance between burner and 

 flame might be determined ; or the distance might be measured, 

 the velocity of issue being maintained constant. In order to 

 do away with the changing velocity of different gases for the 

 same distance from the burner (depending on the nature of 

 the gas itself), it would be better to measure that velocity 

 which is just sufficient to remove the flame from the burner. 



I am here reminded of Bunsen's method for determining 

 the velocity of ignition in the mixed gases evolved in the 

 electrolysis of water *.. The explosive mixture was burned at 

 a small orifice of known area, the velocity with which the 

 gas issued being gradually diminished by reducing the pres- 

 sure until the flame passed backwards through the opening 

 and ignited the mass of the gaseous mixture. 



This point must be reached when the velocity of the issuing 

 gas is an infinitely little less than that with which the ignition 

 is propagated forwards. 



Bunsen calculated the velocity of propagation of ignition, C, 



4V 



from the formula C = — ^- where V denotes the volume of 



gas issuing in t seconds, and d the diameter of the opening. 



In the case of the mixed gases from the electrolysis of water 



C was found to be equal to 34 millims. per second, while for 



* Pogg. Ann. cxxxi. 165. 



