474 



Prof. H. B. Dixon. On the Movements 



[June 5, 



Part II. — Photographic Analysis of Detonation-waves and their Reflections. 

 [In conjunction with E. H. Strange, B.Sc, and E. Graham, B.Sc] 



By throwing the image of the explosion tube on to a photographic 

 film (Eastman's) fixed to a rapidly revolving wheel, we found that the 

 flame could be sharply photographed, and its movements analysed, 

 without the addition of any metallic salts to the tube. 



The first point noticed in the photographs were (1) the sharpness 

 with which the luminosity is set up ; and (2) the uniformity of the 

 detonation-wave. There is no evidence of any gradual heating up of 

 the gases, but, on the contrary, the temperature appears to spring to 

 its maximum with abrupt suddenness. The gas ignited by the 

 detonation (including particles knocked off the tubes) remains luminous 

 for some time after the wave has passed. 



Many of the photographs show very distinctly the movements of 

 the gas en masse, as it follows up the detonation- wave, comes to rest, 

 and swings back again. 



When the detonation-wave hits the closed end of the tube it is 

 reflected back in a distinctly marked luminous wave, remarkable for 

 its great luminosity. As this reflected wave starts back from the 

 closed end it has at first to meet the gas moving bodily forward in the 

 wake of the detonation-wave. As it continues backwards the gas it 

 meets has less forward motion, then becomes stationary, and finally 

 travels back in the same direction as the reflected wave. It follows, 

 therefore, that the velocity of the reflected wave is at first retarded 

 and afterwards increased by the motion of the medium. 



The reflected wave produced by the collision of a detonation- wave 

 with the closed end of the tube is mainly an intense compression- wave. 

 The velocity of the reflection-wave may be readily compared with that 

 of the detonation-wave. In the following table the average velocities 

 observed in several gaseous mixtures are given, the velocity of the 

 reflected wave being taken as nearly as possible at the point where 

 the movement of the gas itself was nil. 



Although the formula for the velocity of sound in gases is strictly 

 valid for small displacements only, nevertheless it appeared of interest 

 to calculate from the observed velocities of these reflection- waves 

 what temperature they indicated in the gas, on the assumption that 

 they were propagated as sound-waves. Of course to calculate the 

 temperature from the velocity of sound it is necessary to know the 

 ratio of the specific heats y, and since in the case of carbonic acid and 

 steam this ratio is very doubtful, a corresponding uncertainty must 

 exist in the temperature calculated. But in the case of cyanogen 

 burning to carbonic oxide, the products of combustion, carbonic oxide, 

 and nitrogen are similar to air, and their specific heats either do not 

 alter, or do not alter greatly, with rise of temperature. The velocity 



