EVENING DISCOURSES. 
THE PHOTOGRAPHIC ANALYSIS OF 
EXPLOSION FLAMES. 
BY 
PROF. WILLIAM A. BONH, F.R.S. 
In principle the method usually employed for the photographic analysis of explosion 
flames is the same as that originally designed by Mallard and Le Chatelier fifty years 
ago. It consists in photographing the movements of the flame along a horizontal 
glass tube on a sensitised plate or film moving vertically at a suitable known velocity, 
thus obtaining (inter alia) a graph compounded of the two velocities, from which 
that of the flame at any point can be deduced. 
Mallard and Le Chatelier employed horizontal tubes of diameters between 
1 and 3 centimetres in sections, each 1 metre long, connected in series by means of 
caoutchouc rings. The whole was focussed, by means of a wide aperture lens, on a 
plate moving vertically with a known uniform velocity of about 1 metre per second. 
In this way, to quote their original memoir,! ‘On obtiendra wne courbe dont chaque 
point aura pour abscissa le chemin parcouru par la flamme dans la tube, et pour ordonné 
le temps écouté depuis Vorigine de la combustion.’ 
As the plates used by Mallard and Le Chatelier were not sufficiently sensitive to 
give satisfactory records with feebly luminous flames, such as those of hydrogen- 
oxygen explosions, they employed explosive mixtures of carbon disulphide with 
either oxygen or nitric oxide, whose flames are much more actinic, believing 
them to be typical of all explosive ‘ oxygen’ or ‘air’ mixtures respectively. The 
behaviour of these mixtures on explosion was found to differ according as they were 
ignited at or near (a) the open, or (b) the closed end of a tube. In the former case 
the flame always proceeded for a certain distance along the tube at a practically 
uniform slow velocity, which was regarded as the true rate of propagation * by 
conduction.’ This initial ‘uniform movement’ was usually succeeded by an 
“ oscillatory period,’ the flame swinging backwards and forwards with increasing ampli- 
tude, and finally either dying out altogether or giving rise to ‘ detonation,’ according 
to circumstances. With some ‘oxygen ’-mixtures, the initial period of uniform 
velocity was short, and appeared to be succeeded abruptly by ‘ detonation,’ without 
passing through any intermediate oscillatory period. When, however, the mixtures 
were ignited near the closed end of the tube, the forward movement of the flame was 
continuously accelerated until finally ‘detonation’ was set up. In ‘ detonation,’ 
where the explosion is propagated from layer to layer by ‘ adiabatic compression,’ 
the flame velocities are both uniform and high, e.g. usually of the order 2,000 to 3,000 
metres per second. 
These features of the flame movement will be illustrated by (i) a series of three 
slides reproducing some of Mallard and Le Chatelier’s original photographs, and 
(ii) experiments showing the movements of explosion-flames through methane-air 
mixtures along a horizontal glass tube about 6 metres long and 6 centimetres internal 
diameter. 
The experimental method was developed and improved by the late Prof. H. B. 
Dixon and his collaborators in Manchester during the ‘nineties’ of last century. 
They used a highly sensitive film rotating vertically on the periphery of a drum with 
1 Annales des Mines 8 Ser. iv. (1883), pp. 274-618. 
