PROFESSOR H. B. DIXON ON THE RATE OF EXPLOSION IN GASES. 163 
velocities greater than those calculated for sound in the undecomposed gas. But this 
is exactly the reverse of what is observed. The formula, which gives the correct 
velocity for explosions of cyanogen, gives numbers which are rather too high for 
acetylene (also a strongly endothermic compound), and too low for ethylene and for 
marsh gas (an exothermic compound). It would appear as if ethylene and marsh gas 
w^ere unstable bodies, and acetylene and cyanogen were stable bodies, in the particular 
condition of the explosion-wave. Let me recall the words used by Professor Ostwald 
in his address last year at Halle, to the sections of Physics and Chemistry, when 
speaking on a cognate subject—the spectra of gases at high temperatures :— 
“ It is held as an undoubted dogma that at the highest temperatures, as, for 
example, in the electric arc, all compounds must be dissociated into their elements. 
This view is certainly not justified. What we do know about the stability of com¬ 
pounds is, on the contrary, that all compounds which are formed with absorption of 
heat become more stable with rising temperature, and the reverse. Because the 
majority of the compounds known to us are formed from the elements with evolution 
of heat, and correspondingly become more unstable with rising temperature, the 
conclusion has been drawn that this is in general the case. But if we reflect that 
cyanogen and acetylene, two compounds formed with great absorption of energy, are 
readily formed in quantity, at the highest temperatures, in the blast furnace and in 
the arc light, we become conscious that spectra occurring at high temperatures may 
belong to compounds which have a fleeting existence confined to those temperatures 
only.”'^^ 
We know, then, that cyanogen and acetylene are formed from their elements at the 
highest temperature ; but Berthelot has shown that these gases are decomposed by 
the shock of a fulminate detonator.! Is it possible that there is an essential difference 
between the two acts—that molecules of acetylene, charged by highly heated mole¬ 
cules, do not break up, but that they are decomposed when they are dashed against 
cold molecules, or the cold walls of the containing vessel ? I have found that the 
decomposition of acetylene by a fulminate is confined to the neighbourhood of the 
detonator. When a charge of fulminate is fired in a steel bomb filled with acetylene, 
the decomposition of the acetylene is not propagated along a tube attached to the 
bomb and filled with the gas. The flame of the detonator is “ carried on” for some 
little distance, but no explosion-ivave is set up in the acetylene. Similar results were 
found with carbon bisulphide vapour, though in this case the decomposition is propa¬ 
gated to a greater distance from the detonator. These experiments show that “ the 
decomposition by shock of acetylene and carbon bisulphide is not propagated like the 
explosion-wave at a constant velocity as far as the gas extends, but the decomposition 
set up by the fulminate dies out at a distance from the detonator, depending on the 
* See ‘Nature,’ April 21, 1892. 
t ‘La Force des Matieres Explosives,’ vol. 1, p. 109. See also Thorpe, ‘ Journ. Cliem. Soc.,’ 1889, 
p. 220. 
Y 2 
