642 
MR. H. B. DIXON ON CONDITIONS OF CHEMICAL CHANGE 
The ignition of a mixture of hydrogen and oxygen and of other explosive mixtures by 
an electric spark, may be stopped either by sufficient rarefaction or sufficient dilution 
with a neutral gas such as carbonic acid or nitrogen. Probably, in the first case, the 
mean distance of the molecules is increased to such an extent that the necessary 
molecular disturbance cannot be communicated from molecule to molecule, and the 
combustion is confined to the space between the platinum wires; in the second case, 
the neutral gas both increases the mean distance of the reacting molecules, and also 
decreases the temperature by absorption of heat. In the reaction under consideration 
—viz., in a mixture of two volumes of carbonic oxide and one of oxygen—although a 
single molecule of water should suffice theoretically for the oxidation of any number of 
carbonic oxide molecules, it is evident that the explosion cannot be propagated unless 
a certain minimum number of steam molecules are present. For when the mean 
distance between the water molecules reaches a certain magnitude, each molecule of 
water becomes surrounded by a crowd of carbonic acid molecules, the product of its 
action on the carbonic oxide and oxygen under the influence of the electric spark ; and 
the incipient combustion dies out, because fresh molecules of carbonic oxide and 
oxygen do not come in contact with it while it has still enough kinetic energy to react 
with them. The slow rate of propagation of explosion in a nearly dry mixture is also 
readily explained. The carbonic oxide molecules have to wait their turn. Though 
probably an enormous number of steam molecules are present in the tube, yet since 
they are comparatively very few, each one must do duty a vast number of times. 
Each steam molecule must in turn present an atom of oxygen to as many molecules 
of carbonic oxide as the number of times the volume of carbonic oxide exceeds the 
volume of the steam. Since each reduction and each oxidation of the oxygen-carrier 
takes time, a given number of carbonic oxide molecules takes longer to burn in presence 
of a comparatively small than in presence of a comparatively large number of steam 
molecules. 
Explosion of dry carbonic oxide and electrolytic gas. 
Having determined the action of aqueous vapour on carbonic oxide at high tem¬ 
peratures, I began in Nov., 1876, a second series of experiments with dry carbonic 
oxide and electrolytic gas in a well dried eudiometer. After each explosion the 
residual gas was analysed. Only those experiments were accepted in which the 
residual analysis confirmed the first part of the operation. The gases were exploded 
under pressures varying between 200 millims. and 300 millims., and at temperatures 
between 6 C and 11°. The following table gives the composition of the mixtures and 
the ratio between the carbonic oxide and hydrogen burnt in the several explosions. 
