MOLECULAR VELOCITY OF REACTIONS. 531 



7. This is a new datum in the problem, and one of the most 

 important, since it determines the real effects of explosive sub- 

 stances in their various applications, such as the velocity com- 

 municated to projectiles in fire-arms, the division and the 

 projection of fragments of bombshells, and, in fine, the various 

 results developed in blasting at the expense either of the rocks 

 required to be dislocated or removed, or of any obstacles which 

 it is proposed to crush or overturn. 



8. The origin of explosive reactions, that is to say, of the 

 preliminary work which determines their beginning, appears to 

 correspond in all cases to an initial heating, which raises the 

 substance to its decomposing temperature, and from which re- 

 action propagates itself. In order for this heating to be 

 efficacious, the heat developed by the decomposition must attain 

 a sufficient intensity to raise gradually, and up to the same 

 degree, the temperature of the adjacent portions ; it is necessary, 

 also, that the heat should not become dissipated meanwhile by 

 radiation, by conduction, or by the expansion of the compressed 

 gases. In other words, the molecular velocity of the reaction 

 in the system regarded as homogeneous, and raised to a uniform 

 temperature throughout, must be sufficiently great, otherwise 

 there would be no explosion. This is noticeable when decom- 

 posing cyanogen by means of the electric spark, or when 

 changing acetylene into benzene by heating. The heat liberated 

 by this last reaction is enormous, and for equal weights is four 

 times that of the explosion of gunpowder, but it is so slowly 

 disengaged that dissipation takes place gradually. 



9. The molecular velocity of a reaction is therefore a main 

 element in the question. Let us summarise the laws which 

 characterise it. 



It increases with the temperature according to a very rapid 

 law. 



It increases also with the condensation of the substance, that 

 is to say, with the pressure in the gaseous systems. 



On the other hand, its action is retarded by the presence of 

 an inert body which lowers the temperature at the same time 

 as it lessens condensation. In this way we can at will modify 

 the character of an explosive substance. For instance, black 

 powder, mixed with sand, will fuse instead of detonating ; 

 dynamite, which is a mixture of silica and nitroglycerin, is less 

 shattering than nitroglycerin ; besides, the shattering character 

 due to the nitroglycerin decreases rapidly in proportion as the 

 quantity of silica is increased. 



10. The velocity of the propagation of reactions developed 

 in consequence of ignition or of a local shock, represents a 

 phenomenon totally distinct from the molecular velocity which 

 we have just defined ; for it expresses the requisite time for the 

 physical conditions of temperature, etc., which have caused the 



2 M 2 



