many of my hearers yet it is true that there is not, and it may safely be said that there never will be, an explosive which can give out nearly as much energy as an equal weight of coal or petroleum. Nitroglycerine stands almost first of our most violent explosives, and yet the power it can generate is less than one sixth of that given by the combustion of an equal weight of good coal. The difference in the effects in the two cases is due to the fact that the coal needs the oxygen of the air for its combustion. It can therefore burn only when the air has access to it. Hence combustion can take place only at its surface and the power developed is dissipated in streams of hot gases which must pass off as fast as they are formed, so as to allow fresh supplies of air to come forward. To put it into a closed chamber is to stop its burning. In an explosive this is not so. This is not because the combustible is different—that which burns in nitroglycerine is the same as that which burns in coal. But the oxygen necessary to burn it is to be found in the explosive itself, it has not
