EXPLOSIVES 375 



or mixture of gases. And as such changes are almost always 

 attended by the production of much heat, the hot gases formed 

 are still further expanded. For the moment the last case must 

 be postponed from consideration, but the reader will easily 

 understand what is referred to by thinking of the disastrous 

 effect in a coal-pit when a mixture of air with inflammable gas 

 from the coal, called fire-damp, comes into contact with a flame. 

 The resulting explosion which, under such circumstances, does 

 nothing but mischief, can in another form be turned to useful 

 practical account when under control in the gas-engine or 

 internal combustion engine of the motor. 



But although explosive substances are familiar in the chemical 

 laboratory, and have multiplied among the products of modern 

 chemical research, it is curious to note that nearly all the ex- 

 plosives employed as propellants or for blasting purposes are 

 produced more or less directly by the use of the " villainous 

 saltpetre " so long an ingredient in old-fashioned black gun- 

 powder. The object in all cases is to introduce into a mixture 

 or compound containing the combustible elements, carbon and 

 hydrogen, so large a quantity of oxygen that the product will 

 burn without the assistance of atmospheric air. 



This is effected in the case of gunpowder through the agency 

 of the nitre or saltpetre which supplies oxygen to the sulphur 

 and charcoal with which it is mixed. Or it may be by bringing 

 cotton or glycerine or phenol or some other compound of this 

 kind into contact with nitric acid. An interchange is then 

 effected whereby a portion of the hydrogen of the original 

 substance is removed in the form of water and the group of atoms, 

 NO 2 , characteristic of the nitrates is introduced. When the 

 nitrated compound is fired the oxygen combines with carbon 

 forming gaseous oxides of carbon, and with the hydrogen form- 

 ing water, which is of course liberated in the form of steam, 

 while the nitrogen is set free in the state of gas and thus con- 

 tributes to the total volume of gas formed in the act of explosion. 



This chapter must be devoted to an account of the chemical 

 composition and action of the modern explosives, some of them 

 of quite recent introduction, but to understand why some of the 

 changes which have taken place of late years have been intro- 

 duced, it is necessary in passing to glance at the changes which 

 have taken place in the construction of military and naval guns. 



At the time of the Crimean War the largest guns ashore or 



