MECHANICS AND USEFUL ARTS. 87 



very intense heat developed, resulting from the chemical transforma- 

 tion ; and that heat, by its expansive effect, contributes far more to 

 the production of the violent explosion than the mere alteration of 

 state resulting from the chemical change. This may readily be ren- 

 dered evident by comparing -the volume of gas which, by theory, 

 would be produced by ignition of the powder, taking into considera- 

 tion the expansive action of the resulting heat, with the compara- 

 tively small volume which the gas would occupy at common tempera- 

 tures. [The difference in volume of the charge of gunpowder and 

 other gases produced under the above circumstances was illustrated 

 by means of cubes.} 



Now, there are several classes of chemical action by which explo- 

 sions may be brought about. First, we are acquainted with a few 

 instances in which explosions result from chemical combination, as 

 in the case of some elementary bodies which possess a tendency to 

 enter into combination with great energy, and consequently with 

 explosive violence. There are numerous instances of combination 

 of a very energetic character, especially between compound bodies, 

 but very few of them, indeed, produce explosive results, simply 

 because the combination proceeds in comparatively a gradual man- 

 ner. As I have said before, it just depends upon the rapidity of 

 action we can establish between bodies upon the intensity of chem- 

 ical affinity exhibited by bodies when they are brought together 

 whether we produce a sufficiently sudden expansion of matter to 

 bring about an explosion. In a case of feeble chemical action, such 

 as that of an acid upon a weak base, heat is developed but slowly 

 and to comparatively a slight extent, because of the very gradual 

 nature of the combination. Thus, if we dissolve this oxide of zinc, 

 which is a weak base, in acid, the heat developed will only gradually 

 melt the very fusible material (paraffin for instance) with which this 

 vessel is coated. This is an example of feeble chemical action. If 

 we proceed a step further, taking, for example, such a substance as 

 this oxide of phosphorus (anhydrous phosphoric acid), which possesses 

 great chemical affinity for water, and we allow it to combine with a 

 small quantity of that liquid, we observe that it will do so much 

 more energetically than the oxide of zinc did with the acid, produc- 

 ing almost an explosive result. We have a proportion of the water 

 suddenly converted into vapor by the heat developed, which is suffi- 

 ciently intense to ignite a highly combustible material, as we find if 

 we allow gun-cotton to come in contact with the oxide of phosphorus 

 which we are combining with water. If two very active elementary 

 bodies, such as bromine on the one hand and potassium (or iron) on 

 the other, are brought together, we find that a still more violently 

 rapid combination takes place. No substance existing naturally in the 

 form of a vapor or gas is produced by their combination ; it is simply 

 here the intense heat, suddenly generated by bringing these two sub- 

 stances together, which suffices to produce a powerfully explosive 

 result, by instantaneously generating a quantity of vapor. And, 

 lastly, if we take a mixture of two gases, such as hydrogen and oxy- 

 gen, or hydrogen and chlorine, and confine them as we have in this 

 glass vessel, we know that the mere momentary contact of some por- 

 tion of the mixture with a body raited to a high temperature, or the 



