Is not that the salt with which gunpowder is made? MRS. B. Yes. Gunpowder is a mixture of five parts of nitre to one of sulphur, and one of charcoal. --Nitre from its great proportion of oxygen, and from the facility with which it yields it, is the basis of most detonating compositions. EMILY. But what is the cause of the violent detonation of gunpowder when set fire to? MRS. B. Detonation may proceed from two causes; the sudden formation or destruction of an elastic fluid. In the first case, when either a solid or liquid is instantaneously converted into an elastic fluid, the prodigious and sudden expansion of the body strikes the air with great violence, and this concussion produces the sound called detonation. CAROLINE. That I comprehend very well; but how can a similar effect be produced by the destruction of a gas? MRS. B. A gas can be destroyed only by condensing it to a liquid or solid state; when this takes place suddenly, the gas, in assuming a new and more compact form, produces a vacuum, into which the surrounding air rushes with great impetuosity; and it is by that rapid and violent motion that the sound is produced. In all detonations, therefore, gases are either suddenly formed, or destroyed. In that of gunpowder, can you tell me which of these two circumstances takes place? EMILY. As gunpowder is a solid, it must, of course, produce the gases in its detonation; but how, I cannot tell. MRS. B. The constituents of gunpowder, when heated to a certain degree, enter into a number of new combinations, and are instantaneously converted into a variety of gases, the sudden expansion of which gives rise to the detonation. CAROLINE. And in what instance does the destruction or condensation of gases produce detonation? MRS. B. I can give you one with which you are well acquainted; the sudden combination of the oxygen and hydrogen gases. CAROLINE. True; I recollect perfectly that hydrogen detonates with oxygen when the two gases are converted into water. MRS. B. But let us return to the nitrat of potash. --This salt is decomposed when exposed to heat, and mixed with any combustible body, such as carbon, sulphur, or metals, these substances oxydating rapidly at the expense of the nitrat. I must show you an instance of this. --I expose to the fire some of the salt in a small iron ladle, and, when it is sufficiently heated, add to it some powdered charcoal; this will attract the oxygen from the salt, and be converted into carbonic acid.-- EMILY. But what occasions that crackling noise, and those vivid flashes that accompany it? MRS. B. The rapidity with which the carbonic acid gas is formed occasions a succession of small detonations, which, together with the emission of flame, is called _deflagration_. _Nitrat of ammonia_ we have already noticed, on account of the gaseous oxyd of nitrogen which is obtained from it.