10. The most common phenomenon proceeding from fire, and yet the most admirable of all others, is the force of gunpowder fired; which being compounded of nitre, brimstone and coals, beaten small, hath from the coals its first taking fire; from the brimstone its nourishment and flame, that is to say, light and motion, and from the nitre the vehemence of both. Now if a piece of nitre, before it is beaten, be laid upon a burning coal, first it melts, and, like water, quencheth that part of the coal it toucheth. Then vapour or air, flying out where the coal and nitre join, bloweth the coal with great swiftness and vehemence on all sides. And from hence it comes to pass, that by two contrary motions, the one, of the particles which go out of the burning coal, the other, of those of the ethereal and watery substance of the nitre, is generated that vehement motion and inflammation. And, lastly, when there is no more action from the nitre, that is to say, when the volatile parts of the nitre are flown out, there is found about the sides a certain white substance, which being thrown again into the fire, will grow red-hot again, but will not be dissipated, at least unless the fire be augmented. If now a possible cause of this be found out, the same will also be a possible cause why a grain of gunpowder set on fire doth expand itself with such vehement motion, and shine. And it may be caused in this manner. Let the particles, of which nitre consisteth, be supposed to be some of them hard, others watery, and the rest ethereal. Also let the hard particles be supposed to be spherically hollow, like small bubbles, so that many of them growing together may constitute a body, whose little caverns are filled with a substance which is either watery, or ethereal, or both. As soon, therefore, as the hard particles are dissipated, the watery and ethereal particles will necessarily fly out; and as they fly, of necessity blow strongly the burning coals and brimstone which are mingled together; whereupon there will follow a great expansion of light, with vehement flame, and a violent dissipation of the particles of the nitre, the brimstone and the coals. Wherefore I have given a possible cause of the force of fired gunpowder. It is manifest from hence, that for the rendering of the cause why a bullet of lead or iron, shot from a piece of ordnance, flies with so great velocity, there is no necessity to introduce such rarefaction, as, by the common definition of it, makes the same matter to have sometimes more, sometimes less quantity; which is inconceivable. For every thing is said to be greater or less, as it hath more or less quantity. The violence with which a bullet is thrust out of a gun, proceeds from the swiftness of the small particles of the fired powder; at least it may proceed from that cause without the supposition of any empty space. [Sidenote: How heat is caused by attrition.]