9. It is known by experience that if hay be laid wet together in a heap, it will after a time begin to smoke, and then burn as it were of itself. The cause whereof seems to be this, that in the air, which is enclosed within the hay, there are those little bodies, which, as I have supposed, are moved freely with simple motion. But this motion being by degrees hindered more and more by the descending moisture, which at the last fills and stops all the passages, the thinner parts of the air ascend by penetrating the water; and those hard little bodies, being so thrust together that they touch and press one another, acquire stronger motion; till at last by the increased strength of this motion the watery parts are first driven outwards, from whence appears vapour; and by the continued increase of this motion, the smallest particles of the dried hay are forced out, and recovering their natural simple motion, they grow hot and shine, that is to say, they are set on fire.
The same also may be the cause of lightning, which happens in the hottest time of the year, when the water is raised up in greatest quantity and carried highest. For after the first clouds are raised, others after others follow them; and being congealed above, they happen, whilst some of them ascend and others descend, to fall one upon another in such manner, as that in some places all their parts are joined together, in others they leave hollow spaces between them; and into these spaces, the ethereal parts being forced out by the compressure of the clouds, many of the harder little bodies are so pent together, as they have not the liberty of such motion as is natural to the air. Wherefore their endeavour grows more vehement, till at last they force their way through the clouds, sometimes in one place, sometimes in another; and, breaking through with great noise, they move the air violently, and striking our eyes, generate light, that is to say, they shine. And this shining is that we call lightning.
The cause of the force of gunpowder; and what is to be ascribed to the coals, what to the brimstone, and what to the nitre.
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.
How heat is caused by attrition.
11. Besides, by the attrition or rubbing of one body against another, as of wood against wood, we find that not only a certain degree of heat, but fire itself is sometimes generated. For such motion is the reciprocation of pressure, sometimes one way, sometimes the other; and by this reciprocation whatsoever is fluid in both the pieces of wood is forced hither and thither; and consequently, to an endeavour of getting out; and at last by breaking out makes fire.
The distinction of light into first, second, &c.
12. Now light is distinguished into, first, second, third, and so on infinitely. And we call that first light, which is in the first lucid body; as the sun, fire, &c.: second, that which is in such bodies, as being not transparent are illuminated by the sun; as the moon, a wall, &c.: and third, that which is in bodies not transparent, but illuminated by second light, &c.