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. [Sidenote: 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.]