_A._ It is manifest; and the cause can be no other but that, except the bow have sense. _B._ And though the bow had sense, and appetite to boot, the cause will be still the same. _A._ Do you think air and water to be pure and homogeneous bodies? _B._ Yes, and many bodies both hard and heavy to be so too, and many liquors also besides water. _A._ Why then do men say they find one air healthy, another infectious? _B._ Not because the nature of the air varies, but because there are in the air, drawn, or rather, beaten up by the sun, many little bodies, whereof some have such motion as is healthful, others such as is hurtful to the life of man. For the sun, as you see in the generation of plants, can fetch up earth as well as water: and from the driest ground any kind of body that lieth loose, so it be small enough, rather than admit any emptiness. By some of these small bodies it is that we live; which being taken in with our breath, pass into our blood, and cause it, by their compounded motion, to circulate through the veins and arteries; which the blood of itself, being a heavy body, without it cannot do. What kind of substance these atoms are, I cannot tell. Some suppose them to be nitre. As for those infectious creatures in the air, whereof so many die in the plague, I have heard that Monsieur Des Cartes, a very ingenious man, was of opinion, that they were little flies. But what grounds he had for it, I know not, though there be many experiments that invite me to believe it. For first, we know that the air is never universally infected over a whole country, but only in or near to some populous town. And therefore the cause must also be partly ascribed to the multitude thronged together, and constrained to carry their excrements into the fields round about and near to their habitation, which in time fermenting breed worms, which commonly in a month or little more, naturally become flies; and though engendered at one town, may fly to another. Secondly, in the beginning of a plague, those that dwell in the suburbs, that is to say, nearest to this corruption, are the poorest of the people, that are nourished for the most part with the roots and herbs which grow in that corrupted dirt; so that the same filth makes both the blood of poor people, and the substance of the fly. And it is said by Aristotle, that everything is nourished by the matter whereof it is generated. Thirdly, when a town is infected, the gentlemen, and those that live on wholsomest food, scarce one of five hundred die of the plague. It seems therefore, whatsoever creatures they be that invade us from the air, they can discern their proper nourishment, and do not enter into the mouth and nostrils with the breath of every man alike, as they would do if they were inanimate. Fourthly, a man may carry the infection with him a great way into the country in his clothes, and infect a village. Shall another man there draw the infection from the