The most remarkable part of the book is the speculative or explanatory part, consisting of an exceedingly ingenious argument, based upon the analogy of admitted facts, to prove that the cause of contagious fevers is some invisible noxious matter in the air. Of the intimate nature of this matter he says: Some consider it to be a sulphurous exhalation from the earth; but this cannot be, for, if so, acrid and sulphurous fumes would increase it, instead of checking or annihilating it. Another theory is that it is due to the products of putrefaction; but how can dead putrid matter ever get such activity as to work such astonishing results? It must therefore be something endowed with a more powerful activity than anything belonging to the mineral kingdom or simply putrefying matter, and must, therefore, be something “actually living.” He further concludes that these living organisms must have an existence independent of the body in which they are found. For this view, surprising and novel enough at first, loses some of its singularity, if we search for resemblances elsewhere. Now, just as it was well known that itch is due to the presence of acari, insects visible by the aid of the microscope, so close attention to these matters in numberless cases during many years, has proved beyond a doubt that the gaol distemper, putrid fever, plague, and infectious epidemics generally, proceed not from matter putrid in itself, but from invisible insects also, that, floating in the air at times, are lodged in the skin in immense quantities; feeding here in clusters, they produce pimples, pustules, etc.; for instance, the eruption of small-pox. He overlooks, or fails to mention, the possibility of their entering by the air passages or digestive system. “Medicines,” he says, “which poison insects without injuring the constitution have always proved specific.” These insects, which constitute contagion, are communicated by air, the raiment, as by contact. He admits that vegetables as well as animals suffer from the ravages of these animalcules. He believed that they originated from eggs and not _de novo_. He advises fumigations with sulphur and frankincense to destroy contagion in rooms, and shows that many diseases in lower animals are cured or prevented by the use of certain agents known to kill insects. In summing up his theory, he says that, generally speaking, there are two sources of these animalcules. First, from subterranean sources, which operate in all sorts of weather and are accompanied by electrical phenomena. Second, from the surface of the earth, swamps, filthy lakes, stagnant ponds, etc. The eggs left on the soil develop in summer, and “the multitudes effluviate into air.”