BUT the Case of a mad Dog entirely comes up to that of a _Contagion_ from a Fever. When the hotter Seasons of the Year throw that Creature into Madness, it is manifestly from a great Increase of Velocity and Motion in the Fluids, which brings on what is equivalent to a _Delirium_, by an additional _Impetus_ upon the Brain, and for want of so much Room through the Skin for Transpiration, as in other Animals; the chief Evacuation is by the Glands of the Mouth: That is, in short, the Dog hath a Fever, which breaks the natural Texture of the Juices, disengages and subtilizes the more rigid or saline Parts, and critically discharges them by the most convenient Outlets the Creature is furnished with, changed into such a poisonous Nature, that wheresoever they come to mix with the Juices of others, they excite in them the like inordinate and mischievous Alterations. THIS Theory likewise might be further illustrated by many Instances from inanimated Fluids, which are capable of being put into Fermentation by a very small Portion of Matter, and which shall by such an Agitation from new Particles, or _Moleculæ_ having Properties of communicating the same Effects to another quiescent Fluid; not much unlike what we see in the Communication and Propagation of Fire, which is excited and carried on in proper Subjects from the minutest Beginnings, and increasing also in its own Force as it spreads. WHEN a Person therefore falls into a Fever from any epidemic, or other more private Cause, and that Fever rises to such a Degree of Malignity as is always supposed in a _Pestilence_, as far as any _Effluvia_ do exhale from that Person, so far may he be said to have round him a contagious and poisonous _Atmosphere_; because there transpires from him such Particles as will excite in other Animals of like Constitution, the same fermentative Motions as those to which they owe their own Origin. ALTHOUGH therefore, in the Beginning of epidemical Diseases, and during the Subsistence of their common Causes, particular Regard is to be had to these Causes, and the Manner whereby they affect the People; yet when it is come to this pass, that the Fever it self is productive of a Poison, or somewhat intirely disagreeable, that communicates the same Impressions upon others, without any Concurrence of the first Cause, then such a Fever is truly a Pestilence by Contagion, and all precautionary Regards ought peculiarly to be suited, to prevent its Infection or Spreading; either by keeping the well Subjects clear from the sick, or destroying the Influence of the poisonous Exhalations, or fortifying the sound against it. But to these Ends, it is necessary to have some tolerable Notion of the Manner how these secret Destroyers are continued, and conveyed to great Distances.