The Causes therefore of these Diseases must either be generated in the Air, or produced from the Effluvia of Animal, Vegetable, or Mineral Substances floating in it. And consequently the Effects of the Contagious Particles must be extreamly various, according to the Qualities of the Bodies from which they are produced. When any of these Causes is of so deleterious a Nature, as not only to be Infectious, but to destroy all or most of those that are affected by it, That Disease is called a Pestilence. But before I proceed to examine the particular Properties and Effects of the Contagious Particles, it will be Necessary to Demonstrate the following Propositions. PROP. I. The Magnitude of the Particles of the Blood being increased, Obstructions will be formed in the Ramifications of the smaller Vessels, which will happen sooner or later, in Proportion to the increased Magnitude of the Particles, and the smallness of the Vessels. _Demonstration._ [Illustration] Let the Canal A be an Artery of a middle Size, sending out the Branches C, D, E, F, G, H; Let the Dotts represent the increased _Moleculæ_ of the Blood, it is evident that these must be stopt some where or other in the Ramifications of the Vessels C, D, E, F, G, H, whenever the Diameters of the _Moleculæ_ exceed those of the containing Vessels. PROP. II. The Magnitude of the Particles of the Blood being increased, those Capillary Vessels nearest the Heart will be soonest obstructed, and _vice versa_; The rest in Proportion to the Velocity of the Blood, Diameters of the Canals, and their Distance from the Heart. _Demonstration._ This is sufficiently evident from the foregoing; For the sooner those _Moleculæ_ arrive at the Capillary Vessels, the sooner those Vessels will be obstructed, and _vice versa_, and consequently _cæteris paribus_ the Capillaries of the Branches C, H, in the preceeding Figure, which are nearest the Heart, will be sooner obstructed than those of E, F. PROP. III. The Magnitude of the Particles of the Blood must be increased either by the Union of a greater Number of them than in a Natural State; Or by the Alteration of their Figure, by which their Surfaces become larger than before. _Demonstration._