It occurred to me, that on the formation of the Epidemiological Society, the first question for consideration should be, What is the nature of those agents, which induce Epidemic Diseases? are they composed of animate or inanimate matter? In other words, do the manifestations of these diseases exhibit the operations of living or of chemical forces. Having, in my study, dwelt on the subject with an earnest desire to find the truth, I put the suggestion, with my ideas, before the public to reject or receive them. If they be rejected, I can but think a full discussion of the enquiry will lead to the most important results. If they be received with favour, I doubt not others, with more ability, will take up the strain and resolve the discords into harmony. J. G. _Wandsworth, September, 1850._ {ix} CONTENTS. PAGE INTRODUCTION 1 CHAPTER I. IS IT PROBABLE THAT EPIDEMIC, ENDEMIC, AND INFECTIOUS DISEASES, DEPEND UPON VITAL GERMS FOR THEIR MANIFESTATIONS? 11 CHAPTER II. THE NUMBER AND VALUE OF FACTS TO SUPPORT THE PROPOSITION. SECTION I.--On Reproduction 22 SECTION II.--Historical Notice of Epidemic Diseases 34 SECTION III.--The Dispersion of Plants and Diseases 64 SECTION IV.--The Relation between Epidemic and Endemic Diseases 96 CHAPTER III. THE REASONABLENESS OF THE APPLICATION OF THE FACTS TO THE INFERENCE. SECTION I.--The Chemical Theory of Epidemics untenable 108 SECTION II.--The Animalcular Theory of Epidemics untenable 128 SECTION III.--Sketch of the Physiology and Pathology of Plants and Animals 138 CHAPTER IV. RESULTS IN PROOF OF THE TENABLENESS OF THE PROPOSITION. SECTION I.--Observations on some of the Laws of Epidemic Diseases 155 SECTION II.--What is the nature of those Poisons which most resemble the Morbid Poisons in their effects on the body? 166 SECTION III.--What results do we obtain from the effects of remedial agents, in proof of the hypothesis? 176 CONCLUSION 189 * * * * * {1} INTRODUCTION. It is one thing for a man to convince himself, but a very different thing to be able to convince others. I am not now speaking of a conviction arising from the impression made by a few startling facts, nor of one forced on the mind by early prejudices, or by the dogmas of the schools, but of a conviction arising from careful enquiry.