"This latent malignity in the atmosphere (says Baron Vansweiten) is known only by its effects, and cannot easily be reduced to any known species of acrimony." In another place he says, "It seems certain that this unknown matter disposes all the humours to a sudden and bad putrefaction[40]." [40] Commentaries on Boerhaave's Aphorisms, vol. v. p. 226, 230. Dr. John Stedman has related many facts, in his Essay upon Insalutary Constitutions of the Air, which prove, that diseases are influenced by a quality in it, which, he says, "is productive of corruption," but which has hitherto eluded the researches of physicians[41]. [41] Page 135. Mr. Lempriere, after mentioning the unusual mortality occasioned by the yellow fever, within the last five or six years, in the island of Jamaica, ascribes it wholly "to that particular constitution of atmosphere upon which the existence of epidemics, at one period rather than another, depend[42]." [42] Vol. ii. p. 31. Not only diseases bear testimony to a change in the atmosphere, but the whole vegetable and animal creation concur in it, proofs of which were mentioned in another place. Three things are remarkable with respect to this inflammatory constitution of the air. 1. It is sometimes of a local nature, and influences the diseases of a city, or country, while adjoining cities and countries are exempted from it. 2. It much oftener pervades a great extent of country. This was evident in the years 1793 and 1794, in the United States. During the same years, the yellow fever prevailed in most of the West-India islands. Many of the epidemics mentioned by Dr. Sims, in the first volume of the Medical Memoirs, affected, in the same years, the most remote parts of the continent of Europe. Even the ocean partakes of a morbid constitution of its atmosphere, and diseases at sea sympathise in violence with those of the land, at an immense distance from each other. This appears in a letter from a surgeon, on board a British ship of war, to Mr. Gooch, published in the third volume of his Medical and Surgical Observations. 3. The predisposing state of the atmosphere to induce malignant diseases continues for several years, under all the circumstances of wet and dry, and of hot and cold weather. This will appear, from attending to the accounts which have been given of the weather, in all the years in which the yellow fever has prevailed in Philadelphia since 1792[43]. The remark is confirmed by all the records of malignant epidemics. [43] Vol. iii. and iv.