These vicissitudes in Italy, and those which are wont to occur in regions within the tropics, are much greater than the variations of weather which are experienced in the British Isles, and which are comparatively harmless; or are hurtful, at least, in a much less degree. In many countries the rivers periodically overflow their banks and cover the surrounding territory. The Nile overflows annually, and when the water has almost disappeared by infiltration into the soil, and by evaporation, and when that which is left is muddy, slimy, and mixed with organized remains, exhalations arise, and a vitiated atmosphere is produced, which is said by medical men, who have lived upon the banks of that river, to be productive of plague. The territory again on the banks of the Canton river in China, is almost constantly under water, and its fertility is thereby much increased. The ground there is used for the growth of rice which delights in a soil covered with water. When the heat is intense, when the water contains organized putrefying materials, and when the weather is close, and the atmosphere is a little agitated, then vapours ascend which, mixing with the air, cause it to be vitiated, and to be productive of malignant remittent fever. The habits of nations are also influential in the production of disease. The privations and penances which devotees endure are followed by a very hurtful influence on the health, whether they be what are enjoined, or whether they be voluntarily suffered, as they suppose, to conciliate the favour of the Deity. The diet, clothing, occupations, pleasures, government, laws, social usages, genius, and ambition of nations, materially influence their health, and give tendencies to particular maladies; but interesting as the subject is, the investigation cannot be pursued here. CHAPTER XI. THE AVOIDANCE OF DISEASES MARKED WITH PALPABLE CONTAGIOUS POISONS—THE LIMITED RANGE OF ACTION OF CONTAGION. It was shewn in the first part of this work, that the contagious poisons of disease, such as the matter of small-pox, are known to act in two modes only, _first_, by application of the palpable matter itself to a person, or by contactual contagion; _secondly_, by application of clothes or other such substances, impregnated with that matter, forming what has been styled fomitic or mediate contagion. It was also shewn, that their action through the medium of the atmosphere, has never been ascertained. Experiments were detailed, which were performed on those poisons, to ascertain their capability to become dissolved in the air, and their evidence was as strong as it possibly could be, against their possessing that attribute. It was, in short, fully ascertained, that contagious diseases do not propagate by atmospheric contagion.